RFC7578: Returning Values from Forms: multipart/form-data
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Obsoletes:
RFC2388
Internet Engineering Task Force (IETF) L. Masinter
Request for Comments: 7578 Adobe
Obsoletes: 2388 July 2015
Category: Standards Track
ISSN: 2070-1721
Returning Values from Forms: multipart/form-data
Abstract
This specification defines the multipart/form-data media type, which
can be used by a wide variety of applications and transported by a
wide variety of protocols as a way of returning a set of values as
the result of a user filling out a form. This document obsoletes
RFC 2388.
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/rfc7578.
Copyright Notice
Copyright (c) 2015 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Percent-Encoding Option . . . . . . . . . . . . . . . . . . . 3
3. Advice for Forms and Form Processing . . . . . . . . . . . . 3
4. Definition of multipart/form-data . . . . . . . . . . . . . . 4
4.1. "Boundary" Parameter of multipart/form-data . . . . . . . 4
4.2. Content-Disposition Header Field for Each Part . . . . . 4
4.3. Multiple Files for One Form Field . . . . . . . . . . . . 5
4.4. Content-Type Header Field for Each Part . . . . . . . . . 5
4.5. The Charset Parameter for "text/plain" Form Data . . . . 5
4.6. The _charset_ Field for Default Charset . . . . . . . . . 6
4.7. Content-Transfer-Encoding Deprecated . . . . . . . . . . 6
4.8. Other "Content-" Header Fields . . . . . . . . . . . . . 7
5. Operability Considerations . . . . . . . . . . . . . . . . . 7
5.1. Non-ASCII Field Names and Values . . . . . . . . . . . . 7
5.1.1. Avoid Non-ASCII Field Names . . . . . . . . . . . . . 7
5.1.2. Interpreting Forms and Creating multipart/form-data
Data . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1.3. Parsing and Interpreting Form Data . . . . . . . . . 8
5.2. Ordered Fields and Duplicated Field Names . . . . . . . . 8
5.3. Interoperability with Web Applications . . . . . . . . . 8
5.4. Correlating Form Data with the Original Form . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Media Type Registration for multipart/form-data . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Changes from RFC 2388 . . . . . . . . . . . . . . . 14
Appendix B. Alternatives . . . . . . . . . . . . . . . . . . . . 14
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
In many applications, it is possible for a user to be presented with
a form. The user will fill out the form, including information that
is typed, generated by user input, or included from files that the
user has selected. When the form is filled out, the data from the
form is sent from the user to the receiving application.
The definition of multipart/form-data is derived from one of those
applications, originally set out in [RFC1867] and subsequently
incorporated into HTML 3.2 [W3C.REC-html32-19970114], where forms are
expressed in HTML, and the form data is sent via HTTP or electronic
mail. This representation is widely implemented in numerous web
browsers and web servers.
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However, multipart/form-data is also used for forms that are
presented using representations other than HTML (spreadsheets, PDF,
etc.) and for transport using means other than electronic mail or
HTTP; it is used in distributed applications that do not involve
forms at all or do not have users filling out the form. For this
reason, this document defines a general syntax and semantics
independent of the application for which it is used, with specific
rules for web applications noted in context.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
2. Percent-Encoding Option
Within this specification, "percent-encoding" (as defined in
[RFC3986]) is offered as a possible way of encoding characters in
file names that are otherwise disallowed, including non-ASCII
characters, spaces, control characters, and so forth. The encoding
is created replacing each non-ASCII or disallowed character with a
sequence, where each byte of the UTF-8 encoding of the character is
represented by a percent-sign (%) followed by the (case-insensitive)
hexadecimal of that byte.
3. Advice for Forms and Form Processing
The representation and interpretation of forms and the nature of form
processing is not specified by this document. However, for forms and
form processing that result in the generation of multipart/form-data,
some suggestions are included.
In a form, there is generally a sequence of fields, where each field
is expected to be supplied with a value, e.g., by a user who fills
out the form. Each field has a name. After a form has been filled
out and the form's data is "submitted", the form processing results
in a set of values for each field -- the "form data".
In forms that work with multipart/form-data, field names could be
arbitrary Unicode strings; however, restricting field names to ASCII
will help avoid some interoperability issues (see Section 5.1).
Within a given form, ensuring field names are unique is also helpful.
Some fields may have default values or presupplied values in the form
itself. Fields with presupplied values might be hidden or invisible;
this allows using generic processing for form data from a variety of
actual forms.
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4. Definition of multipart/form-data
The media type multipart/form-data follows the model of multipart
MIME data streams as specified in Section 5.1 of [RFC2046]; changes
are noted in this document.
A multipart/form-data body contains a series of parts separated by a
boundary.
4.1. "Boundary" Parameter of multipart/form-data
As with other multipart types, the parts are delimited with a
boundary delimiter, constructed using CRLF, "--", and the value of
the "boundary" parameter. The boundary is supplied as a "boundary"
parameter to the multipart/form-data type. As noted in Section 5.1
of [RFC2046], the boundary delimiter MUST NOT appear inside any of
the encapsulated parts, and it is often necessary to enclose the
"boundary" parameter values in quotes in the Content-Type header
field.
4.2. Content-Disposition Header Field for Each Part
Each part MUST contain a Content-Disposition header field [RFC2183]
where the disposition type is "form-data". The Content-Disposition
header field MUST also contain an additional parameter of "name"; the
value of the "name" parameter is the original field name from the
form (possibly encoded; see Section 5.1). For example, a part might
contain a header field such as the following, with the body of the
part containing the form data of the "user" field:
Content-Disposition: form-data; name="user"
For form data that represents the content of a file, a name for the
file SHOULD be supplied as well, by using a "filename" parameter of
the Content-Disposition header field. The file name isn't mandatory
for cases where the file name isn't available or is meaningless or
private; this might result, for example, when selection or drag-and-
drop is used or when the form data content is streamed directly from
a device.
If a "filename" parameter is supplied, the requirements of
Section 2.3 of [RFC2183] for the "receiving MUA" (i.e., the receiving
Mail User Agent) apply to receivers of multipart/form-data as well:
do not use the file name blindly, check and possibly change to match
local file system conventions if applicable, and do not use directory
path information that may be present.
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In most multipart types, the MIME header fields in each part are
restricted to US-ASCII; for compatibility with those systems, file
names normally visible to users MAY be encoded using the percent-
encoding method in Section 2, following how a "file:" URI
[URI-SCHEME] might be encoded.
NOTE: The encoding method described in [RFC5987], which would add a
"filename*" parameter to the Content-Disposition header field, MUST
NOT be used.
Some commonly deployed systems use multipart/form-data with file
names directly encoded including octets outside the US-ASCII range.
The encoding used for the file names is typically UTF-8, although
HTML forms will use the charset associated with the form.
4.3. Multiple Files for One Form Field
The form data for a form field might include multiple files.
[RFC2388] suggested that multiple files for a single form field be
transmitted using a nested "multipart/mixed" part. This usage is
deprecated.
To match widely deployed implementations, multiple files MUST be sent
by supplying each file in a separate part but all with the same
"name" parameter.
Receiving applications intended for wide applicability (e.g.,
multipart/form-data parsing libraries) SHOULD also support the older
method of supplying multiple files.
4.4. Content-Type Header Field for Each Part
Each part MAY have an (optional) "Content-Type" header field, which
defaults to "text/plain". If the contents of a file are to be sent,
the file data SHOULD be labeled with an appropriate media type, if
known, or "application/octet-stream".
4.5. The Charset Parameter for "text/plain" Form Data
In the case where the form data is text, the charset parameter for
the "text/plain" Content-Type MAY be used to indicate the character
encoding used in that part. For example, a form with a text field in
which a user typed "Joe owes <eu>100", where <eu> is the Euro symbol,
might have form data returned as:
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--AaB03x
content-disposition: form-data; name="field1"
content-type: text/plain;charset=UTF-8
content-transfer-encoding: quoted-printable
Joe owes =E2=82=AC100.
--AaB03x
In practice, many widely deployed implementations do not supply a
charset parameter in each part, but rather, they rely on the notion
of a "default charset" for a multipart/form-data instance.
Subsequent sections will explain how the default charset is
established.
4.6. The _charset_ Field for Default Charset
Some form-processing applications (including HTML) have the
convention that the value of a form entry with entry name "_charset_"
and type "hidden" is automatically set when the form is opened; the
value is used as the default charset of text field values (see form-
charset in Section 5.1.2). In such cases, the value of the default
charset for each "text/plain" part without a charset parameter is the
supplied value. For example:
--AaB03x
content-disposition: form-data; name="_charset_"
iso-8859-1
--AaB03x--
content-disposition: form-data; name="field1"
...text encoded in iso-8859-1 ...
AaB03x--
4.7. Content-Transfer-Encoding Deprecated
Previously, it was recommended that senders use a Content-Transfer-
Encoding encoding (such as "quoted-printable") for each non-ASCII
part of a multipart/form-data body because that would allow use in
transports that only support a "7bit" encoding. This use is
deprecated for use in contexts that support binary data such as HTTP.
Senders SHOULD NOT generate any parts with a Content-Transfer-
Encoding header field.
Currently, no deployed implementations that send such bodies have
been discovered.
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4.8. Other "Content-" Header Fields
The multipart/form-data media type does not support any MIME header
fields in parts other than Content-Type, Content-Disposition, and (in
limited circumstances) Content-Transfer-Encoding. Other header
fields MUST NOT be included and MUST be ignored.
5. Operability Considerations
5.1. Non-ASCII Field Names and Values
Normally, MIME header fields in multipart bodies are required to
consist only of 7-bit data in the US-ASCII character set. While
[RFC2388] suggested that non-ASCII field names be encoded according
to the method in [RFC2047], this practice doesn't seem to have been
followed widely.
This specification makes three sets of recommendations for three
different states of workflow.
5.1.1. Avoid Non-ASCII Field Names
For broadest interoperability with existing deployed software, those
creating forms SHOULD avoid non-ASCII field names. This should not
be a burden because, in general, the field names are not visible to
users. The field names in the underlying need not match what the
user sees on the screen.
If non-ASCII field names are unavoidable, form or application
creators SHOULD use UTF-8 uniformly. This will minimize
interoperability problems.
5.1.2. Interpreting Forms and Creating multipart/form-data Data
Some applications of this specification will supply a character
encoding to be used for interpretation of the multipart/form-data
body. In particular, HTML 5 [W3C.REC-html5-20141028] uses
o the content of a "_charset_" field, if there is one;
o the value of an accept-charset attribute of the <form> element, if
there is one;
o the character encoding of the document containing the form, if it
is US-ASCII compatible;
o otherwise, UTF-8.
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Call this value the form-charset. Any text, whether field name,
field value, or ("text/plain") form data that uses characters outside
the ASCII range MAY be represented directly encoded in the form-
charset.
5.1.3. Parsing and Interpreting Form Data
While this specification provides guidance for the creation of
multipart/form-data, parsers and interpreters should be aware of the
variety of implementations. File systems differ as to whether and
how they normalize Unicode names, for example. The matching of form
elements to form-data parts may rely on a fuzzier match. In
particular, some multipart/form-data generators might have followed
the previous advice of [RFC2388] and used the "encoded-word" method
of encoding non-ASCII values, as described in [RFC2047]:
encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
Others have been known to follow [RFC2231], to send unencoded UTF-8,
or even to send strings encoded in the form-charset.
For this reason, interpreting multipart/form-data (even from
conforming generators) may require knowing the charset used in form
encoding in cases where the _charset_ field value or a charset
parameter of a "text/plain" Content-Type header field is not
supplied.
5.2. Ordered Fields and Duplicated Field Names
Form processors given forms with a well-defined ordering SHOULD send
back results in order. (Note that there are some forms that do not
define a natural order.) Intermediaries MUST NOT reorder the
results. Form parts with identical field names MUST NOT be
coalesced.
5.3. Interoperability with Web Applications
Many web applications use the "application/x-www-form-urlencoded"
method for returning data from forms. This format is quite compact,
for example:
name=Xavier+Xantico&verdict=Yes&colour=Blue&happy=sad&Utf%F6r=Send
However, there is no opportunity to label the enclosed data with a
content type, apply a charset, or use other encoding mechanisms.
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Many form-interpreting programs (primarily web browsers) now
implement and generate multipart/form-data, but a receiving
application might also need to support the
"application/x-www-form-urlencoded" format.
5.4. Correlating Form Data with the Original Form
This specification provides no specific mechanism by which multipart/
form-data can be associated with the form that caused it to be
transmitted. This separation is intentional; many different forms
might be used for transmitting the same data. In practice,
applications may supply a specific form processing resource (in HTML,
the ACTION attribute in a FORM tag) for each different form.
Alternatively, data about the form might be encoded in a "hidden
field" (a field that is part of the form but that has a fixed value
to be transmitted back to the form-data processor).
6. IANA Considerations
The media type registration of multipart/form-data has been updated
to point to this document, using the template in Section 8. In
addition, the registrations of the "name" parameter and the "form-
data" value in the "Content Disposition Values and Parameters"
registry have been updated to both point to this document.
7. Security Considerations
All form-processing software should treat user supplied form-data
with sensitivity, as it often contains confidential or personally
identifying information. There is widespread use of form "auto-fill"
features in web browsers; these might be used to trick users to
unknowingly send confidential information when completing otherwise
innocuous tasks. multipart/form-data does not supply any features
for checking integrity, ensuring confidentiality, avoiding user
confusion, or other security features; those concerns must be
addressed by the form-filling and form-data-interpreting
applications.
Applications that receive forms and process them must be careful not
to supply data back to the requesting form-processing site that was
not intended to be sent.
It is important when interpreting the filename of the Content-
Disposition header field to not inadvertently overwrite files in the
recipient's file space.
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User applications that request form information from users must be
careful not to cause a user to send information to the requestor or a
third party unwillingly or unwittingly. For example, a form might
request that spam information be sent to an unintended third party or
private information be sent to someone that the user might not
actually intend. While this is primarily an issue for the
representation and interpretation of forms themselves (rather than
the data representation of the form data), the transportation of
private information must be done in a way that does not expose it to
unwanted prying.
With the introduction of form-data that can reasonably send back the
content of files from a user's file space, the possibility arises
that a user might be sent an automated script that fills out a form
and then sends one of the user's local files to another address.
Thus, additional caution is required when executing automated
scripting where form-data might include a user's files.
Files sent via multipart/form-data may contain arbitrary executable
content, and precautions against malicious content are necessary.
The considerations of Sections 2.3 and 5 of [RFC2183], with respect
to the "filename" parameter of the Content-Disposition header field,
also apply to its usage here.
8. Media Type Registration for multipart/form-data
This section is the media type registration using the template from
[RFC6838].
Type name: multipart
Subtype name: form-data
Required parameters: boundary
Optional parameters: none
Encoding considerations: Common use is BINARY.
In limited use (or transports that restrict the encoding to 7bit
or 8bit), each part is encoded separately using Content-Transfer-
Encoding; see Section 4.7.
Security considerations: See Section 7 of this document.
Interoperability considerations: This document makes several
recommendations for interoperability with deployed
implementations, including Section 4.7.
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Published specification: This document.
Applications that use this media type: Numerous web browsers,
servers, and web applications.
Fragment identifier considerations: None; fragment identifiers are
not defined for this type.
Additional information:
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person & email address to contact for further information: Author of
this document.
Intended usage: COMMON
Restrictions on usage: none
Author: Author of this document.
Change controller: IETF
Provisional registration: N/A
9. References
9.1. Normative References
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<http://www.rfc-editor.org/info/rfc2046>.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, DOI 10.17487/RFC2047, November 1996,
<http://www.rfc-editor.org/info/rfc2047>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC2183] Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183,
DOI 10.17487/RFC2183, August 1997,
<http://www.rfc-editor.org/info/rfc2183>.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, DOI 10.17487/RFC2231, November
1997, <http://www.rfc-editor.org/info/rfc2231>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
9.2. Informative References
[RFC1867] Nebel, E. and L. Masinter, "Form-based File Upload in
HTML", RFC 1867, DOI 10.17487/RFC1867, November 1995,
<http://www.rfc-editor.org/info/rfc1867>.
[RFC2388] Masinter, L., "Returning Values from Forms: multipart/
form-data", RFC 2388, DOI 10.17487/RFC2388, August 1998,
<http://www.rfc-editor.org/info/rfc2388>.
[RFC5987] Reschke, J., "Character Set and Language Encoding for
Hypertext Transfer Protocol (HTTP) Header Field
Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010,
<http://www.rfc-editor.org/info/rfc5987>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<http://www.rfc-editor.org/info/rfc6838>.
[URI-SCHEME]
Kerwin, M., "The file URI Scheme", Work in Progress,
draft-ietf-appsawg-file-scheme-02, May 2015.
[W3C.REC-html32-19970114]
Raggett, D., "HTML 3.2 Reference Specification", W3C
Recommendation REC-html32-19970114, January 1997,
<http://www.w3.org/TR/REC-html32-19970114>.
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[W3C.REC-html5-20141028]
Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Navara, E., O'Connor, E., and S. Pfeiffer, "HTML5", W3C
Recommendation REC-html5-20141028, October 2014,
<http://www.w3.org/TR/2014/REC-html5-20141028>.
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Appendix A. Changes from RFC 2388
The handling of non-ASCII field names has changed -- the method
described in RFC 2047 is no longer recommended; instead, it is
suggested that senders send UTF-8 field names directly and that file
names be sent directly in the form-charset.
The handling of multiple files submitted as the result of a single
form field (e.g., HTML's <input type=file multiple> element) results
in each file having its own top-level part with the same name
parameter; the method of using a nested "multipart/mixed" from
[RFC2388] is no longer recommended for creators and is not required
for receivers as there are no known implementations of senders.
The _charset_ convention and use of an explicit "form-data" charset
is documented; also, "boundary" is now a required parameter in
Content-Type.
The relationship of the ordering of fields within a form and the
ordering of returned values within multipart/form-data was not
defined before, nor was the handling of the case where a form has
multiple fields with the same name.
Various editorial changes were made; they include removing the
obsolete discussion of alternatives from the appendix, updating the
references, and moving the outline of form processing into the
introduction.
Appendix B. Alternatives
There are numerous alternative ways in which form data can be
encoded; many are listed in Section 5.2 of [RFC2388]. The multipart/
form-data encoding is verbose, especially if there are many fields
with short values. In most use cases, this overhead isn't
significant.
More problematic are the differences introduced when implementors
opted to not follow [RFC2388] when encoding non-ASCII field names
(perhaps because "may" should have been "MUST"). As a result,
parsers need to be more complex for matching against the possible
outputs of various encoding methods.
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Acknowledgements
Many thanks to the those who reviewed this document -- Alexey
Melnikov, Salvatore Loreto, Chris Lonvick, Kathleen Moriarty, Barry
Leiba, Julian Reschke, Tom Petch, Ned Freed, Cedric Brancourt, as
well as others, including Ian Hickson, who requested it be produced
in the first place.
Author's Address
Larry Masinter
Adobe
Email: masinter@adobe.com
URI: http://larry.masinter.net
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