Internet Engineering Task Force (IETF) S. Winter
Request for Comments: 7057 RESTENA
Updates: 3748 J. Salowey
Category: Standards Track Cisco
ISSN: 2070-1721 December 2013
Update to the Extensible Authentication Protocol (EAP)
Applicability Statement for
Application Bridging for Federated Access Beyond Web (ABFAB)
Abstract
This document updates the Extensible Authentication Protocol (EAP)
applicability statement from RFC 3748 to reflect recent usage of the
EAP protocol in the Application Bridging for Federated Access Beyond
web (ABFAB) architecture.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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/rfc7057.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
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RFC 7057 EAP Applicability December 2013
Table of Contents
1. Introduction ....................................................2
1.1. Requirements Language ......................................2
2. Uses of EAP for Application-Layer Access ........................2
2.1. Retransmission .............................................4
2.2. Re-authentication ..........................................5
3. Revised EAP Applicability Statement .............................5
4. Security Considerations .........................................6
5. Acknowledgements ................................................6
6. References ......................................................6
6.1. Normative References .......................................6
6.2. Informative References .....................................6
1. Introduction
The EAP applicability statement in [RFC3748] defines the scope of the
Extensible Authentication Protocol to be "for use in network access
authentication, where IP layer connectivity may not be available",
and states that "Use of EAP for other purposes, such as bulk data
transport, is NOT RECOMMENDED".
While some of the reasons for the recommendation against usage of EAP
for bulk data transport are still valid, some of the other provisions
in the applicability statement have turned out to be too narrow.
Section 2 describes the example where EAP is used to authenticate
application-layer access. Section 3 provides new text to update
Section 1.3., "Applicability", in [RFC3748].
1.1. Requirements Language
In this document, several words are used to signify the requirements
of the specification. The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in [RFC2119].
2. Uses of EAP for Application-Layer Access
Ongoing work in the IETF specifies the use of EAP over Generic
Security Service Application Program Interface (GSS-API) for generic
application layer access [RFC7055]. In the past, using EAP in this
context has met resistance due to the lack of channel bindings
[RFC6677]. Without channel bindings, a peer cannot verify if an
authenticator is authorized to provide an advertised service.
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However, as additional services use EAP for authentication, the
distinction of which service is being contacted becomes more
important. Application services might have different properties.
Consider an environment with multiple printers, some of which provide
a confidential service to output documents to a controlled location.
If a peer sent a document to the wrong service, then potentially
sensitive information might be printed in an uncontrolled location
and be disclosed. In addition, it might be more likely that a low-
value service is compromised than some high-value service. If the
high-value service could be impersonated by a low-value service then
the security of the overall system would be limited by the security
of the lower-value service.
This distinction is present in any environment where peers' security
depends on which service they reach. However, it is particularly
acute in a federated environment where multiple organizations are
involved. It is very likely that these organizations will have
different security policies and practices. It is very likely that
the goals of these organizations will not entirely be aligned. In
many situations, one organization could gain value by being able to
impersonate another. In this environment, authenticating the EAP
server is insufficient: the peer must also validate that the
contacted host is authorized to provide the requested service.
In environments where EAP is used for purposes other than network
access authentication:
o All EAP servers and all application access EAP peers MUST support
channel bindings. All network access EAP peers SHOULD support
channel bindings.
o Channel binding MUST be used for all application authentication.
The EAP server MUST require that either the correct EAP lower-
layer attribute or another attribute indicating the purpose of the
authentication be present in the channel binding data for
application authentication.
o Channel binding SHOULD be used for all network access
authentication, and when channel binding data is present, the EAP
server MUST require that it contain the correct EAP lower-layer
attribute to explicitly identify the reason for authentication.
o Any new usage of EAP MUST use channel bindings including the EAP
lower-layer attribute to prevent confusion with network access
usage.
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Operators need to carefully consider the security implications before
relaxing these requirements. One potentially serious attack exists
when channel binding is not required and EAP authentication is
introduced into an existing service other than network access. A
device can be created that impersonates a Network Access Service
(NAS) to peers, but actually proxies the authentication to the new
application service that accepts EAP authentications. This may
decrease the security of this service even for users who previously
used non-EAP means of authentication to the service.
It is REQUIRED for the application layer to prove that both the EAP
peer and EAP authenticator possess the EAP Master Session Key (MSK).
Failing to validate the possession of the EAP MSK can allow an
attacker to insert himself into the conversation and impersonate the
peer or authenticator. In addition, the application should define
channel binding attributes that are sufficient to validate that the
application service is being correctly represented to the peer.
2.1. Retransmission
In EAP, the authenticator is responsible for retransmission. By
default, EAP assumes that the lower layer (the application in this
context) is unreliable. The authenticator can send a packet whenever
its retransmission timer triggers. In this mode, applications need
to be able to receive and process EAP messages at any time during the
authentication conversation.
Alternatively, EAP permits a lower layer to set the retransmission
timer to infinite. When this happens, the lower layer becomes
responsible for reliable delivery of EAP messages. Applications that
use a lock-step or client-driven authentication protocol might
benefit from this approach.
In addition to retransmission behavior, applications need to deal
with discarded EAP messages. For example, whenever some EAP methods
receive erroneous input, these methods discard the input rather than
generating an error response. If the erroneous input was generated
by an attacker, legitimate input can sometimes be received after the
erroneous input. Applications MUST handle discarded EAP messages,
although the specific way in which discarded messages will be handled
depends on the characteristics of the application. Options include
failing the authentication at the application level, requesting an
EAP retransmit and waiting for additional EAP input.
Applications designers that incorporate EAP into their application
need to determine how retransmission and message discards are
handled.
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2.2. Re-authentication
EAP lower layers MAY provide a mechanism for re-authentication to
happen within an existing session [RFC3748]. Re-authentication
permits security associations to be updated without establishing a
new session. For network access, this can be important because
interrupting network access can disrupt connections and media.
Some applications might not need re-authentication support. For
example, if sessions are relatively short-lived or if sessions can be
replaced without significant disruption, re-authentication might not
provide value. Protocols like HyperText Transfer Protocol (HTTP)
[RFC2616] and Simple Mail Transport Protocol (SMTP) [RFC5321] are
examples of protocols where establishing a new connection to update
security associations is likely to be sufficient.
Re-authentication is likely to be valuable if sessions or connections
are long-lived or if there is a significant cost to disrupting them.
Another factor may make re-authentication important. Some protocols
only permit one party in a protocol (for example, the client) to
establish a new connection. If another party in the protocol needs
the security association refreshed, then re-authentication can
provide a mechanism to do so.
Application designers need to determine whether re-authentication
support is needed and which parties can initiate it.
3. Revised EAP Applicability Statement
The following text is appended to the EAP applicability statement in
[RFC3748].
In cases where EAP is used for application authentication, support
for EAP channel bindings is REQUIRED on the EAP peer and EAP server
to validate that the host is authorized to provide the services
requested. In addition, the application MUST define channel binding
attributes that are sufficient to validate that the application
service is being correctly represented to the peer. The protocol
carrying EAP MUST prove possession of the EAP MSK between the EAP
peer and EAP authenticator. In the context of EAP for application
access the application is providing the EAP lower layer.
Applications protocols vary so their specific behavior and transport
characteristics needs to be considered when determining their
retransmission and re-authentication behavior. Circumstances might
require that applications need to perform conversion of identities
from an application specific character set to UTF-8 or another
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RFC 7057 EAP Applicability December 2013
character set required by a particular EAP method. See also
[RADEXT-NAI], Section 2.6, for information about normalization of
identifiers.
4. Security Considerations
In addition to the requirements discussed in the main sections of the
document, applications should take into account how server
authentication is achieved. Some deployments may allow for weak
server authentication that is then validated with an additional
existing exchange that provides mutual authentication. In order to
fully mitigate the risk of NAS impersonation when these mechanisms
are used, it is RECOMMENDED that mutual channel bindings be used to
bind the authentications together as described in [RFC7029]. When
doing channel binding it is REQUIRED that the authenticator is not
able to modify the channel binding data passed between the peer to
the authenticator as part of the authentication process.
5. Acknowledgements
Large amounts of helpful text and insightful thoughts were
contributed by Sam Hartman, Painless Security. David Black
contributed to the text clarifying channel bindings usage.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[RFC6677] Hartman, S., Clancy, T., and K. Hoeper, "Channel-Binding
Support for Extensible Authentication Protocol (EAP)
Methods", RFC 6677, July 2012.
6.2. Informative References
[RADEXT-NAI] DeKok, A., "The Network Access Identifier", Work in
Progress, November 2013.
[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.
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RFC 7057 EAP Applicability December 2013
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC7029] Hartman, S., Wasserman, M., and D. Zhang, "Extensible
Authentication Protocol (EAP) Mutual Cryptographic
Binding", RFC 7029, October 2013.
[RFC7055] Hartman, S., Ed. and J. Howlett, "A GSS-API Mechanism
for the Extensible Authentication Protocol", RFC 7055,
December 2013.
Authors' Addresses
Stefan Winter
Fondation RESTENA
6, rue Richard Coudenhove-Kalergi
Luxembourg 1359
LUXEMBOURG
Phone: +352 424409 1
Fax: +352 422473
EMail: stefan.winter@restena.lu
URI: http://www.restena.lu.
Joseph Salowey
Cisco Systems
2901 3rd Ave
Seattle, Washington 98121
USA
EMail: jsalowey@cisco.com
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