MEDIAMAN M. Sporny
Internet-Draft A. Guy
Intended status: Standards Track Digital Bazaar
Expires: 13 April 2023 10 October 2022
Media Types with Multiple Suffixes
draft-ietf-mediaman-suffixes-02
Abstract
This document updates RFC 6838 "Media Type Specifications and
Registration Procedures" to describe how to interpret subtypes with
multiple suffixes.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in This Document . . . . . . . . . . . . 2
2. Media Types with Multiple Suffixes . . . . . . . . . . . . . 2
2.1. Processing Multiple Suffixes . . . . . . . . . . . . . . 3
2.2. Security Considerations . . . . . . . . . . . . . . . . . 4
2.2.1. Media Type Fibbing . . . . . . . . . . . . . . . . . 4
3. Normative References . . . . . . . . . . . . . . . . . . . . 4
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
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.
1.1. Conventions Used in This Document
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] when they
appear in ALL CAPS. They may also appear in lower or mixed case as
plain English words, without any normative meaning.
2. Media Types with Multiple Suffixes
The following paragraphs are additions to RFC 6838.
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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 (https://www.iana.org/assignments/media-type-structured-
suffix/media-type-structured-suffix.xhtml).
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.
2.1. Processing Multiple 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 and left-most
"+" 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 (https://www.iana.org/assignments/media-type-structured-
suffix/media-type-structured-suffix.xhtml). 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 any of the following processing models: 1) application/
did+ld+json (as specified in the Media Type Registry
(https://www.iana.org/assignments/media-types/media-types.xhtml)), 2)
+ld+json (as specified in the Structured Syntax Suffixes Registry
(https://www.iana.org/assignments/media-type-structured-suffix/media-
type-structured-suffix.xhtml)), or 3) +json (as specified in the
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Structured Syntax Suffixes Registry
(https://www.iana.org/assignments/media-type-structured-suffix/media-
type-structured-suffix.xhtml)). As a further example, for the media
type "image/svg+xml+gzip", applications can choose to process the
underlying representation according any of the following processing
models: 1) image/svg+xml+gzip (as specified in the Media Type
Registry (https://www.iana.org/assignments/media-types/media-
types.xhtml)), 2) +gzip (as specified in the Structured Syntax
Suffixes Registry (https://www.iana.org/assignments/media-type-
structured-suffix/media-type-structured-suffix.xhtml)), and then +xml
(as specified in the Structured Syntax Suffixes Registry
(https://www.iana.org/assignments/media-type-structured-suffix/media-
type-structured-suffix.xhtml)).
If an application choses to utilize a portion of the media type that
is a structured syntax suffix, the specification referred to in the
the "Encoding Considerations" entry of the Structured Syntax Suffixes
Registry (https://www.iana.org/assignments/media-type-structured-
suffix/media-type-structured-suffix.xhtml) MUST be used for both
encoding and decoding the byte stream associated with the media type.
2.2. Security Considerations
2.2.1. Media Type Fibbing
It is possible for 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.
3. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
Appendix A. Acknowledgements
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.
Authors' Addresses
Manu Sporny
Digital Bazaar
203 Roanoke Street W.
Blacksburg, VA 24060
United States of America
Email: msporny@digitalbazaar.com
URI: http://manu.sporny.org/
Amy Guy
Digital Bazaar
203 Roanoke Street W.
Blacksburg, VA 24060
United States of America
Email: rhiaro@digitalbazaar.com
URI: https://rhiaro.co.uk/
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