Internet Engineering Task Force (IETF) T. Lodderstedt, Ed.
Request for Comments: 6819 Deutsche Telekom AG
Category: Informational M. McGloin
ISSN: 2070-1721 IBM
P. Hunt
Oracle Corporation
January 2013
OAuth 2.0 Threat Model and Security Considerations
Abstract
This document gives additional security considerations for OAuth,
beyond those in the OAuth 2.0 specification, based on a comprehensive
threat model for the OAuth 2.0 protocol.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6819.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Lodderstedt, et al. Informational [Page 1]
RFC 6819 OAuth 2.0 Security January 2013
Table of Contents
1. Introduction ....................................................6
2. Overview ........................................................7
2.1. Scope ......................................................7
2.2. Attack Assumptions .........................................7
2.3. Architectural Assumptions ..................................8
2.3.1. Authorization Servers ...............................8
2.3.2. Resource Server .....................................9
2.3.3. Client ..............................................9
3. Security Features ...............................................9
3.1. Tokens ....................................................10
3.1.1. Scope ..............................................11
3.1.2. Limited Access Token Lifetime ......................11
3.2. Access Token ..............................................11
3.3. Refresh Token .............................................11
3.4. Authorization "code" ......................................12
3.5. Redirect URI ..............................................13
3.6. "state" Parameter .........................................13
3.7. Client Identifier .........................................13
4. Threat Model ...................................................15
4.1. Clients ...................................................16
4.1.1. Threat: Obtaining Client Secrets ...................16
4.1.2. Threat: Obtaining Refresh Tokens ...................17
4.1.3. Threat: Obtaining Access Tokens ....................19
4.1.4. Threat: End-User Credentials Phished Using
Compromised or Embedded Browser ....................19
4.1.5. Threat: Open Redirectors on Client .................20
4.2. Authorization Endpoint ....................................21
4.2.1. Threat: Password Phishing by Counterfeit
Authorization Server ...............................21
4.2.2. Threat: User Unintentionally Grants Too
Much Access Scope ..................................21
4.2.3. Threat: Malicious Client Obtains Existing
Authorization by Fraud .............................22
4.2.4. Threat: Open Redirector ............................22
4.3. Token Endpoint ............................................23
4.3.1. Threat: Eavesdropping Access Tokens ................23
4.3.2. Threat: Obtaining Access Tokens from
Authorization Server Database ......................23
4.3.3. Threat: Disclosure of Client Credentials
during Transmission ................................23
4.3.4. Threat: Obtaining Client Secret from
Authorization Server Database ......................24
4.3.5. Threat: Obtaining Client Secret by Online Guessing .24
Lodderstedt, et al. Informational [Page 2]
RFC 6819 OAuth 2.0 Security January 2013
4.4. Obtaining Authorization ...................................25
4.4.1. Authorization "code" ...............................25
4.4.1.1. Threat: Eavesdropping or Leaking
Authorization "codes" .....................25
4.4.1.2. Threat: Obtaining Authorization "codes"
from Authorization Server Database ........26
4.4.1.3. Threat: Online Guessing of
Authorization "codes" .....................27
4.4.1.4. Threat: Malicious Client Obtains
Authorization .............................27
4.4.1.5. Threat: Authorization "code" Phishing .....29
4.4.1.6. Threat: User Session Impersonation ........29
4.4.1.7. Threat: Authorization "code" Leakage
through Counterfeit Client ................30
4.4.1.8. Threat: CSRF Attack against redirect-uri ..32
4.4.1.9. Threat: Clickjacking Attack against
Authorization .............................33
4.4.1.10. Threat: Resource Owner Impersonation .....33
4.4.1.11. Threat: DoS Attacks That Exhaust
Resources ................................34
4.4.1.12. Threat: DoS Using Manufactured
Authorization "codes" ....................35
4.4.1.13. Threat: Code Substitution (OAuth Login) ..36
4.4.2. Implicit Grant .....................................37
4.4.2.1. Threat: Access Token Leak in
Transport/Endpoints .......................37
4.4.2.2. Threat: Access Token Leak in
Browser History ...........................38
4.4.2.3. Threat: Malicious Client Obtains
Authorization .............................38
4.4.2.4. Threat: Manipulation of Scripts ...........38
4.4.2.5. Threat: CSRF Attack against redirect-uri ..39
4.4.2.6. Threat: Token Substitution (OAuth Login) ..39
4.4.3. Resource Owner Password Credentials ................40
4.4.3.1. Threat: Accidental Exposure of
Passwords at Client Site ..................41
4.4.3.2. Threat: Client Obtains Scopes
without End-User Authorization ............42
4.4.3.3. Threat: Client Obtains Refresh
Token through Automatic Authorization .....42
4.4.3.4. Threat: Obtaining User Passwords
on Transport ..............................43
4.4.3.5. Threat: Obtaining User Passwords
from Authorization Server Database ........43
4.4.3.6. Threat: Online Guessing ...................43
4.4.4. Client Credentials .................................44
Lodderstedt, et al. Informational [Page 3]
RFC 6819 OAuth 2.0 Security January 2013
4.5. Refreshing an Access Token ................................44
4.5.1. Threat: Eavesdropping Refresh Tokens from
Authorization Server ...............................44
4.5.2. Threat: Obtaining Refresh Token from
Authorization Server Database ......................44
4.5.3. Threat: Obtaining Refresh Token by Online
Guessing ...........................................45
4.5.4. Threat: Refresh Token Phishing by
Counterfeit Authorization Server ...................45
4.6. Accessing Protected Resources .............................46
4.6.1. Threat: Eavesdropping Access Tokens on Transport ...46
4.6.2. Threat: Replay of Authorized Resource
Server Requests ....................................46
4.6.3. Threat: Guessing Access Tokens .....................46
4.6.4. Threat: Access Token Phishing by
Counterfeit Resource Server ........................47
4.6.5. Threat: Abuse of Token by Legitimate
Resource Server or Client ..........................48
4.6.6. Threat: Leak of Confidential Data in HTTP Proxies ..48
4.6.7. Threat: Token Leakage via Log Files and
HTTP Referrers .....................................48
5. Security Considerations ........................................49
5.1. General ...................................................49
5.1.1. Ensure Confidentiality of Requests .................49
5.1.2. Utilize Server Authentication ......................50
5.1.3. Always Keep the Resource Owner Informed ............50
5.1.4. Credentials ........................................51
5.1.4.1. Enforce Credential Storage
Protection Best Practices .................51
5.1.4.2. Online Attacks on Secrets .................52
5.1.5. Tokens (Access, Refresh, Code) .....................53
5.1.5.1. Limit Token Scope .........................53
5.1.5.2. Determine Expiration Time .................54
5.1.5.3. Use Short Expiration Time .................54
5.1.5.4. Limit Number of Usages or One-Time Usage ..55
5.1.5.5. Bind Tokens to a Particular
Resource Server (Audience) ................55
5.1.5.6. Use Endpoint Address as Token Audience ....56
5.1.5.7. Use Explicitly Defined Scopes for
Audience and Tokens .......................56
5.1.5.8. Bind Token to Client id ...................56
5.1.5.9. Sign Self-Contained Tokens ................56
5.1.5.10. Encrypt Token Content ....................56
5.1.5.11. Adopt a Standard Assertion Format ........57
5.1.6. Access Tokens ......................................57
Lodderstedt, et al. Informational [Page 4]
RFC 6819 OAuth 2.0 Security January 2013
5.2. Authorization Server ......................................57
5.2.1. Authorization "codes" ..............................57
5.2.1.1. Automatic Revocation of Derived
Tokens If Abuse Is Detected ...............57
5.2.2. Refresh Tokens .....................................57
5.2.2.1. Restricted Issuance of Refresh Tokens .....57
5.2.2.2. Binding of Refresh Token to "client_id" ...58
5.2.2.3. Refresh Token Rotation ....................58
5.2.2.4. Revocation of Refresh Tokens ..............58
5.2.2.5. Device Identification .....................59
5.2.2.6. X-FRAME-OPTIONS Header ....................59
5.2.3. Client Authentication and Authorization ............59
5.2.3.1. Don't Issue Secrets to Clients with
Inappropriate Security Policy .............60
5.2.3.2. Require User Consent for Public
Clients without Secret ....................60
5.2.3.3. Issue a "client_id" Only in
Combination with "redirect_uri" ...........61
5.2.3.4. Issue Installation-Specific Client
Secrets ...................................61
5.2.3.5. Validate Pre-Registered "redirect_uri" ....62
5.2.3.6. Revoke Client Secrets .....................63
5.2.3.7. Use Strong Client Authentication
(e.g., client_assertion/client_token) .....63
5.2.4. End-User Authorization .............................63
5.2.4.1. Automatic Processing of Repeated
Authorizations Requires Client Validation .63
5.2.4.2. Informed Decisions Based on Transparency ..63
5.2.4.3. Validation of Client Properties by
End User ..................................64
5.2.4.4. Binding of Authorization "code" to
"client_id" ...............................64
5.2.4.5. Binding of Authorization "code" to
"redirect_uri" ............................64
5.3. Client App Security .......................................65
5.3.1. Don't Store Credentials in Code or
Resources Bundled with Software Packages ...........65
5.3.2. Use Standard Web Server Protection Measures
(for Config Files and Databases) ...................65
5.3.3. Store Secrets in Secure Storage ....................65
5.3.4. Utilize Device Lock to Prevent Unauthorized
Device Access ......................................66
5.3.5. Link the "state" Parameter to User Agent Session ...66
5.4. Resource Servers ..........................................66
5.4.1. Authorization Headers ..............................66
5.4.2. Authenticated Requests .............................67
5.4.3. Signed Requests ....................................67
5.5. A Word on User Interaction and User-Installed Apps ........68
Lodderstedt, et al. Informational [Page 5]
RFC 6819 OAuth 2.0 Security January 2013
6. Acknowledgements ...............................................69
7. References .....................................................69
7.1. Normative References ......................................69
7.2. Informative References ....................................69
1. Introduction
This document gives additional security considerations for OAuth,
beyond those in the OAuth specification, based on a comprehensive
threat model for the OAuth 2.0 protocol [RFC6749]. It contains the
following content:
o Documents any assumptions and scope considered when creating the
threat model.
o Describes the security features built into the OAuth protocol and
how they are intended to thwart attacks.
o Gives a comprehensive threat model for OAuth and describes the
respective countermeasures to thwart those threats.
Threats include any intentional attacks on OAuth tokens and resources
protected by OAuth tokens, as well as security risks introduced if
the proper security measures are not put in place. Threats are
structured along the lines of the protocol structure to help
development teams implement each part of the protocol securely, for
example, all threats for granting access, or all threats for a
particular grant type, or all threats for protecting the resource
server.
Note: This document cannot assess the probability or the risk
associated with a particular threat because those aspects strongly
depend on the particular application and deployment OAuth is used to
protect. Similarly, impacts are given on a rather abstract level.
But the information given here may serve as a foundation for
deployment-specific threat models. Implementors may refine and
detail the abstract threat model in order to account for the specific
properties of their deployment and to come up with a risk analysis.
As this document is based on the base OAuth 2.0 specification, it
does not consider proposed extensions such as client registration or
discovery, many of which are still under discussion.
Lodderstedt, et al. Informational [Page 6]
RFC 6819 OAuth 2.0 Security January 2013
2. Overview
2.1. Scope
This security considerations document only considers clients bound to
a particular deployment as supported by [RFC6749]. Such deployments
have the following characteristics:
o Resource server URLs are static and well-known at development
time; authorization server URLs can be static or discovered.
o Token scope values (e.g., applicable URLs and methods) are well-
known at development time.
o Client registration is out of scope of the current core
specification. Therefore, this document assumes a broad variety
of options, from static registration during development time to
dynamic registration at runtime.
The following are considered out of scope:
o Communication between the authorization server and resource
server.
o Token formats.
o Except for the resource owner password credentials grant type (see
[RFC6749], Section 4.3), the mechanism used by authorization
servers to authenticate the user.
o Mechanism by which a user obtained an assertion and any resulting
attacks mounted as a result of the assertion being false.
o Clients not bound to a specific deployment: An example could be a
mail client with support for contact list access via the portable
contacts API (see [Portable-Contacts]). Such clients cannot be
registered upfront with a particular deployment and should
dynamically discover the URLs relevant for the OAuth protocol.
2.2. Attack Assumptions
The following assumptions relate to an attacker and resources
available to an attacker. It is assumed that:
o the attacker has full access to the network between the client and
authorization servers and the client and the resource server,
respectively. The attacker may eavesdrop on any communications
Lodderstedt, et al. Informational [Page 7]
RFC 6819 OAuth 2.0 Security January 2013
between those parties. He is not assumed to have access to
communication between the authorization server and resource
server.
o an attacker has unlimited resources to mount an attack.
o two of the three parties involved in the OAuth protocol may
collude to mount an attack against the 3rd party. For example,
the client and authorization server may be under control of an
attacker and collude to trick a user to gain access to resources.
2.3. Architectural Assumptions
This section documents assumptions about the features, limitations,
and design options of the different entities of an OAuth deployment
along with the security-sensitive data elements managed by those
entities. These assumptions are the foundation of the threat
analysis.
The OAuth protocol leaves deployments with a certain degree of
freedom regarding how to implement and apply the standard. The core
specification defines the core concepts of an authorization server
and a resource server. Both servers can be implemented in the same
server entity, or they may also be different entities. The latter is
typically the case for multi-service providers with a single
authentication and authorization system and is more typical in
middleware architectures.
2.3.1. Authorization Servers
The following data elements are stored or accessible on the
authorization server:
o usernames and passwords
o client ids and secrets
o client-specific refresh tokens
o client-specific access tokens (in the case of handle-based design;
see Section 3.1)
o HTTPS certificate/key
o per-authorization process (in the case of handle-based design;
Section 3.1): "redirect_uri", "client_id", authorization "code"
Lodderstedt, et al. Informational [Page 8]
RFC 6819 OAuth 2.0 Security January 2013
2.3.2. Resource Server
The following data elements are stored or accessible on the resource
server:
o user data (out of scope)
o HTTPS certificate/key
o either authorization server credentials (handle-based design; see
Section 3.1) or authorization server shared secret/public key
(assertion-based design; see Section 3.1)
o access tokens (per request)
It is assumed that a resource server has no knowledge of refresh
tokens, user passwords, or client secrets.
2.3.3. Client
In OAuth, a client is an application making protected resource
requests on behalf of the resource owner and with its authorization.
There are different types of clients with different implementation
and security characteristics, such as web, user-agent-based, and
native applications. A full definition of the different client types
and profiles is given in [RFC6749], Section 2.1.
The following data elements are stored or accessible on the client:
o client id (and client secret or corresponding client credential)
o one or more refresh tokens (persistent) and access tokens
(transient) per end user or other security-context or delegation
context
o trusted certification authority (CA) certificates (HTTPS)
o per-authorization process: "redirect_uri", authorization "code"
3. Security Features
These are some of the security features that have been built into the
OAuth 2.0 protocol to mitigate attacks and security issues.
Lodderstedt, et al. Informational [Page 9]
RFC 6819 OAuth 2.0 Security January 2013
3.1. Tokens
OAuth makes extensive use of many kinds of tokens (access tokens,
refresh tokens, authorization "codes"). The information content of a
token can be represented in two ways, as follows:
Handle (or artifact) A 'handle' is a reference to some internal data
structure within the authorization server; the internal data
structure contains the attributes of the token, such as user id
(UID), scope, etc. Handles enable simple revocation and do not
require cryptographic mechanisms to protect token content from
being modified. On the other hand, handles require communication
between the issuing and consuming entity (e.g., the authorization
server and resource server) in order to validate the token and
obtain token-bound data. This communication might have a negative
impact on performance and scalability if both entities reside on
different systems. Handles are therefore typically used if the
issuing and consuming entity are the same. A 'handle' token is
often referred to as an 'opaque' token because the resource server
does not need to be able to interpret the token directly; it
simply uses the token.
Assertion (aka self-contained token) An assertion is a parseable
token. An assertion typically has a duration, has an audience,
and is digitally signed in order to ensure data integrity and
origin authentication. It contains information about the user and
the client. Examples of assertion formats are Security Assertion
Markup Language (SAML) assertions [OASIS.saml-core-2.0-os] and
Kerberos tickets [RFC4120]. Assertions can typically be directly
validated and used by a resource server without interactions with
the authorization server. This results in better performance and
scalability in deployments where the issuing and consuming
entities reside on different systems. Implementing token
revocation is more difficult with assertions than with handles.
Tokens can be used in two ways to invoke requests on resource
servers, as follows:
bearer token A 'bearer token' is a token that can be used by any
client who has received the token (e.g., [RFC6750]). Because mere
possession is enough to use the token, it is important that
communication between endpoints be secured to ensure that only
authorized endpoints may capture the token. The bearer token is
convenient for client applications, as it does not require them to
do anything to use them (such as a proof of identity). Bearer
tokens have similar characteristics to web single-sign-on (SSO)
cookies used in browsers.
Lodderstedt, et al. Informational [Page 10]
RFC 6819 OAuth 2.0 Security January 2013
proof token A 'proof token' is a token that can only be used by a
specific client. Each use of the token requires the client to
perform some action that proves that it is the authorized user of
the token. Examples of this are MAC-type access tokens, which
require the client to digitally sign the resource request with a
secret corresponding to the particular token sent with the request
(e.g., [OAuth-HTTP-MAC]).
3.1.1. Scope
A scope represents the access authorization associated with a
particular token with respect to resource servers, resources, and
methods on those resources. Scopes are the OAuth way to explicitly
manage the power associated with an access token. A scope can be
controlled by the authorization server and/or the end user in order
to limit access to resources for OAuth clients that these parties
deem less secure or trustworthy. Optionally, the client can request
the scope to apply to the token but only for a lesser scope than
would otherwise be granted, e.g., to reduce the potential impact if
this token is sent over non-secure channels. A scope is typically
complemented by a restriction on a token's lifetime.
3.1.2. Limited Access Token Lifetime
The protocol parameter "expires_in" allows an authorization server
(based on its policies or on behalf of the end user) to limit the
lifetime of an access token and to pass this information to the
client. This mechanism can be used to issue short-lived tokens to
OAuth clients that the authorization server deems less secure, or
where sending tokens over non-secure channels.
3.2. Access Token
An access token is used by a client to access a resource. Access
tokens typically have short life spans (minutes or hours) that cover
typical session lifetimes. An access token may be refreshed through
the use of a refresh token. The short lifespan of an access token,
in combination with the usage of refresh tokens, enables the
possibility of passive revocation of access authorization on the
expiry of the current access token.
3.3. Refresh Token
A refresh token represents a long-lasting authorization of a certain
client to access resources on behalf of a resource owner. Such
tokens are exchanged between the client and authorization server
only. Clients use this kind of token to obtain ("refresh") new
access tokens used for resource server invocations.
Lodderstedt, et al. Informational [Page 11]
RFC 6819 OAuth 2.0 Security January 2013
A refresh token, coupled with a short access token lifetime, can be
used to grant longer access to resources without involving end-user
authorization. This offers an advantage where resource servers and
authorization servers are not the same entity, e.g., in a distributed
environment, as the refresh token is always exchanged at the
authorization server. The authorization server can revoke the
refresh token at any time, causing the granted access to be revoked
once the current access token expires. Because of this, a short
access token lifetime is important if timely revocation is a high
priority.
The refresh token is also a secret bound to the client identifier and
client instance that originally requested the authorization; the
refresh token also represents the original resource owner grant.
This is ensured by the authorization process as follows:
1. The resource owner and user agent safely deliver the
authorization "code" to the client instance in the first place.
2. The client uses it immediately in secure transport-level
communications to the authorization server and then securely
stores the long-lived refresh token.
3. The client always uses the refresh token in secure transport-
level communications to the authorization server to get an access
token (and optionally roll over the refresh token).
So, as long as the confidentiality of the particular token can be
ensured by the client, a refresh token can also be used as an
alternative means to authenticate the client instance itself.
3.4. Authorization "code"
An authorization "code" represents the intermediate result of a
successful end-user authorization process and is used by the client
to obtain access and refresh tokens. Authorization "codes" are sent
to the client's redirect URI instead of tokens for two purposes:
1. Browser-based flows expose protocol parameters to potential
attackers via URI query parameters (HTTP referrer), the browser
cache, or log file entries, and could be replayed. In order to
reduce this threat, short-lived authorization "codes" are passed
instead of tokens and exchanged for tokens over a more secure
direct connection between the client and the authorization
server.
Lodderstedt, et al. Informational [Page 12]
RFC 6819 OAuth 2.0 Security January 2013
2. It is much simpler to authenticate clients during the direct
request between the client and the authorization server than in
the context of the indirect authorization request. The latter
would require digital signatures.
3.5. Redirect URI
A redirect URI helps to detect malicious clients and prevents
phishing attacks from clients attempting to trick the user into
believing the phisher is the client. The value of the actual
redirect URI used in the authorization request has to be presented
and is verified when an authorization "code" is exchanged for tokens.
This helps to prevent attacks where the authorization "code" is
revealed through redirectors and counterfeit web application clients.
The authorization server should require public clients and
confidential clients using the implicit grant type to pre-register
their redirect URIs and validate against the registered redirect URI
in the authorization request.
3.6. "state" Parameter
The "state" parameter is used to link requests and callbacks to
prevent cross-site request forgery attacks (see Section 4.4.1.8)
where an attacker authorizes access to his own resources and then
tricks a user into following a redirect with the attacker's token.
This parameter should bind to the authenticated state in a user agent
and, as per the core OAuth spec, the user agent must be capable of
keeping it in a location accessible only by the client and user
agent, i.e., protected by same-origin policy.
3.7. Client Identifier
Authentication protocols have typically not taken into account the
identity of the software component acting on behalf of the end user.
OAuth does this in order to increase the security level in delegated
authorization scenarios and because the client will be able to act
without the user being present.
OAuth uses the client identifier to collate associated requests to
the same originator, such as
o a particular end-user authorization process and the corresponding
request on the token's endpoint to exchange the authorization
"code" for tokens, or
Lodderstedt, et al. Informational [Page 13]
RFC 6819 OAuth 2.0 Security January 2013
o the initial authorization and issuance of a token by an end user
to a particular client, and subsequent requests by this client to
obtain tokens without user consent (automatic processing of
repeated authorizations)
This identifier may also be used by the authorization server to
display relevant registration information to a user when requesting
consent for a scope requested by a particular client. The client
identifier may be used to limit the number of requests for a
particular client or to charge the client per request. It may
furthermore be useful to differentiate access by different clients,
e.g., in server log files.
OAuth defines two client types, confidential and public, based on
their ability to authenticate with the authorization server (i.e.,
ability to maintain the confidentiality of their client credentials).
Confidential clients are capable of maintaining the confidentiality
of client credentials (i.e., a client secret associated with the
client identifier) or capable of secure client authentication using
other means, such as a client assertion (e.g., SAML) or key
cryptography. The latter is considered more secure.
The authorization server should determine whether the client is
capable of keeping its secret confidential or using secure
authentication. Alternatively, the end user can verify the identity
of the client, e.g., by only installing trusted applications. The
redirect URI can be used to prevent the delivery of credentials to a
counterfeit client after obtaining end-user authorization in some
cases but can't be used to verify the client identifier.
Clients can be categorized as follows based on the client type,
profile (e.g., native vs. web application; see [RFC6749], Section 9),
and deployment model:
Deployment-independent "client_id" with pre-registered "redirect_uri"
and without "client_secret" Such an identifier is used by
multiple installations of the same software package. The
identifier of such a client can only be validated with the help of
the end-user. This is a viable option for native applications in
order to identify the client for the purpose of displaying meta
information about the client to the user and to differentiate
clients in log files. Revocation of the rights associated with
such a client identifier will affect ALL deployments of the
respective software.
Lodderstedt, et al. Informational [Page 14]
RFC 6819 OAuth 2.0 Security January 2013
Deployment-independent "client_id" with pre-registered "redirect_uri"
and with "client_secret" This is an option for native
applications only, since web applications would require different
redirect URIs. This category is not advisable because the client
secret cannot be protected appropriately (see Section 4.1.1). Due
to its security weaknesses, such client identities have the same
trust level as deployment-independent clients without secrets.
Revocation will affect ALL deployments.
Deployment-specific "client_id" with pre-registered "redirect_uri"
and with "client_secret" The client registration process ensures
the validation of the client's properties, such as redirect URI,
web site URL, web site name, and contacts. Such a client
identifier can be utilized for all relevant use cases cited above.
This level can be achieved for web applications in combination
with a manual or user-bound registration process. Achieving this
level for native applications is much more difficult. Either the
installation of the application is conducted by an administrator,
who validates the client's authenticity, or the process from
validating the application to the installation of the application
on the device and the creation of the client credentials is
controlled end-to-end by a single entity (e.g., application market
provider). Revocation will affect a single deployment only.
Deployment-specific "client_id" with "client_secret" without
validated properties Such a client can be recognized by the
authorization server in transactions with subsequent requests
(e.g., authorization and token issuance, refresh token issuance,
and access token refreshment). The authorization server cannot
assure any property of the client to end users. Automatic
processing of re-authorizations could be allowed as well. Such
client credentials can be generated automatically without any
validation of client properties, which makes it another option,
especially for native applications. Revocation will affect a
single deployment only.
4. Threat Model
This section gives a comprehensive threat model of OAuth 2.0.
Threats are grouped first by attacks directed against an OAuth
component, which are the client, authorization server, and resource
server. Subsequently, they are grouped by flow, e.g., obtain token
or access protected resources. Every countermeasure description
refers to a detailed description in Section 5.
Lodderstedt, et al. Informational [Page 15]
RFC 6819 OAuth 2.0 Security January 2013
4.1. Clients
This section describes possible threats directed to OAuth clients.
4.1.1. Threat: Obtaining Client Secrets
The attacker could try to get access to the secret of a particular
client in order to:
o replay its refresh tokens and authorization "codes", or
o obtain tokens on behalf of the attacked client with the privileges
of that "client_id" acting as an instance of the client.
The resulting impact would be the following:
o Client authentication of access to the authorization server can be
bypassed.
o Stolen refresh tokens or authorization "codes" can be replayed.
Depending on the client category, the following attacks could be
utilized to obtain the client secret.
Attack: Obtain Secret From Source Code or Binary:
This applies for all client types. For open source projects, secrets
can be extracted directly from source code in their public
repositories. Secrets can be extracted from application binaries
just as easily when the published source is not available to the
attacker. Even if an application takes significant measures to
obfuscate secrets in their application distribution, one should
consider that the secret can still be reverse-engineered by anyone
with access to a complete functioning application bundle or binary.
Countermeasures:
o Don't issue secrets to public clients or clients with
inappropriate security policy (Section 5.2.3.1).
o Require user consent for public clients (Section 5.2.3.2).
o Use deployment-specific client secrets (Section 5.2.3.4).
o Revoke client secrets (Section 5.2.3.6).
Lodderstedt, et al. Informational [Page 16]
RFC 6819 OAuth 2.0 Security January 2013
Attack: Obtain a Deployment-Specific Secret:
An attacker may try to obtain the secret from a client installation,
either from a web site (web server) or a particular device (native
application).
Countermeasures:
o Web server: Apply standard web server protection measures (for
config files and databases) (see Section 5.3.2).
o Native applications: Store secrets in secure local storage
(Section 5.3.3).
o Revoke client secrets (Section 5.2.3.6).
4.1.2. Threat: Obtaining Refresh Tokens
Depending on the client type, there are different ways that refresh
tokens may be revealed to an attacker. The following sub-sections
give a more detailed description of the different attacks with
respect to different client types and further specialized
countermeasures. Before detailing those threats, here are some
generally applicable countermeasures:
o The authorization server should validate the client id associated
with the particular refresh token with every refresh request
(Section 5.2.2.2).
o Limit token scope (Section 5.1.5.1).
o Revoke refresh tokens (Section 5.2.2.4).
o Revoke client secrets (Section 5.2.3.6).
o Refresh tokens can automatically be replaced in order to detect
unauthorized token usage by another party (see "Refresh Token
Rotation", Section 5.2.2.3).
Attack: Obtain Refresh Token from Web Application:
An attacker may obtain the refresh tokens issued to a web application
by way of overcoming the web server's security controls.
Impact: Since a web application manages the user accounts of a
certain site, such an attack would result in an exposure of all
refresh tokens on that site to the attacker.
Lodderstedt, et al. Informational [Page 17]
RFC 6819 OAuth 2.0 Security January 2013
Countermeasures:
o Standard web server protection measures (Section 5.3.2).
o Use strong client authentication (e.g., client_assertion/
client_token) so the attacker cannot obtain the client secret
required to exchange the tokens (Section 5.2.3.7).
Attack: Obtain Refresh Token from Native Clients:
On native clients, leakage of a refresh token typically affects a
single user only.
Read from local file system: The attacker could try to get file
system access on the device and read the refresh tokens. The
attacker could utilize a malicious application for that purpose.
Countermeasures:
o Store secrets in secure storage (Section 5.3.3).
o Utilize device lock to prevent unauthorized device access
(Section 5.3.4).
Attack: Steal Device:
The host device (e.g., mobile phone) may be stolen. In that case,
the attacker gets access to all applications under the identity of
the legitimate user.
Countermeasures:
o Utilize device lock to prevent unauthorized device access
(Section 5.3.4).
o Where a user knows the device has been stolen, they can revoke the
affected tokens (Section 5.2.2.4).
Attack: Clone Device:
All device data and applications are copied to another device.
Applications are used as-is on the target device.
Lodderstedt, et al. Informational [Page 18]
RFC 6819 OAuth 2.0 Security January 2013
Countermeasures:
o Utilize device lock to prevent unauthorized device access
(Section 5.3.4).
o Combine refresh token request with device identification
(Section 5.2.2.5).
o Refresh token rotation (Section 5.2.2.3).
o Where a user knows the device has been cloned, they can use
refresh token revocation (Section 5.2.2.4).
4.1.3. Threat: Obtaining Access Tokens
Depending on the client type, there are different ways that access
tokens may be revealed to an attacker. Access tokens could be stolen
from the device if the application stores them in a storage device
that is accessible to other applications.
Impact: Where the token is a bearer token and no additional mechanism
is used to identify the client, the attacker can access all resources
associated with the token and its scope.
Countermeasures:
o Keep access tokens in transient memory and limit grants
(Section 5.1.6).
o Limit token scope (Section 5.1.5.1).
o Keep access tokens in private memory or apply same protection
means as for refresh tokens (Section 5.2.2).
o Keep access token lifetime short (Section 5.1.5.3).
4.1.4. Threat: End-User Credentials Phished Using Compromised or
Embedded Browser
A malicious application could attempt to phish end-user passwords by
misusing an embedded browser in the end-user authorization process,
or by presenting its own user interface instead of allowing a trusted
system browser to render the authorization user interface. By doing
so, the usual visual trust mechanisms may be bypassed (e.g.,
Transport Layer Security (TLS) confirmation, web site mechanisms).
By using an embedded or internal client application user interface,
the client application has access to additional information to which
it should not have access (e.g., UID/password).
Lodderstedt, et al. Informational [Page 19]
RFC 6819 OAuth 2.0 Security January 2013
Impact: If the client application or the communication is
compromised, the user would not be aware of this, and all information
in the authorization exchange, such as username and password, could
be captured.
Countermeasures:
o The OAuth flow is designed so that client applications never need
to know user passwords. Client applications should avoid directly
asking users for their credentials. In addition, end users could
be educated about phishing attacks and best practices, such as
only accessing trusted clients, as OAuth does not provide any
protection against malicious applications and the end user is
solely responsible for the trustworthiness of any native
application installed.
o Client applications could be validated prior to publication in an
application market for users to access. That validation is out of
scope for OAuth but could include validating that the client
application handles user authentication in an appropriate way.
o Client developers should not write client applications that
collect authentication information directly from users and should
instead delegate this task to a trusted system component, e.g.,
the system browser.
4.1.5. Threat: Open Redirectors on Client
An open redirector is an endpoint using a parameter to automatically
redirect a user agent to the location specified by the parameter
value without any validation. If the authorization server allows the
client to register only part of the redirect URI, an attacker can use
an open redirector operated by the client to construct a redirect URI
that will pass the authorization server validation but will send the
authorization "code" or access token to an endpoint under the control
of the attacker.
Impact: An attacker could gain access to authorization "codes" or
access tokens.
Countermeasures:
o Require clients to register full redirect URI (Section 5.2.3.5).
Lodderstedt, et al. Informational [Page 20]
RFC 6819 OAuth 2.0 Security January 2013
4.2. Authorization Endpoint
4.2.1. Threat: Password Phishing by Counterfeit Authorization Server
OAuth makes no attempt to verify the authenticity of the
authorization server. A hostile party could take advantage of this
by intercepting the client's requests and returning misleading or
otherwise incorrect responses. This could be achieved using DNS or
Address Resolution Protocol (ARP) spoofing. Wide deployment of OAuth
and similar protocols may cause users to become inured to the
practice of being redirected to web sites where they are asked to
enter their passwords. If users are not careful to verify the
authenticity of these web sites before entering their credentials, it
will be possible for attackers to exploit this practice to steal
users' passwords.
Countermeasures:
o Authorization servers should consider such attacks when developing
services based on OAuth and should require the use of transport-
layer security for any requests where the authenticity of the
authorization server or of request responses is an issue (see
Section 5.1.2).
o Authorization servers should attempt to educate users about the
risks posed by phishing attacks and should provide mechanisms that
make it easy for users to confirm the authenticity of their sites.
4.2.2. Threat: User Unintentionally Grants Too Much Access Scope
When obtaining end-user authorization, the end user may not
understand the scope of the access being granted and to whom, or they
may end up providing a client with access to resources that should
not be permitted.
Countermeasures:
o Explain the scope (resources and the permissions) the user is
about to grant in an understandable way (Section 5.2.4.2).
o Narrow the scope, based on the client. When obtaining end-user
authorization and where the client requests scope, the
authorization server may want to consider whether to honor that
scope based on the client identifier. That decision is between
the client and authorization server and is outside the scope of
this spec. The authorization server may also want to consider
what scope to grant based on the client type, e.g., providing
lower scope to public clients (Section 5.1.5.1).
Lodderstedt, et al. Informational [Page 21]
RFC 6819 OAuth 2.0 Security January 2013
4.2.3. Threat: Malicious Client Obtains Existing Authorization by Fraud
Authorization servers may wish to automatically process authorization
requests from clients that have been previously authorized by the
user. When the user is redirected to the authorization server's end-
user authorization endpoint to grant access, the authorization server
detects that the user has already granted access to that particular
client. Instead of prompting the user for approval, the
authorization server automatically redirects the user back to the
client.
A malicious client may exploit that feature and try to obtain such an
authorization "code" instead of the legitimate client.
Countermeasures:
o Authorization servers should not automatically process repeat
authorizations to public clients unless the client is validated
using a pre-registered redirect URI (Section 5.2.3.5).
o Authorization servers can mitigate the risks associated with
automatic processing by limiting the scope of access tokens
obtained through automated approvals (Section 5.1.5.1).
4.2.4. Threat: Open Redirector
An attacker could use the end-user authorization endpoint and the
redirect URI parameter to abuse the authorization server as an open
redirector. An open redirector is an endpoint using a parameter to
automatically redirect a user agent to the location specified by the
parameter value without any validation.
Impact: An attacker could utilize a user's trust in an authorization
server to launch a phishing attack.
Countermeasures:
o Require clients to register any full redirect URIs
(Section 5.2.3.5).
o Don't redirect to a redirect URI if the client identifier or
redirect URI can't be verified (Section 5.2.3.5).
Lodderstedt, et al. Informational [Page 22]
RFC 6819 OAuth 2.0 Security January 2013
4.3. Token Endpoint
4.3.1. Threat: Eavesdropping Access Tokens
Attackers may attempt to eavesdrop access tokens in transit from the
authorization server to the client.
Impact: The attacker is able to access all resources with the
permissions covered by the scope of the particular access token.
Countermeasures:
o As per the core OAuth spec, the authorization servers must ensure
that these transmissions are protected using transport-layer
mechanisms such as TLS (see Section 5.1.1).
o If end-to-end confidentiality cannot be guaranteed, reducing scope
(see Section 5.1.5.1) and expiry time (Section 5.1.5.3) for access
tokens can be used to reduce the damage in case of leaks.
4.3.2. Threat: Obtaining Access Tokens from Authorization Server
Database
This threat is applicable if the authorization server stores access
tokens as handles in a database. An attacker may obtain access
tokens from the authorization server's database by gaining access to
the database or launching a SQL injection attack.
Impact: Disclosure of all access tokens.
Countermeasures:
o Enforce system security measures (Section 5.1.4.1.1).
o Store access token hashes only (Section 5.1.4.1.3).
o Enforce standard SQL injection countermeasures
(Section 5.1.4.1.2).
4.3.3. Threat: Disclosure of Client Credentials during Transmission
An attacker could attempt to eavesdrop the transmission of client
credentials between the client and server during the client
authentication process or during OAuth token requests.
Impact: Revelation of a client credential enabling phishing or
impersonation of a client service.
Lodderstedt, et al. Informational [Page 23]
RFC 6819 OAuth 2.0 Security January 2013
Countermeasures:
o The transmission of client credentials must be protected using
transport-layer mechanisms such as TLS (see Section 5.1.1).
o Use alternative authentication means that do not require the
sending of plaintext credentials over the wire (e.g., Hash-based
Message Authentication Code).
4.3.4. Threat: Obtaining Client Secret from Authorization Server
Database
An attacker may obtain valid "client_id"/secret combinations from the
authorization server's database by gaining access to the database or
launching a SQL injection attack.
Impact: Disclosure of all "client_id"/secret combinations. This
allows the attacker to act on behalf of legitimate clients.
Countermeasures:
o Enforce system security measures (Section 5.1.4.1.1).
o Enforce standard SQL injection countermeasures
(Section 5.1.4.1.2).
o Ensure proper handling of credentials as per "Enforce Credential
Storage Protection Best Practices" (Section 5.1.4.1).
4.3.5. Threat: Obtaining Client Secret by Online Guessing
An attacker may try to guess valid "client_id"/secret pairs.
Impact: Disclosure of a single "client_id"/secret pair.
Countermeasures:
o Use high entropy for secrets (Section 5.1.4.2.2).
o Lock accounts (Section 5.1.4.2.3).
o Use strong client authentication (Section 5.2.3.7).
Lodderstedt, et al. Informational [Page 24]
RFC 6819 OAuth 2.0 Security January 2013
4.4. Obtaining Authorization
This section covers threats that are specific to certain flows
utilized to obtain access tokens. Each flow is characterized by
response types and/or grant types on the end-user authorization and
token endpoint, respectively.
4.4.1. Authorization "code"
4.4.1.1. Threat: Eavesdropping or Leaking Authorization "codes"
An attacker could try to eavesdrop transmission of the authorization
"code" between the authorization server and client. Furthermore,
authorization "codes" are passed via the browser, which may
unintentionally leak those codes to untrusted web sites and attackers
in different ways:
o Referrer headers: Browsers frequently pass a "referer" header when
a web page embeds content, or when a user travels from one web
page to another web page. These referrer headers may be sent even
when the origin site does not trust the destination site. The
referrer header is commonly logged for traffic analysis purposes.
o Request logs: Web server request logs commonly include query
parameters on requests.
o Open redirectors: Web sites sometimes need to send users to
another destination via a redirector. Open redirectors pose a
particular risk to web-based delegation protocols because the
redirector can leak verification codes to untrusted destination
sites.
o Browser history: Web browsers commonly record visited URLs in the
browser history. Another user of the same web browser may be able
to view URLs that were visited by previous users.
Note: A description of similar attacks on the SAML protocol can be
found at [OASIS.sstc-saml-bindings-1.1], Section 4.1.1.9.1;
[Sec-Analysis]; and [OASIS.sstc-sec-analysis-response-01].
Lodderstedt, et al. Informational [Page 25]
RFC 6819 OAuth 2.0 Security January 2013
Countermeasures:
o As per the core OAuth spec, the authorization server as well as
the client must ensure that these transmissions are protected
using transport-layer mechanisms such as TLS (see Section 5.1.1).
o The authorization server will require the client to authenticate
wherever possible, so the binding of the authorization "code" to a
certain client can be validated in a reliable way (see
Section 5.2.4.4).
o Use short expiry time for authorization "codes" (Section 5.1.5.3).
o The authorization server should enforce a one-time usage
restriction (see Section 5.1.5.4).
o If an authorization server observes multiple attempts to redeem an
authorization "code", the authorization server may want to revoke
all tokens granted based on the authorization "code" (see
Section 5.2.1.1).
o In the absence of these countermeasures, reducing scope
(Section 5.1.5.1) and expiry time (Section 5.1.5.3) for access
tokens can be used to reduce the damage in case of leaks.
o The client server may reload the target page of the redirect URI
in order to automatically clean up the browser cache.
4.4.1.2. Threat: Obtaining Authorization "codes" from Authorization
Server Database
This threat is applicable if the authorization server stores
authorization "codes" as handles in a database. An attacker may
obtain authorization "codes" from the authorization server's database
by gaining access to the database or launching a SQL injection
attack.
Impact: Disclosure of all authorization "codes", most likely along
with the respective "redirect_uri" and "client_id" values.
Countermeasures:
o Best practices for credential storage protection should be
employed (Section 5.1.4.1).
o Enforce system security measures (Section 5.1.4.1.1).
Lodderstedt, et al. Informational [Page 26]
RFC 6819 OAuth 2.0 Security January 2013
o Store access token hashes only (Section 5.1.4.1.3).
o Enforce standard SQL injection countermeasures
(Section 5.1.4.1.2).
4.4.1.3. Threat: Online Guessing of Authorization "codes"
An attacker may try to guess valid authorization "code" values and
send the guessed code value using the grant type "code" in order to
obtain a valid access token.
Impact: Disclosure of a single access token and probably also an
associated refresh token.
Countermeasures:
o Handle-based tokens must use high entropy (Section 5.1.4.2.2).
o Assertion-based tokens should be signed (Section 5.1.5.9).
o Authenticate the client; this adds another value that the attacker
has to guess (Section 5.2.3.4).
o Bind the authorization "code" to the redirect URI; this adds
another value that the attacker has to guess (Section 5.2.4.5).
o Use short expiry time for tokens (Section 5.1.5.3).
4.4.1.4. Threat: Malicious Client Obtains Authorization
A malicious client could pretend to be a valid client and obtain an
access authorization in this way. The malicious client could even
utilize screen-scraping techniques in order to simulate a user's
consent in the authorization flow.
Assumption: It is not the task of the authorization server to protect
the end-user's device from malicious software. This is the
responsibility of the platform running on the particular device,
probably in cooperation with other components of the respective
ecosystem (e.g., an application management infrastructure). The sole
responsibility of the authorization server is to control access to
the end-user's resources maintained in resource servers and to
prevent unauthorized access to them via the OAuth protocol. Based on
this assumption, the following countermeasures are available to cope
with the threat.
Lodderstedt, et al. Informational [Page 27]
RFC 6819 OAuth 2.0 Security January 2013
Countermeasures:
o The authorization server should authenticate the client, if
possible (see Section 5.2.3.4). Note: The authentication takes
place after the end user has authorized the access.
o The authorization server should validate the client's redirect URI
against the pre-registered redirect URI, if one exists (see
Section 5.2.3.5). Note: An invalid redirect URI indicates an
invalid client, whereas a valid redirect URI does not necessarily
indicate a valid client. The level of confidence depends on the
client type. For web applications, the level of confidence is
high, since the redirect URI refers to the globally unique network
endpoint of this application, whose fully qualified domain name
(FQDN) is also validated using HTTPS server authentication by the
user agent. In contrast, for native clients, the redirect URI
typically refers to device local resources, e.g., a custom scheme.
So, a malicious client on a particular device can use the valid
redirect URI the legitimate client uses on all other devices.
o After authenticating the end user, the authorization server should
ask him/her for consent. In this context, the authorization
server should explain to the end user the purpose, scope, and
duration of the authorization the client asked for. Moreover, the
authorization server should show the user any identity information
it has for that client. It is up to the user to validate the
binding of this data to the particular application (e.g., Name)
and to approve the authorization request (see Section 5.2.4.3).
o The authorization server should not perform automatic
re-authorizations for clients it is unable to reliably
authenticate or validate (see Section 5.2.4.1).
o If the authorization server automatically authenticates the end
user, it may nevertheless require some user input in order to
prevent screen scraping. Examples are CAPTCHAs (Completely
Automated Public Turing tests to tell Computers and Humans Apart)
or other multi-factor authentication techniques such as random
questions, token code generators, etc.
o The authorization server may also limit the scope of tokens it
issues to clients it cannot reliably authenticate (see
Section 5.1.5.1).
Lodderstedt, et al. Informational [Page 28]
RFC 6819 OAuth 2.0 Security January 2013
4.4.1.5. Threat: Authorization "code" Phishing
A hostile party could impersonate the client site and get access to
the authorization "code". This could be achieved using DNS or ARP
spoofing. This applies to clients, which are web applications; thus,
the redirect URI is not local to the host where the user's browser is
running.
Impact: This affects web applications and may lead to a disclosure of
authorization "codes" and, potentially, the corresponding access and
refresh tokens.
Countermeasures:
It is strongly recommended that one of the following countermeasures
be utilized in order to prevent this attack:
o The redirect URI of the client should point to an HTTPS-protected
endpoint, and the browser should be utilized to authenticate this
redirect URI using server authentication (see Section 5.1.2).
o The authorization server should require that the client be
authenticated, i.e., confidential client, so the binding of the
authorization "code" to a certain client can be validated in a
reliable way (see Section 5.2.4.4).
4.4.1.6. Threat: User Session Impersonation
A hostile party could impersonate the client site and impersonate the
user's session on this client. This could be achieved using DNS or
ARP spoofing. This applies to clients, which are web applications;
thus, the redirect URI is not local to the host where the user's
browser is running.
Impact: An attacker who intercepts the authorization "code" as it is
sent by the browser to the callback endpoint can gain access to
protected resources by submitting the authorization "code" to the
client. The client will exchange the authorization "code" for an
access token and use the access token to access protected resources
for the benefit of the attacker, delivering protected resources to
the attacker, or modifying protected resources as directed by the
attacker. If OAuth is used by the client to delegate authentication
to a social site (e.g., as in the implementation of a "Login" button
on a third-party social network site), the attacker can use the
intercepted authorization "code" to log into the client as the user.
Lodderstedt, et al. Informational [Page 29]
RFC 6819 OAuth 2.0 Security January 2013
Note: Authenticating the client during authorization "code" exchange
will not help to detect such an attack, as it is the legitimate
client that obtains the tokens.
Countermeasures:
o In order to prevent an attacker from impersonating the end-user's
session, the redirect URI of the client should point to an HTTPS
protected endpoint, and the browser should be utilized to
authenticate this redirect URI using server authentication (see
Section 5.1.2).
4.4.1.7. Threat: Authorization "code" Leakage through Counterfeit
Client
The attacker leverages the authorization "code" grant type in an
attempt to get another user (victim) to log in, authorize access to
his/her resources, and subsequently obtain the authorization "code"
and inject it into a client application using the attacker's account.
The goal is to associate an access authorization for resources of the
victim with the user account of the attacker on a client site.
The attacker abuses an existing client application and combines it
with his own counterfeit client web site. The attacker depends on
the victim expecting the client application to request access to a
certain resource server. The victim, seeing only a normal request
from an expected application, approves the request. The attacker
then uses the victim's authorization to gain access to the
information unknowingly authorized by the victim.
The attacker conducts the following flow:
1. The attacker accesses the client web site (or application) and
initiates data access to a particular resource server. The
client web site in turn initiates an authorization request to the
resource server's authorization server. Instead of proceeding
with the authorization process, the attacker modifies the
authorization server end-user authorization URL as constructed by
the client to include a redirect URI parameter referring to a web
site under his control (attacker's web site).
2. The attacker tricks another user (the victim) into opening that
modified end-user authorization URI and authorizing access (e.g.,
via an email link or blog link). The way the attacker achieves
this goal is out of scope.
3. Having clicked the link, the victim is requested to authenticate
and authorize the client site to have access.
Lodderstedt, et al. Informational [Page 30]
RFC 6819 OAuth 2.0 Security January 2013
4. After completion of the authorization process, the authorization
server redirects the user agent to the attacker's web site
instead of the original client web site.
5. The attacker obtains the authorization "code" from his web site
by means that are out of scope of this document.
6. He then constructs a redirect URI to the target web site (or
application) based on the original authorization request's
redirect URI and the newly obtained authorization "code", and
directs his user agent to this URL. The authorization "code" is
injected into the original client site (or application).
7. The client site uses the authorization "code" to fetch a token
from the authorization server and associates this token with the
attacker's user account on this site.
8. The attacker may now access the victim's resources using the
client site.
Impact: The attacker gains access to the victim's resources as
associated with his account on the client site.
Countermeasures:
o The attacker will need to use another redirect URI for its
authorization process rather than the target web site because it
needs to intercept the flow. So, if the authorization server
associates the authorization "code" with the redirect URI of a
particular end-user authorization and validates this redirect URI
with the redirect URI passed to the token's endpoint, such an
attack is detected (see Section 5.2.4.5).
o The authorization server may also enforce the usage and validation
of pre-registered redirect URIs (see Section 5.2.3.5). This will
allow for early recognition of authorization "code" disclosure to
counterfeit clients.
o For native applications, one could also consider using deployment-
specific client ids and secrets (see Section 5.2.3.4), along with
the binding of authorization "codes" to "client_ids" (see
Section 5.2.4.4) to detect such an attack because the attacker
does not have access to the deployment-specific secret. Thus, he
will not be able to exchange the authorization "code".
Lodderstedt, et al. Informational [Page 31]
RFC 6819 OAuth 2.0 Security January 2013
o The client may consider using other flows that are not vulnerable
to this kind of attack, such as the implicit grant type (see
Section 4.4.2) or resource owner password credentials (see
Section 4.4.3).
4.4.1.8. Threat: CSRF Attack against redirect-uri
Cross-site request forgery (CSRF) is a web-based attack whereby HTTP
requests are transmitted from a user that the web site trusts or has
authenticated (e.g., via HTTP redirects or HTML forms). CSRF attacks
on OAuth approvals can allow an attacker to obtain authorization to
OAuth protected resources without the consent of the user.
This attack works against the redirect URI used in the authorization
"code" flow. An attacker could authorize an authorization "code" to
their own protected resources on an authorization server. He then
aborts the redirect flow back to the client on his device and tricks
the victim into executing the redirect back to the client. The
client receives the redirect, fetches the token(s) from the
authorization server, and associates the victim's client session with
the resources accessible using the token.
Impact: The user accesses resources on behalf of the attacker. The
effective impact depends on the type of resource accessed. For
example, the user may upload private items to an attacker's
resources. Or, when using OAuth in 3rd-party login scenarios, the
user may associate his client account with the attacker's identity at
the external Identity Provider. In this way, the attacker could
easily access the victim's data at the client by logging in from
another device with his credentials at the external Identity
Provider.
Countermeasures:
o The "state" parameter should be used to link the authorization
request with the redirect URI used to deliver the access token
(Section 5.3.5).
o Client developers and end users can be educated to not follow
untrusted URLs.
Lodderstedt, et al. Informational [Page 32]
RFC 6819 OAuth 2.0 Security January 2013
4.4.1.9. Threat: Clickjacking Attack against Authorization
With clickjacking, a malicious site loads the target site in a
transparent iFrame (see [iFrame]) overlaid on top of a set of dummy
buttons that are carefully constructed to be placed directly under
important buttons on the target site. When a user clicks a visible
button, they are actually clicking a button (such as an "Authorize"
button) on the hidden page.
Impact: An attacker can steal a user's authentication credentials and
access their resources.
Countermeasures:
o For newer browsers, avoidance of iFrames during authorization can
be enforced on the server side by using the X-FRAME-OPTIONS header
(Section 5.2.2.6).
o For older browsers, JavaScript frame-busting (see [Framebusting])
techniques can be used but may not be effective in all browsers.
4.4.1.10. Threat: Resource Owner Impersonation
When a client requests access to protected resources, the
authorization flow normally involves the resource owner's explicit
response to the access request, either granting or denying access to
the protected resources. A malicious client can exploit knowledge of
the structure of this flow in order to gain authorization without the
resource owner's consent, by transmitting the necessary requests
programmatically and simulating the flow against the authorization
server. That way, the client may gain access to the victim's
resources without her approval. An authorization server will be
vulnerable to this threat if it uses non-interactive authentication
mechanisms or splits the authorization flow across multiple pages.
The malicious client might embed a hidden HTML user agent, interpret
the HTML forms sent by the authorization server, and automatically
send the corresponding form HTTP POST requests. As a prerequisite,
the attacker must be able to execute the authorization process in the
context of an already-authenticated session of the resource owner
with the authorization server. There are different ways to achieve
this:
o The malicious client could abuse an existing session in an
external browser or cross-browser cookies on the particular
device.
Lodderstedt, et al. Informational [Page 33]
RFC 6819 OAuth 2.0 Security January 2013
o The malicious client could also request authorization for an
initial scope acceptable to the user and then silently abuse the
resulting session in his browser instance to "silently" request
another scope.
o Alternatively, the attacker might exploit an authorization
server's ability to authenticate the resource owner automatically
and without user interactions, e.g., based on certificates.
In all cases, such an attack is limited to clients running on the
victim's device, either within the user agent or as a native app.
Please note: Such attacks cannot be prevented using CSRF
countermeasures, since the attacker just "executes" the URLs as
prepared by the authorization server including any nonce, etc.
Countermeasures:
Authorization servers should decide, based on an analysis of the risk
associated with this threat, whether to detect and prevent this
threat.
In order to prevent such an attack, the authorization server may
force a user interaction based on non-predictable input values as
part of the user consent approval. The authorization server could
o combine password authentication and user consent in a single form,
o make use of CAPTCHAs, or
o use one-time secrets sent out of band to the resource owner (e.g.,
via text or instant message).
Alternatively, in order to allow the resource owner to detect abuse,
the authorization server could notify the resource owner of any
approval by appropriate means, e.g., text or instant message, or
email.
4.4.1.11. Threat: DoS Attacks That Exhaust Resources
If an authorization server includes a nontrivial amount of entropy in
authorization "codes" or access tokens (limiting the number of
possible codes/tokens) and automatically grants either without user
intervention and has no limit on codes or access tokens per user, an
attacker could exhaust the pool of authorization "codes" by
repeatedly directing the user's browser to request authorization
"codes" or access tokens.
Lodderstedt, et al. Informational [Page 34]
RFC 6819 OAuth 2.0 Security January 2013
Countermeasures:
o The authorization server should consider limiting the number of
access tokens granted per user.
o The authorization server should include a nontrivial amount of
entropy in authorization "codes".
4.4.1.12. Threat: DoS Using Manufactured Authorization "codes"
An attacker who owns a botnet can locate the redirect URIs of clients
that listen on HTTP, access them with random authorization "codes",
and cause a large number of HTTPS connections to be concentrated onto
the authorization server. This can result in a denial-of-service
(DoS) attack on the authorization server.
This attack can still be effective even when CSRF defense/the "state"
parameter (see Section 4.4.1.8) is deployed on the client side. With
such a defense, the attacker might need to incur an additional HTTP
request to obtain a valid CSRF code/"state" parameter. This
apparently cuts down the effectiveness of the attack by a factor of
2. However, if the HTTPS/HTTP cost ratio is higher than 2 (the cost
factor is estimated to be around 3.5x at [SSL-Latency]), the attacker
still achieves a magnification of resource utilization at the expense
of the authorization server.
Impact: There are a few effects that the attacker can accomplish with
this OAuth flow that they cannot easily achieve otherwise.
1. Connection laundering: With the clients as the relay between the
attacker and the authorization server, the authorization server
learns little or no information about the identity of the
attacker. Defenses such as rate-limiting on the offending
attacker machines are less effective because it is difficult to
identify the attacking machines. Although an attacker could also
launder its connections through an anonymizing system such as
Tor, the effectiveness of that approach depends on the capacity
of the anonymizing system. On the other hand, a potentially
large number of OAuth clients could be utilized for this attack.
2. Asymmetric resource utilization: The attacker incurs the cost of
an HTTP connection and causes an HTTPS connection to be made on
the authorization server; the attacker can coordinate the timing
of such HTTPS connections across multiple clients relatively
easily. Although the attacker could achieve something similar,
say, by including an iFrame pointing to the HTTPS URL of the
authorization server in an HTTP web page and luring web users to
visit that page, timing attacks using such a scheme may be more
Lodderstedt, et al. Informational [Page 35]
RFC 6819 OAuth 2.0 Security January 2013
difficult, as it seems nontrivial to synchronize a large number
of users to simultaneously visit a particular site under the
attacker's control.
Countermeasures:
o Though not a complete countermeasure by themselves, CSRF defense
and the "state" parameter created with secure random codes should
be deployed on the client side. The client should forward the
authorization "code" to the authorization server only after both
the CSRF token and the "state" parameter are validated.
o If the client authenticates the user, either through a single-
sign-on protocol or through local authentication, the client
should suspend the access by a user account if the number of
invalid authorization "codes" submitted by this user exceeds a
certain threshold.
o The authorization server should send an error response to the
client reporting an invalid authorization "code" and rate-limit or
disallow connections from clients whose number of invalid requests
exceeds a threshold.
4.4.1.13. Threat: Code Substitution (OAuth Login)
An attacker could attempt to log into an application or web site
using a victim's identity. Applications relying on identity data
provided by an OAuth protected service API to login users are
vulnerable to this threat. This pattern can be found in so-called
"social login" scenarios.
As a prerequisite, a resource server offers an API to obtain personal
information about a user that could be interpreted as having obtained
a user identity. In this sense, the client is treating the resource
server API as an "identity" API. A client utilizes OAuth to obtain
an access token for the identity API. It then queries the identity
API for an identifier and uses it to look up its internal user
account data (login). The client assumes that, because it was able
to obtain information about the user, the user has been
authenticated.
If the client uses the grant type "code", the attacker needs to
gather a valid authorization "code" of the respective victim from the
same Identity Provider used by the target client application. The
attacker tricks the victim into logging into a malicious app (which
may appear to be legitimate to the Identity Provider) using the same
Identity Provider as the target application. This results in the
Identity Provider's authorization server issuing an authorization
Lodderstedt, et al. Informational [Page 36]
RFC 6819 OAuth 2.0 Security January 2013
"code" for the respective identity API. The malicious app then sends
this code to the attacker, which in turn triggers a login process
within the target application. The attacker now manipulates the
authorization response and substitutes their code (bound to their
identity) for the victim's code. This code is then exchanged by the
client for an access token, which in turn is accepted by the identity
API, since the audience, with respect to the resource server, is
correct. But since the identifier returned by the identity API is
determined by the identity in the access token (issued based on the
victim's code), the attacker is logged into the target application
under the victim's identity.
Impact: The attacker gains access to an application and user-specific
data within the application.
Countermeasures:
o All clients must indicate their client ids with every request to
exchange an authorization "code" for an access token. The
authorization server must validate whether the particular
authorization "code" has been issued to the particular client. If
possible, the client shall be authenticated beforehand.
o Clients should use an appropriate protocol, such as OpenID (cf.
[OPENID]) or SAML (cf. [OASIS.sstc-saml-bindings-1.1]) to
implement user login. Both support audience restrictions on
clients.
4.4.2. Implicit Grant
In the implicit grant type flow, the access token is directly
returned to the client as a fragment part of the redirect URI. It is
assumed that the token is not sent to the redirect URI target, as
HTTP user agents do not send the fragment part of URIs to HTTP
servers. Thus, an attacker cannot eavesdrop the access token on this
communication path, and the token cannot leak through HTTP referrer
headers.
4.4.2.1. Threat: Access Token Leak in Transport/Endpoints
This token might be eavesdropped by an attacker. The token is sent
from the server to the client via a URI fragment of the redirect URI.
If the communication is not secured or the endpoint is not secured,
the token could be leaked by parsing the returned URI.
Impact: The attacker would be able to assume the same rights granted
by the token.
Lodderstedt, et al. Informational [Page 37]
RFC 6819 OAuth 2.0 Security January 2013
Countermeasures:
o The authorization server should ensure confidentiality (e.g.,
using TLS) of the response from the authorization server to the
client (see Section 5.1.1).
4.4.2.2. Threat: Access Token Leak in Browser History
An attacker could obtain the token from the browser's history. Note
that this means the attacker needs access to the particular device.
Countermeasures:
o Use short expiry time for tokens (see Section 5.1.5.3). Reduced
scope of the token may reduce the impact of that attack (see
Section 5.1.5.1).
o Make responses non-cacheable.
4.4.2.3. Threat: Malicious Client Obtains Authorization
A malicious client could attempt to obtain a token by fraud.
The same countermeasures as for Section 4.4.1.4 are applicable,
except client authentication.
4.4.2.4. Threat: Manipulation of Scripts
A hostile party could act as the client web server and replace or
modify the actual implementation of the client (script). This could
be achieved using DNS or ARP spoofing. This applies to clients
implemented within the web browser in a scripting language.
Impact: The attacker could obtain user credential information and
assume the full identity of the user.
Countermeasures:
o The authorization server should authenticate the server from which
scripts are obtained (see Section 5.1.2).
o The client should ensure that scripts obtained have not been
altered in transport (see Section 5.1.1).
Lodderstedt, et al. Informational [Page 38]
RFC 6819 OAuth 2.0 Security January 2013
o Introduce one-time, per-use secrets (e.g., "client_secret") values
that can only be used by scripts in a small time window once
loaded from a server. The intention would be to reduce the
effectiveness of copying client-side scripts for re-use in an
attacker's modified code.
4.4.2.5. Threat: CSRF Attack against redirect-uri
CSRF attacks (see Section 4.4.1.8) also work against the redirect URI
used in the implicit grant flow. An attacker could acquire an access
token to their own protected resources. He could then construct a
redirect URI and embed their access token in that URI. If he can
trick the user into following the redirect URI and the client does
not have protection against this attack, the user may have the
attacker's access token authorized within their client.
Impact: The user accesses resources on behalf of the attacker. The
effective impact depends on the type of resource accessed. For
example, the user may upload private items to an attacker's
resources. Or, when using OAuth in 3rd-party login scenarios, the
user may associate his client account with the attacker's identity at
the external Identity Provider. In this way, the attacker could
easily access the victim's data at the client by logging in from
another device with his credentials at the external Identity
Provider.
Countermeasures:
o The "state" parameter should be used to link the authorization
request with the redirect URI used to deliver the access token.
This will ensure that the client is not tricked into completing
any redirect callback unless it is linked to an authorization
request initiated by the client. The "state" parameter should not
be guessable, and the client should be capable of keeping the
"state" parameter secret.
o Client developers and end users can be educated to not follow
untrusted URLs.
4.4.2.6. Threat: Token Substitution (OAuth Login)
An attacker could attempt to log into an application or web site
using a victim's identity. Applications relying on identity data
provided by an OAuth protected service API to login users are
vulnerable to this threat. This pattern can be found in so-called
"social login" scenarios.
Lodderstedt, et al. Informational [Page 39]
RFC 6819 OAuth 2.0 Security January 2013
As a prerequisite, a resource server offers an API to obtain personal
information about a user that could be interpreted as having obtained
a user identity. In this sense, the client is treating the resource
server API as an "identity" API. A client utilizes OAuth to obtain
an access token for the identity API. It then queries the identity
API for an identifier and uses it to look up its internal user
account data (login). The client assumes that, because it was able
to obtain information about the user, the user has been
authenticated.
To succeed, the attacker needs to gather a valid access token of the
respective victim from the same Identity Provider used by the target
client application. The attacker tricks the victim into logging into
a malicious app (which may appear to be legitimate to the Identity
Provider) using the same Identity Provider as the target application.
This results in the Identity Provider's authorization server issuing
an access token for the respective identity API. The malicious app
then sends this access token to the attacker, which in turn triggers
a login process within the target application. The attacker now
manipulates the authorization response and substitutes their access
token (bound to their identity) for the victim's access token. This
token is accepted by the identity API, since the audience, with
respect to the resource server, is correct. But since the identifier
returned by the identity API is determined by the identity in the
access token, the attacker is logged into the target application
under the victim's identity.
Impact: The attacker gains access to an application and user-specific
data within the application.
Countermeasures:
o Clients should use an appropriate protocol, such as OpenID (cf.
[OPENID]) or SAML (cf. [OASIS.sstc-saml-bindings-1.1]) to
implement user login. Both support audience restrictions on
clients.
4.4.3. Resource Owner Password Credentials
The resource owner password credentials grant type (see [RFC6749],
Section 4.3), often used for legacy/migration reasons, allows a
client to request an access token using an end-user's user id and
password along with its own credential. This grant type has higher
risk because it maintains the UID/password anti-pattern.
Additionally, because the user does not have control over the
authorization process, clients using this grant type are not limited
Lodderstedt, et al. Informational [Page 40]
RFC 6819 OAuth 2.0 Security January 2013
by scope but instead have potentially the same capabilities as the
user themselves. As there is no authorization step, the ability to
offer token revocation is bypassed.
Because passwords are often used for more than 1 service, this
anti-pattern may also put at risk whatever else is accessible with
the supplied credential. Additionally, any easily derived equivalent
(e.g., joe@example.com and joe@example.net) might easily allow
someone to guess that the same password can be used elsewhere.
Impact: The resource server can only differentiate scope based on the
access token being associated with a particular client. The client
could also acquire long-lived tokens and pass them up to an
attacker's web service for further abuse. The client, eavesdroppers,
or endpoints could eavesdrop the user id and password.
Countermeasures:
o Except for migration reasons, minimize use of this grant type.
o The authorization server should validate the client id associated
with the particular refresh token with every refresh request
(Section 5.2.2.2).
o As per the core OAuth specification, the authorization server must
ensure that these transmissions are protected using transport-
layer mechanisms such as TLS (see Section 5.1.1).
o Rather than encouraging users to use a UID and password, service
providers should instead encourage users not to use the same
password for multiple services.
o Limit use of resource owner password credential grants to
scenarios where the client application and the authorizing service
are from the same organization.
4.4.3.1. Threat: Accidental Exposure of Passwords at Client Site
If the client does not provide enough protection, an attacker or
disgruntled employee could retrieve the passwords for a user.
Countermeasures:
o Use other flows that do not rely on the client's cooperation for
secure resource owner credential handling.
o Use digest authentication instead of plaintext credential
processing.
Lodderstedt, et al. Informational [Page 41]
RFC 6819 OAuth 2.0 Security January 2013
o Obfuscate passwords in logs.
4.4.3.2. Threat: Client Obtains Scopes without End-User Authorization
All interaction with the resource owner is performed by the client.
Thus it might, intentionally or unintentionally, happen that the
client obtains a token with scope unknown for, or unintended by, the
resource owner. For example, the resource owner might think the
client needs and acquires read-only access to its media storage only
but the client tries to acquire an access token with full access
permissions.
Countermeasures:
o Use other flows that do not rely on the client's cooperation for
resource owner interaction.
o The authorization server may generally restrict the scope of
access tokens (Section 5.1.5.1) issued by this flow. If the
particular client is trustworthy and can be authenticated in a
reliable way, the authorization server could relax that
restriction. Resource owners may prescribe (e.g., in their
preferences) what the maximum scope is for clients using this
flow.
o The authorization server could notify the resource owner by an
appropriate medium, e.g., email, of the grant issued (see
Section 5.1.3).
4.4.3.3. Threat: Client Obtains Refresh Token through Automatic
Authorization
All interaction with the resource owner is performed by the client.
Thus it might, intentionally or unintentionally, happen that the
client obtains a long-term authorization represented by a refresh
token even if the resource owner did not intend so.
Countermeasures:
o Use other flows that do not rely on the client's cooperation for
resource owner interaction.
o The authorization server may generally refuse to issue refresh
tokens in this flow (see Section 5.2.2.1). If the particular
client is trustworthy and can be authenticated in a reliable way
(see client authentication), the authorization server could relax
Lodderstedt, et al. Informational [Page 42]
RFC 6819 OAuth 2.0 Security January 2013
that restriction. Resource owners may allow or deny (e.g., in
their preferences) the issuing of refresh tokens using this flow
as well.
o The authorization server could notify the resource owner by an
appropriate medium, e.g., email, of the refresh token issued (see
Section 5.1.3).
4.4.3.4. Threat: Obtaining User Passwords on Transport
An attacker could attempt to eavesdrop the transmission of end-user
credentials with the grant type "password" between the client and
server.
Impact: Disclosure of a single end-user's password.
Countermeasures:
o Ensure confidentiality of requests (Section 5.1.1).
o Use alternative authentication means that do not require the
sending of plaintext credentials over the wire (e.g., Hash-based
Message Authentication Code).
4.4.3.5. Threat: Obtaining User Passwords from Authorization Server
Database
An attacker may obtain valid username/password combinations from the
authorization server's database by gaining access to the database or
launching a SQL injection attack.
Impact: Disclosure of all username/password combinations. The impact
may exceed the domain of the authorization server, since many users
tend to use the same credentials on different services.
Countermeasures:
o Enforce credential storage protection best practices
(Section 5.1.4.1).
4.4.3.6. Threat: Online Guessing
An attacker may try to guess valid username/password combinations
using the grant type "password".
Impact: Revelation of a single username/password combination.
Lodderstedt, et al. Informational [Page 43]
RFC 6819 OAuth 2.0 Security January 2013
Countermeasures:
o Utilize secure password policy (Section 5.1.4.2.1).
o Lock accounts (Section 5.1.4.2.3).
o Use tar pit (Section 5.1.4.2.4).
o Use CAPTCHAs (Section 5.1.4.2.5).
o Consider not using the grant type "password".
o Client authentication (see Section 5.2.3) will provide another
authentication factor and thus hinder the attack.
4.4.4. Client Credentials
Client credentials (see [RFC6749], Section 3) consist of an
identifier (not secret) combined with an additional means (such as a
matching client secret) of authenticating a client. The threats to
this grant type are similar to those described in Section 4.4.3.
4.5. Refreshing an Access Token
4.5.1. Threat: Eavesdropping Refresh Tokens from Authorization Server
An attacker may eavesdrop refresh tokens when they are transmitted
from the authorization server to the client.
Countermeasures:
o As per the core OAuth spec, the authorization servers must ensure
that these transmissions are protected using transport-layer
mechanisms such as TLS (see Section 5.1.1).
o If end-to-end confidentiality cannot be guaranteed, reducing scope
(see Section 5.1.5.1) and expiry time (see Section 5.1.5.3) for
issued access tokens can be used to reduce the damage in case of
leaks.
4.5.2. Threat: Obtaining Refresh Token from Authorization Server
Database
This threat is applicable if the authorization server stores refresh
tokens as handles in a database. An attacker may obtain refresh
tokens from the authorization server's database by gaining access to
the database or launching a SQL injection attack.
Lodderstedt, et al. Informational [Page 44]
RFC 6819 OAuth 2.0 Security January 2013
Impact: Disclosure of all refresh tokens.
Countermeasures:
o Enforce credential storage protection best practices
(Section 5.1.4.1).
o Bind token to client id, if the attacker cannot obtain the
required id and secret (Section 5.1.5.8).
4.5.3. Threat: Obtaining Refresh Token by Online Guessing
An attacker may try to guess valid refresh token values and send it
using the grant type "refresh_token" in order to obtain a valid
access token.
Impact: Exposure of a single refresh token and derivable access
tokens.
Countermeasures:
o For handle-based designs (Section 5.1.4.2.2).
o For assertion-based designs (Section 5.1.5.9).
o Bind token to client id, because the attacker would guess the
matching client id, too (see Section 5.1.5.8).
o Authenticate the client; this adds another element that the
attacker has to guess (see Section 5.2.3.4).
4.5.4. Threat: Refresh Token Phishing by Counterfeit Authorization
Server
An attacker could try to obtain valid refresh tokens by proxying
requests to the authorization server. Given the assumption that the
authorization server URL is well-known at development time or can at
least be obtained from a well-known resource server, the attacker
must utilize some kind of spoofing in order to succeed.
Countermeasures:
o Utilize server authentication (as described in Section 5.1.2).
Lodderstedt, et al. Informational [Page 45]
RFC 6819 OAuth 2.0 Security January 2013
4.6. Accessing Protected Resources
4.6.1. Threat: Eavesdropping Access Tokens on Transport
An attacker could try to obtain a valid access token on transport
between the client and resource server. As access tokens are shared
secrets between the authorization server and resource server, they
should be treated with the same care as other credentials (e.g., end-
user passwords).
Countermeasures:
o Access tokens sent as bearer tokens should not be sent in the
clear over an insecure channel. As per the core OAuth spec,
transmission of access tokens must be protected using transport-
layer mechanisms such as TLS (see Section 5.1.1).
o A short lifetime reduces impact in case tokens are compromised
(see Section 5.1.5.3).
o The access token can be bound to a client's identifier and require
the client to prove legitimate ownership of the token to the
resource server (see Section 5.4.2).
4.6.2. Threat: Replay of Authorized Resource Server Requests
An attacker could attempt to replay valid requests in order to obtain
or to modify/destroy user data.
Countermeasures:
o The resource server should utilize transport security measures
(e.g., TLS) in order to prevent such attacks (see Section 5.1.1).
This would prevent the attacker from capturing valid requests.
o Alternatively, the resource server could employ signed requests
(see Section 5.4.3) along with nonces and timestamps in order to
uniquely identify requests. The resource server should detect and
refuse every replayed request.
4.6.3. Threat: Guessing Access Tokens
Where the token is a handle, the attacker may attempt to guess the
access token values based on knowledge they have from other access
tokens.
Impact: Access to a single user's data.
Lodderstedt, et al. Informational [Page 46]
RFC 6819 OAuth 2.0 Security January 2013
Countermeasures:
o Handle tokens should have a reasonable level of entropy (see
Section 5.1.4.2.2) in order to make guessing a valid token value
infeasible.
o Assertion (or self-contained token) token contents should be
protected by a digital signature (see Section 5.1.5.9).
o Security can be further strengthened by using a short access token
duration (see Sections 5.1.5.2 and 5.1.5.3).
4.6.4. Threat: Access Token Phishing by Counterfeit Resource Server
An attacker may pretend to be a particular resource server and to
accept tokens from a particular authorization server. If the client
sends a valid access token to this counterfeit resource server, the
server in turn may use that token to access other services on behalf
of the resource owner.
Countermeasures:
o Clients should not make authenticated requests with an access
token to unfamiliar resource servers, regardless of the presence
of a secure channel. If the resource server URL is well-known to
the client, it may authenticate the resource servers (see
Section 5.1.2).
o Associate the endpoint URL of the resource server the client
talked to with the access token (e.g., in an audience field) and
validate the association at a legitimate resource server. The
endpoint URL validation policy may be strict (exact match) or more
relaxed (e.g., same host). This would require telling the
authorization server about the resource server endpoint URL in the
authorization process.
o Associate an access token with a client and authenticate the
client with resource server requests (typically via a signature,
in order to not disclose a secret to a potential attacker). This
prevents the attack because the counterfeit server is assumed to
lack the capability to correctly authenticate on behalf of the
legitimate client to the resource server (Section 5.4.2).
o Restrict the token scope (see Section 5.1.5.1) and/or limit the
token to a certain resource server (Section 5.1.5.5).
Lodderstedt, et al. Informational [Page 47]
RFC 6819 OAuth 2.0 Security January 2013
4.6.5. Threat: Abuse of Token by Legitimate Resource Server or Client
A legitimate resource server could attempt to use an access token to
access another resource server. Similarly, a client could try to use
a token obtained for one server on another resource server.
Countermeasures:
o Tokens should be restricted to particular resource servers (see
Section 5.1.5.5).
4.6.6. Threat: Leak of Confidential Data in HTTP Proxies
An OAuth HTTP authentication scheme as discussed in [RFC6749] is
optional. However, [RFC2616] relies on the Authorization and
WWW-Authenticate headers to distinguish authenticated content so that
it can be protected. Proxies and caches, in particular, may fail to
adequately protect requests not using these headers. For example,
private authenticated content may be stored in (and thus be
retrievable from) publicly accessible caches.
Countermeasures:
o Clients and resource servers not using an OAuth HTTP
authentication scheme (see Section 5.4.1) should take care to use
Cache-Control headers to minimize the risk that authenticated
content is not protected. Such clients should send a
Cache-Control header containing the "no-store" option [RFC2616].
Resource server success (2XX status) responses to these requests
should contain a Cache-Control header with the "private" option
[RFC2616].
o Reducing scope (see Section 5.1.5.1) and expiry time
(Section 5.1.5.3) for access tokens can be used to reduce the
damage in case of leaks.
4.6.7. Threat: Token Leakage via Log Files and HTTP Referrers
If access tokens are sent via URI query parameters, such tokens may
leak to log files and the HTTP "referer".
Countermeasures:
o Use Authorization headers or POST parameters instead of URI
request parameters (see Section 5.4.1).
o Set logging configuration appropriately.
Lodderstedt, et al. Informational [Page 48]
RFC 6819 OAuth 2.0 Security January 2013
o Prevent unauthorized persons from access to system log files (see
Section 5.1.4.1.1).
o Abuse of leaked access tokens can be prevented by enforcing
authenticated requests (see Section 5.4.2).
o The impact of token leakage may be reduced by limiting scope (see
Section 5.1.5.1) and duration (see Section 5.1.5.3) and by
enforcing one-time token usage (see Section 5.1.5.4).
5. Security Considerations
This section describes the countermeasures as recommended to mitigate
the threats described in Section 4.
5.1. General
This section covers considerations that apply generally across all
OAuth components (client, resource server, token server, and user
agents).
5.1.1. Ensure Confidentiality of Requests
This is applicable to all requests sent from the client to the
authorization server or resource server. While OAuth provides a
mechanism for verifying the integrity of requests, it provides no
guarantee of request confidentiality. Unless further precautions are
taken, eavesdroppers will have full access to request content and may
be able to mount interception or replay attacks by using the contents
of requests, e.g., secrets or tokens.
Attacks can be mitigated by using transport-layer mechanisms such as
TLS [RFC5246]. A virtual private network (VPN), e.g., based on IPsec
VPNs [RFC4301], may be considered as well.
Note: This document assumes end-to-end TLS protected connections
between the respective protocol entities. Deployments deviating from
this assumption by offloading TLS in between (e.g., on the data
center edge) must refine this threat model in order to account for
the additional (mainly insider) threat this may cause.
This is a countermeasure against the following threats:
o Replay of access tokens obtained on the token's endpoint or the
resource server's endpoint
o Replay of refresh tokens obtained on the token's endpoint
Lodderstedt, et al. Informational [Page 49]
RFC 6819 OAuth 2.0 Security January 2013
o Replay of authorization "codes" obtained on the token's endpoint
(redirect?)
o Replay of user passwords and client secrets
5.1.2. Utilize Server Authentication
HTTPS server authentication or similar means can be used to
authenticate the identity of a server. The goal is to reliably bind
the fully qualified domain name of the server to the public key
presented by the server during connection establishment (see
[RFC2818]).
The client should validate the binding of the server to its domain
name. If the server fails to prove that binding, the communication
is considered a man-in-the-middle attack. This security measure
depends on the certification authorities the client trusts for that
purpose. Clients should carefully select those trusted CAs and
protect the storage for trusted CA certificates from modifications.
This is a countermeasure against the following threats:
o Spoofing
o Proxying
o Phishing by counterfeit servers
5.1.3. Always Keep the Resource Owner Informed
Transparency to the resource owner is a key element of the OAuth
protocol. The user should always be in control of the authorization
processes and get the necessary information to make informed
decisions. Moreover, user involvement is a further security
countermeasure. The user can probably recognize certain kinds of
attacks better than the authorization server. Information can be
presented/exchanged during the authorization process, after the
authorization process, and every time the user wishes to get informed
by using techniques such as:
o User consent forms.
o Notification messages (e.g., email, SMS, ...). Note that
notifications can be a phishing vector. Messages should be such
that look-alike phishing messages cannot be derived from them.
Lodderstedt, et al. Informational [Page 50]
RFC 6819 OAuth 2.0 Security January 2013
o Activity/event logs.
o User self-care applications or portals.
5.1.4. Credentials
This section describes countermeasures used to protect all kinds of
credentials from unauthorized access and abuse. Credentials are
long-term secrets, such as client secrets and user passwords as well
as all kinds of tokens (refresh and access tokens) or authorization
"codes".
5.1.4.1. Enforce Credential Storage Protection Best Practices
Administrators should undertake industry best practices to protect
the storage of credentials (for example, see [OWASP]). Such
practices may include but are not limited to the following
sub-sections.
5.1.4.1.1. Enforce Standard System Security Means
A server system may be locked down so that no attacker may get access
to sensitive configuration files and databases.
5.1.4.1.2. Enforce Standard SQL Injection Countermeasures
If a client identifier or other authentication component is queried
or compared against a SQL database, it may become possible for an
injection attack to occur if parameters received are not validated
before submission to the database.
o Ensure that server code is using the minimum database privileges
possible to reduce the "surface" of possible attacks.
o Avoid dynamic SQL using concatenated input. If possible, use
static SQL.
o When using dynamic SQL, parameterize queries using bind arguments.
Bind arguments eliminate the possibility of SQL injections.
o Filter and sanitize the input. For example, if an identifier has
a known format, ensure that the supplied value matches the
identifier syntax rules.
Lodderstedt, et al. Informational [Page 51]
RFC 6819 OAuth 2.0 Security January 2013
5.1.4.1.3. No Cleartext Storage of Credentials
The authorization server should not store credentials in clear text.
Typical approaches are to store hashes instead or to encrypt
credentials. If the credential lacks a reasonable entropy level
(because it is a user password), an additional salt will harden the
storage to make offline dictionary attacks more difficult.
Note: Some authentication protocols require the authorization server
to have access to the secret in the clear. Those protocols cannot be
implemented if the server only has access to hashes. Credentials
should be strongly encrypted in those cases.
5.1.4.1.4. Encryption of Credentials
For client applications, insecurely persisted client credentials are
easy targets for attackers to obtain. Store client credentials using
an encrypted persistence mechanism such as a keystore or database.
Note that compiling client credentials directly into client code
makes client applications vulnerable to scanning as well as difficult
to administer should client credentials change over time.
5.1.4.1.5. Use of Asymmetric Cryptography
Usage of asymmetric cryptography will free the authorization server
of the obligation to manage credentials.
5.1.4.2. Online Attacks on Secrets
5.1.4.2.1. Utilize Secure Password Policy
The authorization server may decide to enforce a complex user
password policy in order to increase the user passwords' entropy to
hinder online password attacks. Note that too much complexity can
increase the likelihood that users re-use passwords or write them
down, or otherwise store them insecurely.
5.1.4.2.2. Use High Entropy for Secrets
When creating secrets not intended for usage by human users (e.g.,
client secrets or token handles), the authorization server should
include a reasonable level of entropy in order to mitigate the risk
of guessing attacks. The token value should be >=128 bits long and
constructed from a cryptographically strong random or pseudo-random
number sequence (see [RFC4086] for best current practice) generated
by the authorization server.
Lodderstedt, et al. Informational [Page 52]
RFC 6819 OAuth 2.0 Security January 2013
5.1.4.2.3. Lock Accounts
Online attacks on passwords can be mitigated by locking the
respective accounts after a certain number of failed attempts.
Note: This measure can be abused to lock down legitimate service
users.
5.1.4.2.4. Use Tar Pit
The authorization server may react on failed attempts to authenticate
by username/password by temporarily locking the respective account
and delaying the response for a certain duration. This duration may
increase with the number of failed attempts. The objective is to
slow the attacker's attempts on a certain username down.
Note: This may require a more complex and stateful design of the
authorization server.
5.1.4.2.5. Use CAPTCHAs
The idea is to prevent programs from automatically checking a huge
number of passwords, by requiring human interaction.
Note: This has a negative impact on user experience.
5.1.5. Tokens (Access, Refresh, Code)
5.1.5.1. Limit Token Scope
The authorization server may decide to reduce or limit the scope
associated with a token. The basis of this decision is out of scope;
examples are:
o a client-specific policy, e.g., issue only less powerful tokens to
public clients,
o a service-specific policy, e.g., it is a very sensitive service,
o a resource-owner-specific setting, or
o combinations of such policies and preferences.
Lodderstedt, et al. Informational [Page 53]
RFC 6819 OAuth 2.0 Security January 2013
The authorization server may allow different scopes dependent on the
grant type. For example, end-user authorization via direct
interaction with the end user (authorization "code") might be
considered more reliable than direct authorization via grant type
"username"/"password". This means will reduce the impact of the
following threats:
o token leakage
o token issuance to malicious software
o unintended issuance of powerful tokens with resource owner
credentials flow
5.1.5.2. Determine Expiration Time
Tokens should generally expire after a reasonable duration. This
complements and strengthens other security measures (such as
signatures) and reduces the impact of all kinds of token leaks.
Depending on the risk associated with token leakage, tokens may
expire after a few minutes (e.g., for payment transactions) or stay
valid for hours (e.g., read access to contacts).
The expiration time is determined by several factors, including:
o risk associated with token leakage,
o duration of the underlying access grant,
o duration until the modification of an access grant should take
effect, and
o time required for an attacker to guess or produce a valid token.
5.1.5.3. Use Short Expiration Time
A short expiration time for tokens is a means of protection against
the following threats:
o replay
o token leak (a short expiration time will reduce impact)
o online guessing (a short expiration time will reduce the
likelihood of success)
Lodderstedt, et al. Informational [Page 54]
RFC 6819 OAuth 2.0 Security January 2013
Note: Short token duration requires more precise clock
synchronization between the authorization server and resource server.
Furthermore, shorter duration may require more token refreshes
(access token) or repeated end-user authorization processes
(authorization "code" and refresh token).
5.1.5.4. Limit Number of Usages or One-Time Usage
The authorization server may restrict the number of requests or
operations that can be performed with a certain token. This
mechanism can be used to mitigate the following threats:
o replay of tokens
o guessing
For example, if an authorization server observes more than one
attempt to redeem an authorization "code", the authorization server
may want to revoke all access tokens granted based on the
authorization "code" as well as reject the current request.
As with the authorization "code", access tokens may also have a
limited number of operations. This either forces client applications
to re-authenticate and use a refresh token to obtain a fresh access
token, or forces the client to re-authorize the access token by
involving the user.
5.1.5.5. Bind Tokens to a Particular Resource Server (Audience)
Authorization servers in multi-service environments may consider
issuing tokens with different content to different resource servers
and to explicitly indicate in the token the target server to which a
token is intended to be sent. SAML assertions (see
[OASIS.saml-core-2.0-os]) use the Audience element for this purpose.
This countermeasure can be used in the following situations:
o It reduces the impact of a successful replay attempt, since the
token is applicable to a single resource server only.
o It prevents abuse of a token by a rogue resource server or client,
since the token can only be used on that server. It is rejected
by other servers.
o It reduces the impact of leakage of a valid token to a counterfeit
resource server.
Lodderstedt, et al. Informational [Page 55]
RFC 6819 OAuth 2.0 Security January 2013
5.1.5.6. Use Endpoint Address as Token Audience
This may be used to indicate to a resource server which endpoint URL
has been used to obtain the token. This measure will allow the
detection of requests from a counterfeit resource server, since such
a token will contain the endpoint URL of that server.
5.1.5.7. Use Explicitly Defined Scopes for Audience and Tokens
Deployments may consider only using tokens with explicitly defined
scopes, where every scope is associated with a particular resource
server. This approach can be used to mitigate attacks where a
resource server or client uses a token for a different purpose than
the one intended.
5.1.5.8. Bind Token to Client id
An authorization server may bind a token to a certain client
identifier. This identifier should be validated for every request
with that token. This technique can be used to
o detect token leakage and
o prevent token abuse.
Note: Validating the client identifier may require the target server
to authenticate the client's identifier. This authentication can be
based on secrets managed independently of the token (e.g.,
pre-registered client id/secret on authorization server) or sent with
the token itself (e.g., as part of the encrypted token content).
5.1.5.9. Sign Self-Contained Tokens
Self-contained tokens should be signed in order to detect any attempt
to modify or produce faked tokens (e.g., Hash-based Message
Authentication Code or digital signatures).
5.1.5.10. Encrypt Token Content
Self-contained tokens may be encrypted for confidentiality reasons or
to protect system internal data. Depending on token format, keys
(e.g., symmetric keys) may have to be distributed between server
nodes. The method of distribution should be defined by the token and
the encryption used.
Lodderstedt, et al. Informational [Page 56]
RFC 6819 OAuth 2.0 Security January 2013
5.1.5.11. Adopt a Standard Assertion Format
For service providers intending to implement an assertion-based token
design, it is highly recommended to adopt a standard assertion format
(such as SAML [OASIS.saml-core-2.0-os] or the JavaScript Object
Notation Web Token (JWT) [OAuth-JWT]).
5.1.6. Access Tokens
The following measures should be used to protect access tokens:
o Keep them in transient memory (accessible by the client
application only).
o Pass tokens securely using secure transport (TLS).
o Ensure that client applications do not share tokens with 3rd
parties.
5.2. Authorization Server
This section describes considerations related to the OAuth
authorization server endpoint.
5.2.1. Authorization "codes"
5.2.1.1. Automatic Revocation of Derived Tokens If Abuse Is Detected
If an authorization server observes multiple attempts to redeem an
authorization grant (e.g., such as an authorization "code"), the
authorization server may want to revoke all tokens granted based on
the authorization grant.
5.2.2. Refresh Tokens
5.2.2.1. Restricted Issuance of Refresh Tokens
The authorization server may decide, based on an appropriate policy,
not to issue refresh tokens. Since refresh tokens are long-term
credentials, they may be subject to theft. For example, if the
authorization server does not trust a client to securely store such
tokens, it may refuse to issue such a client a refresh token.
Lodderstedt, et al. Informational [Page 57]
RFC 6819 OAuth 2.0 Security January 2013
5.2.2.2. Binding of Refresh Token to "client_id"
The authorization server should match every refresh token to the
identifier of the client to whom it was issued. The authorization
server should check that the same "client_id" is present for every
request to refresh the access token. If possible (e.g., confidential
clients), the authorization server should authenticate the respective
client.
This is a countermeasure against refresh token theft or leakage.
Note: This binding should be protected from unauthorized
modifications.
5.2.2.3. Refresh Token Rotation
Refresh token rotation is intended to automatically detect and
prevent attempts to use the same refresh token in parallel from
different apps/devices. This happens if a token gets stolen from the
client and is subsequently used by both the attacker and the
legitimate client. The basic idea is to change the refresh token
value with every refresh request in order to detect attempts to
obtain access tokens using old refresh tokens. Since the
authorization server cannot determine whether the attacker or the
legitimate client is trying to access, in case of such an access
attempt the valid refresh token and the access authorization
associated with it are both revoked.
The OAuth specification supports this measure in that the token's
response allows the authorization server to return a new refresh
token even for requests with grant type "refresh_token".
Note: This measure may cause problems in clustered environments,
since usage of the currently valid refresh token must be ensured. In
such an environment, other measures might be more appropriate.
5.2.2.4. Revocation of Refresh Tokens
The authorization server may allow clients or end users to explicitly
request the invalidation of refresh tokens. A mechanism to revoke
tokens is specified in [OAuth-REVOCATION].
Lodderstedt, et al. Informational [Page 58]
RFC 6819 OAuth 2.0 Security January 2013
This is a countermeasure against:
o device theft,
o impersonation of a resource owner, or
o suspected compromised client applications.
5.2.2.5. Device Identification
The authorization server may require the binding of authentication
credentials to a device identifier. The International Mobile Station
Equipment Identity [IMEI] is one example of such an identifier; there
are also operating system-specific identifiers. The authorization
server could include such an identifier when authenticating user
credentials in order to detect token theft from a particular device.
Note: Any implementation should consider potential privacy
implications of using device identifiers.
5.2.2.6. X-FRAME-OPTIONS Header
For newer browsers, avoidance of iFrames can be enforced on the
server side by using the X-FRAME-OPTIONS header (see
[X-Frame-Options]). This header can have two values, "DENY" and
"SAMEORIGIN", which will block any framing or any framing by sites
with a different origin, respectively. The value "ALLOW-FROM"
specifies a list of trusted origins that iFrames may originate from.
This is a countermeasure against the following threat:
o Clickjacking attacks
5.2.3. Client Authentication and Authorization
As described in Section 3 (Security Features), clients are
identified, authenticated, and authorized for several purposes, such
as to:
o Collate requests to the same client,
o Indicate to the user that the client is recognized by the
authorization server,
o Authorize access of clients to certain features on the
authorization server or resource server, and
o Log a client identifier to log files for analysis or statistics.
Lodderstedt, et al. Informational [Page 59]
RFC 6819 OAuth 2.0 Security January 2013
Due to the different capabilities and characteristics of the
different client types, there are different ways to support these
objectives, which will be described in this section. Authorization
server providers should be aware of the security policy and
deployment of a particular client and adapt its treatment
accordingly. For example, one approach could be to treat all clients
as less trustworthy and unsecure. On the other extreme, a service
provider could activate every client installation individually by an
administrator and in that way gain confidence in the identity of the
software package and the security of the environment in which the
client is installed. There are several approaches in between.
5.2.3.1. Don't Issue Secrets to Clients with Inappropriate Security
Policy
Authorization servers should not issue secrets to clients that cannot
protect secrets ("public" clients). This reduces the probability of
the server treating the client as strongly authenticated.
For example, it is of limited benefit to create a single client id
and secret that are shared by all installations of a native
application. Such a scenario requires that this secret must be
transmitted from the developer via the respective distribution
channel, e.g., an application market, to all installations of the
application on end-user devices. A secret, burned into the source
code of the application or an associated resource bundle, is not
protected from reverse engineering. Secondly, such secrets cannot be
revoked, since this would immediately put all installations out of
work. Moreover, since the authorization server cannot really trust
the client's identifier, it would be dangerous to indicate to end
users the trustworthiness of the client.
There are other ways to achieve a reasonable security level, as
described in the following sections.
5.2.3.2. Require User Consent for Public Clients without Secret
Authorization servers should not allow automatic authorization for
public clients. The authorization server may issue an individual
client id but should require that all authorizations are approved by
the end user. For clients without secrets, this is a countermeasure
against the following threat:
o Impersonation of public client applications.
Lodderstedt, et al. Informational [Page 60]
RFC 6819 OAuth 2.0 Security January 2013
5.2.3.3. Issue a "client_id" Only in Combination with "redirect_uri"
The authorization server may issue a "client_id" and bind the
"client_id" to a certain pre-configured "redirect_uri". Any
authorization request with another redirect URI is refused
automatically. Alternatively, the authorization server should not
accept any dynamic redirect URI for such a "client_id" and instead
should always redirect to the well-known pre-configured redirect URI.
This is a countermeasure for clients without secrets against the
following threats:
o Cross-site scripting attacks
o Impersonation of public client applications
5.2.3.4. Issue Installation-Specific Client Secrets
An authorization server may issue separate client identifiers and
corresponding secrets to the different installations of a particular
client (i.e., software package). The effect of such an approach
would be to turn otherwise "public" clients back into "confidential"
clients.
For web applications, this could mean creating one "client_id" and
"client_secret" for each web site on which a software package is
installed. So, the provider of that particular site could request a
client id and secret from the authorization server during the setup
of the web site. This would also allow the validation of some of the
properties of that web site, such as redirect URI, web site URL, and
whatever else proves useful. The web site provider has to ensure the
security of the client secret on the site.
For native applications, things are more complicated because every
copy of a particular application on any device is a different
installation. Installation-specific secrets in this scenario will
require obtaining a "client_id" and "client_secret" either
1. during the download process from the application market, or
2. during installation on the device.
Either approach will require an automated mechanism for issuing
client ids and secrets, which is currently not defined by OAuth.
The first approach would allow the achievement of a certain level of
trust in the authenticity of the application, whereas the second
option only allows the authentication of the installation but not the
validation of properties of the client. But this would at least help
Lodderstedt, et al. Informational [Page 61]
RFC 6819 OAuth 2.0 Security January 2013
to prevent several replay attacks. Moreover, installation-specific
"client_ids" and secrets allow the selective revocation of all
refresh tokens of a specific installation at once.
5.2.3.5. Validate Pre-Registered "redirect_uri"
An authorization server should require all clients to register their
"redirect_uri", and the "redirect_uri" should be the full URI as
defined in [RFC6749]. The way that this registration is performed is
out of scope of this document. As per the core spec, every actual
redirect URI sent with the respective "client_id" to the end-user
authorization endpoint must match the registered redirect URI. Where
it does not match, the authorization server should assume that the
inbound GET request has been sent by an attacker and refuse it.
Note: The authorization server should not redirect the user agent
back to the redirect URI of such an authorization request.
Validating the pre-registered "redirect_uri" is a countermeasure
against the following threats:
o Authorization "code" leakage through counterfeit web site: allows
authorization servers to detect attack attempts after the first
redirect to an end-user authorization endpoint (Section 4.4.1.7).
o Open redirector attack via a client redirection endpoint
(Section 4.1.5).
o Open redirector phishing attack via an authorization server
redirection endpoint (Section 4.2.4).
The underlying assumption of this measure is that an attacker will
need to use another redirect URI in order to get access to the
authorization "code". Deployments might consider the possibility of
an attacker using spoofing attacks to a victim's device to circumvent
this security measure.
Note: Pre-registering clients might not scale in some deployments
(manual process) or require dynamic client registration (not
specified yet). With the lack of dynamic client registration, a
pre-registered "redirect_uri" only works for clients bound to certain
deployments at development/configuration time. As soon as dynamic
resource server discovery is required, the pre-registered
"redirect_uri" may no longer be feasible.
Lodderstedt, et al. Informational [Page 62]
RFC 6819 OAuth 2.0 Security January 2013
5.2.3.6. Revoke Client Secrets
An authorization server may revoke a client's secret in order to
prevent abuse of a revealed secret.
Note: This measure will immediately invalidate any authorization
"code" or refresh token issued to the respective client. This might
unintentionally impact client identifiers and secrets used across
multiple deployments of a particular native or web application.
This a countermeasure against:
o Abuse of revealed client secrets for private clients
5.2.3.7. Use Strong Client Authentication (e.g., client_assertion/
client_token)
By using an alternative form of authentication such as client
assertion [OAuth-ASSERTIONS], the need to distribute a
"client_secret" is eliminated. This may require the use of a secure
private key store or other supplemental authentication system as
specified by the client assertion issuer in its authentication
process.
5.2.4. End-User Authorization
This section includes considerations for authorization flows
involving the end user.
5.2.4.1. Automatic Processing of Repeated Authorizations Requires
Client Validation
Authorization servers should NOT automatically process repeat
authorizations where the client is not authenticated through a client
secret or some other authentication mechanism such as a signed
authentication assertion certificate (Section 5.2.3.7) or validation
of a pre-registered redirect URI (Section 5.2.3.5).
5.2.4.2. Informed Decisions Based on Transparency
The authorization server should clearly explain to the end user what
happens in the authorization process and what the consequences are.
For example, the user should understand what access he is about to
grant to which client for what duration. It should also be obvious
to the user whether the server is able to reliably certify certain
client properties (web site URL, security policy).
Lodderstedt, et al. Informational [Page 63]
RFC 6819 OAuth 2.0 Security January 2013
5.2.4.3. Validation of Client Properties by End User
In the authorization process, the user is typically asked to approve
a client's request for authorization. This is an important security
mechanism by itself because the end user can be involved in the
validation of client properties, such as whether the client name
known to the authorization server fits the name of the web site or
the application the end user is using. This measure is especially
helpful in situations where the authorization server is unable to
authenticate the client. It is a countermeasure against:
o A malicious application
o A client application masquerading as another client
5.2.4.4. Binding of Authorization "code" to "client_id"
The authorization server should bind every authorization "code" to
the id of the respective client that initiated the end-user
authorization process. This measure is a countermeasure against:
o Replay of authorization "codes" with different client credentials,
since an attacker cannot use another "client_id" to exchange an
authorization "code" into a token
o Online guessing of authorization "codes"
Note: This binding should be protected from unauthorized
modifications (e.g., using protected memory and/or a secure
database).
5.2.4.5. Binding of Authorization "code" to "redirect_uri"
The authorization server should be able to bind every authorization
"code" to the actual redirect URI used as the redirect target of the
client in the end-user authorization process. This binding should be
validated when the client attempts to exchange the respective
authorization "code" for an access token. This measure is a
countermeasure against authorization "code" leakage through
counterfeit web sites, since an attacker cannot use another redirect
URI to exchange an authorization "code" into a token.
Lodderstedt, et al. Informational [Page 64]
RFC 6819 OAuth 2.0 Security January 2013
5.3. Client App Security
This section deals with considerations for client applications.
5.3.1. Don't Store Credentials in Code or Resources Bundled with
Software Packages
Because of the number of copies of client software, there is limited
benefit in creating a single client id and secret that is shared by
all installations of an application. Such an application by itself
would be considered a "public" client, as it cannot be presumed to be
able to keep client secrets. A secret, burned into the source code
of the application or an associated resource bundle, cannot be
protected from reverse engineering. Secondly, such secrets cannot be
revoked, since this would immediately put all installations out of
work. Moreover, since the authorization server cannot really trust
the client's identifier, it would be dangerous to indicate to end
users the trustworthiness of the client.
5.3.2. Use Standard Web Server Protection Measures (for Config Files
and Databases)
Use standard web server protection and configuration measures to
protect the integrity of the server, databases, configuration files,
and other operational components of the server.
5.3.3. Store Secrets in Secure Storage
There are different ways to store secrets of all kinds (tokens,
client secrets) securely on a device or server.
Most multi-user operating systems segregate the personal storage of
different system users. Moreover, most modern smartphone operating
systems even support the storage of application-specific data in
separate areas of file systems and protect the data from access by
other applications. Additionally, applications can implement
confidential data by using a user-supplied secret, such as a PIN or
password.
Another option is to swap refresh token storage to a trusted backend
server. This option in turn requires a resilient authentication
mechanism between the client and backend server. Note: Applications
should ensure that confidential data is kept confidential even after
reading from secure storage, which typically means keeping this data
in the local memory of the application.
Lodderstedt, et al. Informational [Page 65]
RFC 6819 OAuth 2.0 Security January 2013
5.3.4. Utilize Device Lock to Prevent Unauthorized Device Access
On a typical modern phone, there are many "device lock" options that
can be utilized to provide additional protection when a device is
stolen or misplaced. These include PINs, passwords, and other
biometric features such as "face recognition". These are not equal
in the level of security they provide.
5.3.5. Link the "state" Parameter to User Agent Session
The "state" parameter is used to link client requests and prevent
CSRF attacks, for example, attacks against the redirect URI. An
attacker could inject their own authorization "code" or access token,
which can result in the client using an access token associated with
the attacker's protected resources rather than the victim's (e.g.,
save the victim's bank account information to a protected resource
controlled by the attacker).
The client should utilize the "state" request parameter to send the
authorization server a value that binds the request to the user
agent's authenticated state (e.g., a hash of the session cookie used
to authenticate the user agent) when making an authorization request.
Once authorization has been obtained from the end user, the
authorization server redirects the end-user's user agent back to the
client with the required binding value contained in the "state"
parameter.
The binding value enables the client to verify the validity of the
request by matching the binding value to the user agent's
authenticated state.
5.4. Resource Servers
The following section details security considerations for resource
servers.
5.4.1. Authorization Headers
Authorization headers are recognized and specially treated by HTTP
proxies and servers. Thus, the usage of such headers for sending
access tokens to resource servers reduces the likelihood of leakage
or unintended storage of authenticated requests in general, and
especially Authorization headers.
Lodderstedt, et al. Informational [Page 66]
RFC 6819 OAuth 2.0 Security January 2013
5.4.2. Authenticated Requests
An authorization server may bind tokens to a certain client
identifier and enable resource servers to validate that association
on resource access. This will require the resource server to
authenticate the originator of a request as the legitimate owner of a
particular token. There are several options to implement this
countermeasure:
o The authorization server may associate the client identifier with
the token (either internally or in the payload of a self-contained
token). The client then uses client certificate-based HTTP
authentication on the resource server's endpoint to authenticate
its identity, and the resource server validates the name with the
name referenced by the token.
o Same as the option above, but the client uses his private key to
sign the request to the resource server (the public key is either
contained in the token or sent along with the request).
o Alternatively, the authorization server may issue a token-bound
key, which the client uses in a Holder-of-Key proof to
authenticate the client's use of the token. The resource server
obtains the secret directly from the authorization server, or the
secret is contained in an encrypted section of the token. In that
way, the resource server does not "know" the client but is able to
validate whether the authorization server issued the token to that
client.
Authenticated requests are a countermeasure against abuse of tokens
by counterfeit resource servers.
5.4.3. Signed Requests
A resource server may decide to accept signed requests only, either
to replace transport-level security measures or to complement such
measures. Every signed request should be uniquely identifiable and
should not be processed twice by the resource server. This
countermeasure helps to mitigate:
o modifications of the message and
o replay attempts
Lodderstedt, et al. Informational [Page 67]
RFC 6819 OAuth 2.0 Security January 2013
5.5. A Word on User Interaction and User-Installed Apps
OAuth, as a security protocol, is distinctive in that its flow
usually involves significant user interaction, making the end user a
part of the security model. This creates some important difficulties
in defending against some of the threats discussed above. Some of
these points have already been made, but it's worth repeating and
highlighting them here.
o End users must understand what they are being asked to approve
(see Section 5.2.4.2). Users often do not have the expertise to
understand the ramifications of saying "yes" to an authorization
request and are likely not to be able to see subtle differences in
the wording of requests. Malicious software can confuse the user,
tricking the user into approving almost anything.
o End-user devices are prone to software compromise. This has been
a long-standing problem, with frequent attacks on web browsers and
other parts of the user's system. But with the increasing
popularity of user-installed "apps", the threat posed by
compromised or malicious end-user software is very strong and is
one that is very difficult to mitigate.
o Be aware that users will demand to install and run such apps, and
that compromised or malicious ones can steal credentials at many
points in the data flow. They can intercept the very user login
credentials that OAuth is designed to protect. They can request
authorization far beyond what they have led the user to understand
and approve. They can automate a response on behalf of the user,
hiding the whole process. No solution is offered here, because
none is known; this remains in the space between better security
and better usability.
o Addressing these issues by restricting the use of user-installed
software may be practical in some limited environments and can be
used as a countermeasure in those cases. Such restrictions are
not practical in the general case, and mechanisms for after-the-
fact recovery should be in place.
o While end users are mostly incapable of properly vetting
applications they load onto their devices, those who deploy
authorization servers might have tools at their disposal to
mitigate malicious clients. For example, a well-run authorization
server must only assert client properties to the end user it is
effectively capable of validating, explicitly point out which
properties it cannot validate, and indicate to the end user the
risk associated with granting access to the particular client.
Lodderstedt, et al. Informational [Page 68]
RFC 6819 OAuth 2.0 Security January 2013
6. Acknowledgements
We would like to thank Stephen Farrell, Barry Leiba, Hui-Lan Lu,
Francisco Corella, Peifung E. Lam, Shane B. Weeden, Skylar Woodward,
Niv Steingarten, Tim Bray, and James H. Manger for their comments and
contributions.
7. References
7.1. Normative References
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework",
RFC 6749, October 2012.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750, October 2012.
7.2. Informative References
[Framebusting]
Rydstedt, G., Bursztein, Boneh, D., and C. Jackson,
"Busting Frame Busting: a Study of Clickjacking
Vulnerabilities on Popular Sites", IEEE 3rd Web 2.0
Security and Privacy Workshop, May 2010, <http://elie.im/
publication/busting-frame-busting-a-study-of-
clickjacking-vulnerabilities-on-popular-sites>.
[IMEI] 3GPP, "International Mobile station Equipment Identities
(IMEI)", 3GPP TS 22.016 11.0.0, September 2012,
<http://www.3gpp.org/ftp/Specs/html-info/22016.htm>.
[OASIS.saml-core-2.0-os]
Cantor, S., Ed., Kemp, J., Ed., Philpott, R., Ed., and E.
Maler, Ed., "Assertions and Protocols for the OASIS
Security Assertion Markup Language (SAML) V2.0", OASIS
Standard saml-core-2.0-os, March 2005,
<http://docs.oasis-open.org/security/saml/
v2.0/saml-core-2.0-os.pdf>.
[OASIS.sstc-saml-bindings-1.1]
Maler, E., Ed., Mishra, P., Ed., and R. Philpott, Ed.,
"Bindings and Profiles for the OASIS Security Assertion
Markup Language (SAML) V1.1", September 2003,
<http://www.oasis-open.org/committees/download.php/3405/
oasis-sstc-saml-bindings-1.1.pdf>.
Lodderstedt, et al. Informational [Page 69]
RFC 6819 OAuth 2.0 Security January 2013
[OASIS.sstc-sec-analysis-response-01]
Linn, J., Ed., and P. Mishra, Ed., "SSTC Response to
"Security Analysis of the SAML Single Sign-on Browser/
Artifact Profile"", January 2005,
<http://www.oasis-open.org/committees/download.php/
11191/sstc-gross-sec-analysis-response-01.pdf>.
[OAuth-ASSERTIONS]
Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
"Assertion Framework for OAuth 2.0", Work in Progress,
December 2012.
[OAuth-HTTP-MAC]
Richer, J., Ed., Mills, W., Ed., and H. Tschofenig, Ed.,
"OAuth 2.0 Message Authentication Code (MAC) Tokens", Work
in Progress, November 2012.
[OAuth-JWT]
Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", Work in Progress, December 2012.
[OAuth-REVOCATION]
Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "Token
Revocation", Work in Progress, November 2012.
[OPENID] "OpenID Foundation Home Page", <http://openid.net/>.
[OWASP] "Open Web Application Security Project Home Page",
<https://www.owasp.org/>.
[Portable-Contacts]
Smarr, J., "Portable Contacts 1.0 Draft C", August 2008,
<http://portablecontacts.net/>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
Lodderstedt, et al. Informational [Page 70]
RFC 6819 OAuth 2.0 Security January 2013
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[SSL-Latency]
Sissel, J., Ed., "SSL handshake latency and HTTPS
optimizations", June 2010.
[Sec-Analysis]
Gross, T., "Security Analysis of the SAML Single Sign-on
Browser/Artifact Profile", 19th Annual Computer Security
Applications Conference, Las Vegas, December 2003.
[X-Frame-Options]
Ross, D. and T. Gondrom, "HTTP Header X-Frame-Options",
Work in Progress, October 2012.
[iFrame] World Wide Web Consortium, "Frames in HTML documents",
W3C HTML 4.01, December 1999,
<http://www.w3.org/TR/html4/present/frames.html#h-16.5>.
Authors' Addresses
Torsten Lodderstedt (editor)
Deutsche Telekom AG
EMail: torsten@lodderstedt.net
Mark McGloin
IBM
EMail: mark.mcgloin@ie.ibm.com
Phil Hunt
Oracle Corporation
EMail: phil.hunt@yahoo.com
Lodderstedt, et al. Informational [Page 71]