| Internet-Draft | Media Types with Multiple Suffixes | July 2023 |
| Sporny & Guy | Expires 11 January 2024 | [Page] |
This document updates RFC 6838 "Media Type Specifications and Registration Procedures" to describe how to interpret subtypes with multiple suffixes.¶
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As written, RFC 6838 [RFC6838] permits the registration of media type subtype names which contain any number of occurrences of the "+" character. RFC 6838 defines the characters following the final "+" to be a structured syntax suffix, but does not define anything further about how to interpret subtype names containing more than one "+" character.¶
This document updates RFC 6838 to clarify how to interpret subtype names containing more than one "+" character as subtypes with multiple suffixes.¶
As registration of media types which use a structured suffix has become widely supported, this enables further specialization of media types that build on already registered and well-defined media types which themselves use a structured suffix.¶
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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The following paragraphs are additions to RFC 6838.¶
Media types MAY be registered with more than one structured suffix appended to the base subtype name. Characters on the left-most side of the left-most "+" in a subtype name specify the base subtype name. The entire structured suffix is all of the characters to the right of the first "+" sign in the media type, including the initial "+" sign itself. The entire structured suffix MAY be composed of one or more other structured suffixes. As an example, given the "application/foo+bar+baz" media type: "application" is the top-level type, "foo" is the base subtype name, "+bar+baz" is the entire structured suffix, and "+baz" is the other structured suffix contained in the entire structured suffix.¶
When the entire structured suffix is composed of multiple structured suffixes, those structured suffixes MUST be interpreted as ordered. For example, presume a media type that uses two suffixes, such as "application/foo+bar+baz", where "+bar+baz", "+bar", and "+baz" are registered structured suffixes. A processor is expected to process either the entire media type, or "+baz", or "+bar+baz". The processor is never expected to process "+bar" alone when presented with a "application/foo+bar+baz" media type, as that would be considered interpreting multiple structured suffixes out of order.¶
Media types containing more than one structured suffix MUST be registered according to the procedure defined in [RFC6838]. A new media type that utilizes a structured suffix MUST only be registered if the entire structured suffix is already registered in the Structured Syntax Suffixes registry. For example, a media type that uses two suffixes, such as "application/foo+bar+baz" is only permitted insofar as "+baz" and "+bar+baz" are already registered structured syntax suffixes.¶
Registered media types have clear processing rules. In cases where specific handling of the exact media type is not required, receivers of the media type MAY do generic processing on the underlying representation according to their ability to process any subset of the suffix(es) from right to left inclusive. In other words, an application can choose to ignore the base subtype name from a media type with multiple suffixes, and process according to the remaining media type suffix(es).¶
This sort of generic processing of a portion of the media type MAY be utilized by a processor that is capable of applying decoding rules associated with the portion of the structured syntax suffix in the Structured Syntax Suffixes Registry.¶
For example, for the media type "application/did+ld+json", applications can choose to process the underlying representation according to any of the following processing models:¶
If an application choses to utilize a portion of the media type that is a structured syntax suffix, the suffix MUST exist as an entry in the Structured Syntax Suffixes Registry and the specification referred to in the "Encoding Considerations" entry of the registry MUST be used for both encoding and decoding the byte stream associated with the media type.¶
The syntax and semantics for fragment identifiers are specified in the "Fragment Identifier Considerations" column in the IANA Structured Syntax Suffixes registry. In general, when processing fragment identifiers associated with a structured syntax suffix, the following rules SHOULD be followed:¶
Other advisory information, such as fragment processing not being defined in any existing specification, MAY be provided in the "Fragment Identifier Considerations" column in the IANA Structured Syntax Suffixes registry as long as the text is terse in nature.¶
If a toolchain chooses to process a provided media type by using the selected structured suffix processing rules, it cannot presume that a document that is valid per the decoding rules associated with the structured suffix will be valid for a recognized subset of the structured suffix. For example, presuming a media type of "application/foo+bar+baz", a toolchain cannot presume that a valid "+baz" document will also be a valid "+bar+baz" document or a valid "application/foo+bar+baz" document.¶
If a toolchain chooses to process a provided media type by using the selected structured suffix processing rules, it cannot presume that fragment identifier semantics will be the same across a recognized subset of the structured suffix. For example, presuming a media type of "application/foo+bar+baz", a toolchain cannot presume that the fragment semantics for a "+baz" document will be the same as for a "+bar+baz" document or the same as for an "application/foo+bar+baz" document.¶
Toolchains cannot assume that the security characteristics of processing based on structured suffixes will be the same for the entire media type or any combination of recognized structured suffixes. For example, presuming a media type of "application/foo+bar+baz", a toolchain cannot presume that the security considerations for a "+baz" document will be the same as for a "+bar+baz" document or the same for an "application/foo+bar+baz" document.¶
It is possible for an attacker to utilize multiple structured suffixes in a way that tricks unsuspecting toolchains into skipping important security checks and allowing viruses to propagate. For example, an attacker might utilize an "application/vnd.ms-excel.addin.macroEnabled.12+zip" structured suffix to trigger an unzip process that would then invoke Microsoft Excel directly, bypassing anti-virus tooling that would otherwise block a macro-enabled MS Excel file containing a virus of some kind from being scanned or opened.¶
Enterprising attackers might take advantage of toolchains that carelessly process media types in this manner. Toolchains that process media types based purely on a structured suffix need to ensure that further processing does not blindly trust the decoded data and that proper magic header or file structure checking is performed before allowing the decoded data to drive operations that might negatively impact the application environment or operating system.¶
The editors would like to thank the following individuals for feedback on the specification (in alphabetical order): Harald Alvestrand, Martin J. Dürst, Ivan Herman, Graham Klyne, Murray S. Kucherawy, Mark Nottingham, Roberto Polli, Orie Steele, and Ted Thibodeau Jr.¶