RFC 9512 | YAML Media Type | February 2024
Polli, et al. | Informational | [Page]

Stream:
  Internet Engineering Task Force (IETF)
RFC:
  9512
Category:
  Informational
Published:
  February 2024
ISSN:
  2070-1721
Authors:
  R. Polli
  DTD, Italian Government
  E. Wilde
  Axway
  E. Aro
  Mozilla

# RFC 9512

# YAML Media Type

## Abstract

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 https://www.rfc-editor.org/info/rfc9512.¶

## 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 (https://trustee.ietf.org/license-info) 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.¶

## 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", "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 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 3.2.2.2 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
     ---
     &document_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 document.¶

Additional information:

  Deprecated alias names for this type:
    application/x-yaml, text/yaml, and text/x-yaml. These names are used but are not registered.¶
  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:

  httpapi@ietf.org or art@ietf.org¶

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 3.2.3.3 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.
     Title:
       type: string
       maxLength: &text_limit 64

     Name:
       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 versions¶
- 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
     ---
     non-json-keys:
       0: a number
       [0, 1]: a sequence
       ? {k: v}
       : a map
     ---
     non-json-keys:
       !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 issues.¶

### 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 time.¶

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, <https://www.rfc-editor.org/info/rfc9110>.
[JSON]
  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, <https://www.rfc-editor.org/info/rfc8259>.
[JSON-POINTER]
  Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed., "JavaScript Object Notation (JSON) Pointer", RFC 6901, DOI 10.17487/RFC6901, April 2013, <https://www.rfc-editor.org/info/rfc6901>.
[MEDIATYPE]
  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>.
[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, <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174]
  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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, <https://www.rfc-editor.org/info/rfc3986>.
[UTF-8]
  Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <https://www.rfc-editor.org/info/rfc3629>.
[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, <https://yaml.org/spec/1.2.2/>.

### 6.2. Informative References

[RFC7464]
  Williams, N., "JavaScript Object Notation (JSON) Text Sequences", RFC 7464, DOI 10.17487/RFC7464, February 2015, <https://www.rfc-editor.org/info/rfc7464>.

## 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 strings.¶

     %YAML 1.2
     ---
     a-map-cannot:
       ? {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 node.¶

     %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
       author:
         given_name: Federico
         family_name: De Roberto
     book:
       <<: *the-viceroys
       title: The Illusion

Figure 9: Example of YAML Merge Keys

## Acknowledgements

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