Indicating Character Encoding and Language for HTTP Header Field Parameters

By default, header field values in Hypertext Transfer Protocol (HTTP) messages cannot easily carry characters outside the US-ASCII coded character set. RFC 2231 defines an encoding mechanism for use in parameters inside Multipurpose Internet Mail Extensions (MIME) header field values. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a simplified profile of the encoding defined in RFC 2231.¶

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• The HTTP specification discourages use of non-US-ASCII characters in field values, placing them into the "obs-text" ABNF production ([RFC7230], Section 3.2).
• Furthermore, it stays silent about default character encoding schemes for field values, so any use of non-US-ASCII characters would need to be specific to the field definition, or would require some other kind of out-of-band information.
• Finally, some APIs assume a default character encoding scheme in order to map from the octet sequences (obtained from the HTTP message) to character sequences: for instance, the XMLHttpRequest API ([XMLHttpRequest]) uses the Interface Definition Language type "ByteString", effectively resulting in the ISO-8859-1 character encoding scheme [ISO-8859-1] being used.

On the other hand, RFC 2231 defines an encoding mechanism for parameters inside MIME header fields ([RFC2231]), which, as opposed to HTTP messages, do need to be sent over non-binary transports. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a simplified profile of the encoding defined in RFC 2231. It can be applied to any HTTP header field that uses the common "parameter" ("name=value") syntax.¶

This document obsoletes [RFC5987] and moves it to "historic" status; the changes are summarized in Appendix A.¶

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 [RFC2119].¶

This specification uses the ABNF (Augmented Backus-Naur Form) notation defined in [RFC5234]. The following core rules are included by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters), DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f), and LWSP (linear whitespace).¶

This specification uses terminology defined in [RFC6365], namely: "character encoding scheme" (below abbreviated to "character encoding"), "charset" and "coded character set".¶

Note that this differs from RFC 2231, which uses the term "character set" for "character encoding scheme".¶

RFC 2231 defines several extensions to MIME. The sections below discuss if and how they apply to HTTP header fields.¶

In short: ¶

• Parameter Continuations aren't needed (Section 3.1),
• Character Encoding and Language Information are useful, therefore a simple subset is specified (Section 3.2), and
• Language Specifications in Encoded Words aren't needed (Section 3.3).

Specifications of HTTP header fields that use the extensions defined in Section 3.2 ought to clearly state that. A simple way to achieve this is to normatively reference this specification, and to include the ext-value production into the ABNF for specific header field parameters.¶

For instance:

  foo         = token ";" LWSP title-param
  title-param = "title" LWSP "=" LWSP value
              / "title*" LWSP "=" LWSP ext-value
  ext-value   = <see draft-ietf-httpbis-rfc5987bis, Section 3.2>

[pub: Upon publication as RFC, the string "draft-ietf-httpbis-rfc5987bis" needs to be replaced with the RFC name, and this comment needs to be removed.]

The format described in this document makes it possible to transport non-ASCII characters, and thus enables character "spoofing" scenarios, in which a displayed value appears to be something other than it is.¶

Furthermore, there are known attack scenarios relating to decoding UTF-8.¶

See Section 10 of [RFC3629] for more information on both topics.¶

In addition, the extension specified in this document makes it possible to transport multiple language variants for a single parameter, and such use might allow spoofing attacks, where different language versions of the same parameter are not equivalent. Whether this attack is useful as an attack depends on the parameter specified.¶

There are no IANA Considerations related to this specification.¶

• The document title was changed to "Indicating Character Encoding and Language for HTTP Header Field Parameters".
• The introduction was rewritten to better explain the issues around non-ASCII characters in field values.
• The requirement to support the "ISO-8859-1" encoding was removed.
• The document does not attempt to re-define a generic "parameter" ABNF anymore (it turned out that there really isn't a generic definition of parameters in HTTP; for instance, there are subtle differences with respect to whitespace handling).
• A note about defects in error handling in current implementations was removed, as it wasn't accurate anymore.

• "Authorization" (as used in HTTP Digest Authentication, defined in [RFC7616]),
• "Content-Disposition", defined in [RFC6266], and
• "Link", defined in [RFC5988].

As the encoding is a profile/clarification of the one defined in [RFC2231] in 1997, many user agents already supported it for use in "Content-Disposition" when [RFC5987] got published.¶

Since the publication of [RFC5987], three more popular desktop user agents have added support for this encoding; see <http://purl.org/NET/http/content-disposition-tests#encoding-2231-char> for details. At this time, the current versions of all major desktop user agents support it.¶

Note that the implementation in Internet Explorer 9 does not support the ISO-8859-1 character encoding; this document revision acknowledges that UTF-8 is sufficient for expressing all code points, and removes the requirement to support ISO-8859-1.¶

The "Link" header field, on the other hand, was more recently specified in [RFC5988]. At the time of this writing, no User Agent except Firefox supported the "title*" parameter (starting with release 15).¶

Section 3.4 of [RFC7616] defines the "username*" parameter for use in HTTP Digest Authentication. At the time of writing, no User Agent implemented this extension.¶

Thanks to Martin Dürst and Frank Ellermann for help figuring out ABNF details, to Graham Klyne and Alexey Melnikov for general review, to Chris Newman for pointing out an RFC 2231 incompatibility, and to Benjamin Carlyle, Roar Lauritzsen, Eric Lawrence, and James Manger for implementer's feedback.¶