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YAML Media Type
RFC 9512

Document Type RFC - Informational (February 2024)
Authors Roberto Polli , Erik Wilde , Eemeli Aro
Last updated 2024-02-14
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Zaheduzzaman Sarker
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RFC 9512

Internet Engineering Task Force (IETF)                          R. Polli
Request for Comments: 9512                       DTD, Italian Government
Category: Informational                                         E. Wilde
ISSN: 2070-1721                                                    Axway
                                                                  E. Aro
                                                           February 2024

                            YAML Media Type


   This document registers the application/yaml media type and the +yaml
   structured syntax suffix with IANA.  Both identify document
   components that are serialized according to the YAML specification.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2024 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
   ( 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Notational Conventions
     1.2.  Fragment Identification
       1.2.1.  Fragment Identification via Alias Nodes
   2.  Media Type and Structured Syntax Suffix Registrations
     2.1.  Media Type application/yaml
     2.2.  The +yaml Structured Syntax Suffix
   3.  Interoperability Considerations
     3.1.  YAML Is an Evolving Language
     3.2.  YAML Streams
     3.3.  Filename Extension
     3.4.  YAML and JSON
     3.5.  Fragment Identifiers
   4.  Security Considerations
     4.1.  Arbitrary Code Execution
     4.2.  Resource Exhaustion
     4.3.  YAML Streams
     4.4.  Expressing Booleans
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  Examples Related to Fragment Identifier
     A.1.  Unreferenceable Nodes
     A.2.  Referencing a Missing Node
     A.3.  Representation Graph with Anchors and Cyclic References
   Authors' Addresses

1.  Introduction

   YAML [YAML] is a data serialization format that is capable of
   conveying one or multiple documents in a single presentation stream
   (e.g., a file or a network resource).  It is widely used on the
   Internet, including in the API sector (e.g., see [OAS]), but a
   corresponding media type and structured syntax suffix had not
   previously been registered by IANA.

   To increase interoperability when exchanging YAML streams and
   leverage content negotiation mechanisms when exchanging YAML
   resources, this specification registers the application/yaml media
   type and the +yaml structured syntax suffix [MEDIATYPE].

   Moreover, it provides security considerations and interoperability
   considerations related to [YAML], including its relation with [JSON].

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 terms "content negotiation" and "resource" in this document are
   to be interpreted as in [HTTP].

   The terms "fragment" and "fragment identifier" in this document are
   to be interpreted as in [URI].

   The terms "presentation", "stream", "YAML document", "representation
   graph", "tag", "serialization detail", "node", "alias node",
   "anchor", and "anchor name" in this document are to be interpreted as
   in [YAML].

   Figures containing YAML code always start with the %YAML directive to
   improve readability.

1.2.  Fragment Identification

   A fragment identifies a node in a stream.

   A fragment identifier starting with "*" is to be interpreted as a
   YAML alias node (see Section 1.2.1).

   For single-document YAML streams, a fragment identifier that is empty
   or that starts with "/" is to be interpreted as a JSON Pointer
   [JSON-POINTER] and is evaluated on the YAML representation graph,
   traversing alias nodes; in particular, the empty fragment identifier
   references the root node.  This syntax can only reference the YAML
   nodes that are on a path that is made up of nodes interoperable with
   the JSON data model (see Section 3.4).

   A fragment identifier is not guaranteed to reference an existing
   node.  Therefore, applications SHOULD define how an unresolved alias
   node ought to be handled.

1.2.1.  Fragment Identification via Alias Nodes

   This section describes how to use alias nodes (see Sections
   and 7.1 of [YAML]) as fragment identifiers to designate nodes.

   A YAML alias node can be represented in a URI fragment identifier by
   encoding it into bytes using UTF-8 [UTF-8], but percent-encoding of
   those characters is not allowed by the fragment rule in Section 3.5
   of [URI].

   If multiple nodes match a fragment identifier, the first occurrence
   of such a match is selected.

   Users concerned with interoperability of fragment identifiers:

   *  SHOULD limit alias nodes to a set of characters that do not
      require encoding to be expressed as URI fragment identifiers (this
      is generally possible since anchor names are a serialization
      detail), and

   *  SHOULD NOT use alias nodes that match multiple nodes.

   In the example resource below, the relative reference (see
   Section 4.2 of [URI]) file.yaml#*foo identifies the first alias node
   *foo pointing to the node with value scalar and not to the one in the
   second document, whereas the relative reference file.yaml#*document_2
   identifies the root node of the second document {one: [a, sequence]}.

    %YAML 1.2
    one: &foo scalar
    two: &bar
      - some
      - sequence
      - items
    %YAML 1.2
    one: &foo [a, sequence]

           Figure 1: A YAML Stream Containing Two YAML Documents

2.  Media Type and Structured Syntax Suffix Registrations

   This section includes the information required for IANA to register
   the application/yaml media type and the +yaml structured syntax
   suffix per [MEDIATYPE].

2.1.  Media Type application/yaml

   The media type for YAML is application/yaml; the following
   information serves as the registration form for this media type.

   Type name:  application

   Subtype name:  yaml

   Required parameters:  N/A

   Optional parameters:  N/A; unrecognized parameters should be ignored.

   Encoding considerations:  binary

   Security considerations:  See Section 4 of this document.

   Interoperability considerations:  See Section 3 of this document.

   Published specification:  [YAML], this document

   Applications that use this media type:  Applications that need a
      human-friendly, cross-language, and Unicode-based data
      serialization language designed around the common data types of
      dynamic programming languages.

   Fragment identifier considerations:  See Section 1.2 of this

   Additional information:

      Deprecated alias names for this type:  application/x-yaml, text/
         yaml, and text/x-yaml.  These names are used but are not
      Magic number(s):  N/A
      File extension(s):  "yaml" (preferred) and "yml".  See Section 3.3
         of this document.
      Macintosh file type code(s):  N/A
      Windows Clipboard Name:  YAML

   Person and email address to contact for further information:  See the
      Authors' Addresses section of this document.

   Intended usage:  COMMON

   Restrictions on usage:  None

   Author:  See the Authors' Addresses section of this document.

   Change controller:  IETF

2.2.  The +yaml Structured Syntax Suffix

   The suffix +yaml MAY be used with any media type whose representation
   follows that established for application/yaml.  The structured syntax
   suffix registration form follows.  See [MEDIATYPE] for definitions of
   each part of the registration form.

   Name:  YAML Ain't Markup Language (YAML)

   +suffix:  +yaml

   References:  [YAML], this document

   Encoding considerations:  Same as application/yaml

   Interoperability considerations:  Same as application/yaml

   Fragment identifier considerations:  Unlike application/yaml, there
      is no fragment identification syntax defined for +yaml.

      A specific xxx/yyy+yaml media type needs to define the syntax and
      semantics for fragment identifiers because the ones defined for
      application/yaml do not apply unless explicitly expressed.

   Security considerations:  Same as application/yaml

   Contact: or

   Author:  See the Authors' Addresses section of this document.

   Change controller:  IETF

3.  Interoperability Considerations

3.1.  YAML Is an Evolving Language

   YAML is an evolving language, and over time, some features have been
   added and others removed.

   The application/yaml media type registration is independent of the
   YAML version.  This allows content negotiation of version-independent
   YAML resources.

   Implementers concerned about features related to a specific YAML
   version can specify it in YAML documents using the %YAML directive
   (see Section 6.8.1 of [YAML]).

3.2.  YAML Streams

   A YAML stream can contain zero or more YAML documents.

   When receiving a multi-document stream, an application that only
   expects single-document streams should signal an error instead of
   ignoring the extra documents.

   Current implementations consider different documents in a stream
   independent, similarly to JSON text sequences (see [RFC7464]);
   elements such as anchors are not guaranteed to be referenceable
   across different documents.

3.3.  Filename Extension

   The "yaml" filename extension is the preferred one; it is the most
   popular and widely used on the web.  The "yml" filename extension is
   still used.  The simultaneous usage of two filename extensions in the
   same context might cause interoperability issues (e.g., when both a
   "config.yaml" and a "config.yml" are present).

3.4.  YAML and JSON

   When using flow collection styles (see Section 7.4 of [YAML]), a YAML
   document could look like JSON [JSON]; thus, similar interoperability
   considerations apply.

   When using YAML as a more efficient format to serialize information
   intended to be consumed as JSON, information not reflected in the
   representation graph and classified as presentation or serialization
   details (see Section 3.2 of [YAML]) can be discarded.  This includes
   comments (see Section of [YAML]), directives, and alias nodes
   (see Section 7.1 of [YAML]) that do not have a JSON counterpart.

    %YAML 1.2
    # This comment will be lost
    # when serializing in JSON.
      type: string
      maxLength: &text_limit 64

      type: string
      maxLength: *text_limit  # Replaced by the value 64.

           Figure 2: JSON Replaces Alias Nodes with Static Values

   Implementers need to ensure that relevant information will not be
   lost during processing.  For example, they might consider alias nodes
   being replaced by static values as acceptable.

   In some cases, an implementer may want to define a list of allowed
   YAML features, taking into account that the following features might
   have interoperability issues with [JSON]:

   *  multi-document YAML streams

   *  non-UTF-8 encoding.  Before encoding YAML streams in UTF-16 or
      UTF-32, it is important to note that Section 8.1 of [JSON]
      mandates the use of UTF-8 when exchanging JSON texts between
      systems that are not part of a closed ecosystem and that
      Section 5.2 of [YAML] recommends the use of UTF-8.

   *  mapping keys that are not strings

   *  cyclic references represented using anchors (see Section 4.2 and
      Figure 4)

   *  .inf and .nan float values, since JSON does not support them

   *  non-JSON types, including the ones associated with tags like
      !!timestamp that were included in the default schema of older YAML

   *  tags in general, specifically ones that do not map to JSON types,
      e.g., custom and local tags such as !!python/object and !mytag
      (see Section 2.4 of [YAML])

    %YAML 1.2
      0: a number
      [0, 1]: a sequence
      ? {k: v}
      : a map
      !date 2020-01-01: a timestamp
    non-json-value: !date 2020-01-01

       Figure 3: Example of Mapping Keys and Values Not Supported in
                    JSON in a Multi-Document YAML Stream

3.5.  Fragment Identifiers

   To allow fragment identifiers to traverse alias nodes, the YAML
   representation graph needs to be generated before the fragment
   identifier evaluation.  It is important that this evaluation does not
   cause the issues mentioned in Sections 3.4 and 4, such as infinite
   loops and unexpected code execution.

   Implementers need to consider that the YAML version and supported
   features (e.g., merge keys) can affect the generation of the
   representation graph (see Figure 9).

   In Section 1.2, this document extends the use of specifications based
   on the JSON data model with support for YAML fragment identifiers.
   This is to improve the interoperability of already-consolidated
   practices, such as writing OpenAPI documents [OAS] in YAML.

   Appendix A provides a non-exhaustive list of examples to help readers
   understand interoperability issues related to fragment identifiers.

4.  Security Considerations

   Security requirements for both media types and media type suffixes
   are discussed in Section 4.6 of [MEDIATYPE].

4.1.  Arbitrary Code Execution

   Care should be used when using YAML tags because their resolution
   might trigger unexpected code execution.

   Code execution in deserializers should be disabled by default and
   only be enabled explicitly.  In the latter case, the implementation
   should ensure (for example, via specific functions) that the code
   execution results in strictly bounded time/memory limits.

   Many implementations provide safe deserializers that address these

4.2.  Resource Exhaustion

   YAML documents are rooted, connected, directed graphs and can contain
   reference cycles, so they can't be treated as simple trees (see
   Section 3.2.1 of [YAML]).  An implementation that treats them as
   simple trees risks going into an infinite loop while traversing the
   YAML representation graph.  This can happen:

   *  when trying to serialize it as JSON or

   *  when searching/identifying nodes using specifications based on the
      JSON data model (e.g., [JSON-POINTER]).

    %YAML 1.2
    x: &x
      y: *x

                        Figure 4: A Cyclic Document

   Even if a representation graph is not cyclic, treating it as a simple
   tree could lead to improper behaviors, such as triggering an
   Exponential Data Expansion (e.g., a Billion Laughs Attack).

    %YAML 1.2
    x1: &a1 ["a", "a"]
    x2: &a2 [*a1, *a1]
    x3: &a3 [*a2, *a2]

                    Figure 5: A Billion Laughs Document

   This can be addressed using processors that limit the anchor
   recursion depth and validate the input before processing it; even in
   these cases, it is important to carefully test the implementation you
   are going to use.  The same considerations apply when serializing a
   YAML representation graph in a format that does not support reference
   cycles (see Section 3.4).

4.3.  YAML Streams

   Incremental parsing and processing of a YAML stream can produce
   partial results and later indicate failure to parse the remainder of
   the stream; to prevent partial processing, implementers might prefer
   validating and processing all the documents in a stream at the same

   Repeated parsing and re-encoding of a YAML stream can result in the
   addition or removal of document delimiters (e.g., --- or ...) as well
   as the modification of anchor names and other serialization details
   that can break signature validation.

4.4.  Expressing Booleans

   Section 10.3.2 of [YAML] specifies that only the scalars matching the
   regular expression true|True|TRUE|false|False|FALSE are interpreted
   as booleans.  Older YAML versions were more tolerant (e.g.,
   interpreting NO and N as False and interpreting YES and Y as True).
   When the older syntax is used, a YAML implementation could then
   interpret {insecure: n} as {insecure: "n"} instead of {insecure:
   false}. Using the syntax defined in Section 10.3.2 of [YAML] prevents
   these issues.

5.  IANA Considerations

   IANA has updated the "Media Types" registry
   ( with the registration
   information in Section 2.1 for the media type application/yaml.

   IANA has updated the "Structured Syntax Suffixes" registry
   ( with
   the registration information in Section 2.2 for the structured syntax
   suffix +yaml.

6.  References

6.1.  Normative References

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

   [JSON]     Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

              Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,

              Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

   [OAS]      Miller, D., Whitlock, J., Gardiner, M., Ralphson, M.,
              Ratovsky, R., and U. Sarid, "OpenAPI Specification",
              v3.0.0, 26 July 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <>.

   [YAML]     Ben-Kiki, O., Evans, C., dot Net, I., Müller, T.,
              Antoniou, P., Aro, E., and T. Smith, "YAML Ain't Markup
              Language Version 1.2", 1 October 2021,

6.2.  Informative References

   [RFC7464]  Williams, N., "JavaScript Object Notation (JSON) Text
              Sequences", RFC 7464, DOI 10.17487/RFC7464, February 2015,

Appendix A.  Examples Related to Fragment Identifier Interoperability

A.1.  Unreferenceable Nodes

   This example shows a couple of YAML nodes that cannot be referenced
   based on the JSON data model since their mapping keys are not

    %YAML 1.2
      ? {be: expressed}
      : with a JSON Pointer

    0: no numeric mapping keys in JSON

      Figure 6: Example of YAML Nodes That Are Not Referenceable Based
                             on JSON Data Model

A.2.  Referencing a Missing Node

   In this example, the fragment #/0 does not reference an existing

    %YAML 1.2
    0: "JSON Pointer `#/0` references a string mapping key."

       Figure 7: Example of a JSON Pointer That Does Not Reference an
                               Existing Node

A.3.  Representation Graph with Anchors and Cyclic References

   In this YAML document, the #/foo/bar/baz fragment identifier
   traverses the representation graph and references the string you.
   Moreover, the presence of a cyclic reference implies that there are
   infinite fragment identifiers #/foo/bat/../bat/bar referencing the
   &anchor node.

    %YAML 1.2
    anchor: &anchor
      baz: you
    foo: &foo
      bar: *anchor
      bat: *foo

          Figure 8: Example of a Cyclic Reference and Alias Nodes

   Many YAML implementations will resolve the merge key "<<:"
   ( defined in YAML 1.1 in the
   representation graph.  This means that the fragment #/book/author/
   given_name references the string Federico and that the fragment
   #/book/<< will not reference any existing node.

    %YAML 1.1
    # Many implementations use merge keys.
    the-viceroys: &the-viceroys
      title: The Viceroys
        given_name: Federico
        family_name: De Roberto
      <<: *the-viceroys
      title: The Illusion

                    Figure 9: Example of YAML Merge Keys


   Thanks to Erik Wilde and David Biesack for being the initial
   contributors to this specification and to Darrel Miller and Rich Salz
   for their support during the adoption phase.

   In addition, this document owes a lot to the extensive discussion
   inside and outside the HTTPAPI Working Group.  The following
   contributors helped improve this specification by opening pull
   requests, reporting bugs, asking smart questions, drafting or
   reviewing text, and evaluating open issues: Tina (tinita) Müller, Ben
   Hutton, Carsten Bormann, Manu Sporny, and Jason Desrosiers.

Authors' Addresses

   Roberto Polli
   Digital Transformation Department, Italian Government

   Erik Wilde

   Eemeli Aro