| Internet-Draft | Media Types with Multiple Suffixes | January 2023 |
| Sporny & Guy | Expires 5 July 2023 | [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 suffix appended to the base subtype name. The suffixes MUST be interpreted as ordered. Valid media type names containing a structured suffix are built from right to left (not left to right). Characters on the left-most side of the left-most "+" in a subtype name specify the base subtype name. Characters to the right of each "+" in a subtype name denote additional structured syntax suffixes.¶
Media types with more than one suffix MUST be registered according to the procedure defined in [RFC6838]. A new base subtype name MUST only be registered with suffix combinations that are already registered in their own right in the Structured Syntax Suffixes registry.¶
For example, a media type that uses two suffixes, such as "application/foo+xml+gzip" is only permitted insofar as "+gzip" and "+xml" 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 MAY be utilized in a processing pipeline where each segment of the pipeline handles a particular structured syntax suffix by applying decoding rules associated with the structured syntax suffix in the Structured Syntax Suffixes Registry. The segment of the pipleine could then remove the structured syntax suffix from the media type and then pass the output of the decoding operation as well as the modified media type further down the pipeline.¶
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: 1) application/did+ld+json (as specified in the Media Type Registry), 2) +ld+json (as specified in the Structured Syntax Suffixes Registry), or 3) +json (as specified in the Structured Syntax Suffixes Registry). As a further example, for the media type "image/svg+xml+gzip", applications can choose to process the underlying representation according to any of the following processing models: 1) image/svg+xml+gzip (as specified in the Media Type Registry), 2) +gzip (as specified in the Structured Syntax Suffixes Registry), and then +xml (as specified in the Structured Syntax Suffixes Registry).¶
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 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.¶
Given this generic structured syntax processing approach, it is possible to perform structured syntax suffix processing on structured syntax suffixes that result in an invalid media type that cannot be processed further. For example, when processing image/svg+xml+gzip, a processor could choose to process using the +gzip, and then the +xml structured syntax suffixes rules which would result in a meaningless application/svg media type. Application developers are advised to ensure that the last structured syntax suffix, or valid media type, processed is the last one that is expected to be meaningfully processed by their application. Thus, an application that processes the +gzip and then the +xml structured syntax suffixes from an image/svg+xml+gzip media type expects that the +xml data is the last meaningful piece of information that it hopes to extract from the processing pipeline. That is, the application processor is expected to make a choice between processing as +xml or as image/svg+xml, and by making a choice, other choices might be removed from further processing pipeline stages.¶
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.¶
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.¶
While the liklihood of these sorts of attacks are low, they are not zero and enterprising attackers might take advantage of applications that carelessly register themselves in a structured suffix processing toolchain. These sorts of toolchains need to ensure that the incoming media type is not blindly trusted and that proper magic header or file structure checking is performed before allowing the encoded 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): Martin J. Duerst, Ivan Herman, Graham Klyne, Murray S. Kucherawy, Mark Nottingham, and Ted Thibodeau Jr.¶