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 (seeSection 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 (seeSection 3.4).¶

A fragment identifier is not guaranteed to reference an existing node.Therefore, applicationsSHOULD define how an unresolved alias nodeought to be handled.¶

 %YAML 1.2 --- one: &foo scalar two: &bar   - some   - sequence   - items ... %YAML 1.2 --- &document_2 one: &foo [a, sequence]

2.1.Media Typeapplication/yaml

The media type for YAML isapplication/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:

SeeSection 4 of this document.¶

Interoperability considerations:

SeeSection 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:

SeeSection 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". SeeSection 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+yamlMAY be used with any media type whose representation followsthat established forapplication/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 asapplication/yaml¶

Interoperability considerations:

Same asapplication/yaml¶

Fragment identifier considerations:

Unlikeapplication/yaml,there is no fragment identification syntax definedfor+yaml.¶

A specificxxx/yyy+yaml media typeneeds to define the syntax and semantics for fragment identifiersbecause the ones defined forapplication/yamldo not apply unless explicitly expressed.¶

Security considerations:

Same asapplication/yaml¶

Contact:

httpapi@ietf.org or art@ietf.org¶

Author:

See the Authors' Addresses section of this document.¶

Change controller:

IETF¶

3.1.YAML Is an Evolving Language

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

Theapplication/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 versioncan 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 streamsshould 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 referenceableacross 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 contextmight 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 formatto serialize information intended to be consumed as JSON,information not reflected in the representation graphand 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.

Implementers need to ensure that relevantinformation 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 todefine a list of allowed YAML features,taking into account that the following features might have interoperabilityissues 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 thatSection 8.1 of [JSON] mandates the use of UTF-8when exchanging JSON texts between systems that are not part of a closed ecosystemand that Section 5.2 of[YAML] recommends the use of UTF-8.¶
• mapping keys that are not strings¶
• cyclic references represented using anchors (seeSection 4.2andFigure 4)¶
• .inf and.nan float values, since JSON does not support them¶
• non-JSON types,including the ones associated with tags like!!timestampthat 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 ...

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 Sections3.4 and4, 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 (seeFigure 9).¶

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

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

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 defaultand 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 graphsand 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 treesrisks 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

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]

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

4.3.YAML Streams

Incremental parsing and processing of a YAML stream can produce partial resultsand 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 resultin 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 theregular expressiontrue|True|TRUE|false|False|FALSE are interpreted as booleans.Older YAML versions were more tolerant (e.g., interpretingNO andN asFalse and interpretingYES andY asTrue).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.¶

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

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."

A.3.Representation Graph with Anchors and Cyclic References

In this YAML document, the#/foo/bar/baz fragment identifiertraverses the representation graph and references the stringyou.Moreover, the presence of a cyclic reference implies thatthere are infinite fragment identifiers#/foo/bat/../bat/barreferencing the&anchor node.¶

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

Many YAML implementations will resolvethe merge key "<<:" defined in YAML 1.1in the representation graph.This means that the fragment#/book/author/given_name references the stringFedericoand 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