Digital Transformation Department, Italian Government

Italy robipolli@gmail.com

Axway

Switzerland erik.wilde@dret.net

Mozilla

Finland eemeli@gmail.com

art httpapi 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

• .  Introduction
  • .  Notational Conventions
  • .  Fragment Identification
    • .  Fragment Identification via Alias Nodes
• .  Media Type and Structured Syntax Suffix Registrations
  • .  Media Type application/yaml
  • .  The +yaml Structured Syntax Suffix
• .  Interoperability Considerations
  • .  YAML Is an Evolving Language
  • .  YAML Streams
  • .  Filename Extension
  • .  YAML and JSON
  • .  Fragment Identifiers
• .  Security Considerations
  • .  Arbitrary Code Execution
  • .  Resource Exhaustion
  • .  YAML Streams
  • .  Expressing Booleans
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• .  Examples Related to Fragment Identifier Interoperability
  • .  Unreferenceable Nodes
  • .  Referencing a Missing Node
  • .  Representation Graph with Anchors and Cyclic References
• Acknowledgements
• Authors' Addresses

Introduction 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 ), 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 . Moreover, it provides security considerations and interoperability considerations related to , including its relation with .

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 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 . The terms "fragment" and "fragment identifier" in this document are to be interpreted as in . 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 . Figures containing YAML code always start with the %YAML directive to improve readability.

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 ). For single-document YAML streams, a fragment identifier that is empty or that starts with "/" is to be interpreted as a 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 ). A fragment identifier is not guaranteed to reference an existing node. Therefore, applications SHOULD define how an unresolved alias node ought to be handled.

Fragment Identification via Alias Nodes This section describes how to use alias nodes (see Sections 3.2.2.2 and 7.1 of ) 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 , but percent-encoding of those characters is not allowed by the fragment rule in . 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 ) 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]}.

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

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 .

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 of this document.

Interoperability considerations:

See of this document.

Published specification:

, 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 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 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

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 for definitions of each part of the registration form.

Name:

YAML Ain't Markup Language (YAML)

+suffix:

+yaml

References:

, 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

Interoperability Considerations

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 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 ); elements such as anchors are not guaranteed to be referenceable across different documents.

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).

YAML and JSON When using flow collection styles (see Section 7.4 of ), a YAML document could look like 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 ) can be discarded. This includes comments (see Section 3.2.3.3 of ), directives, and alias nodes (see Section 7.1 of ) that do not have a JSON counterpart.

JSON Replaces Alias Nodes with Static Values %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 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 :

• multi-document YAML streams
• non-UTF-8 encoding. Before encoding YAML streams in UTF-16 or UTF-32, it is important to note that 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 recommends the use of UTF-8.
• mapping keys that are not strings
• cyclic references represented using anchors (see and )
• .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 )

Example of Mapping Keys and Values Not Supported in JSON in a Multi‑Document YAML Stream %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 ...

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 and , 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 ). In , 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 in YAML. provides a non-exhaustive list of examples to help readers understand interoperability issues related to fragment identifiers.

Security Considerations Security requirements for both media types and media type suffixes are discussed in .

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.

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 ). 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., ).

A Cyclic Document %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).

A Billion Laughs Document %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 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 ).

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.

Expressing Booleans Section 10.3.2 of 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 prevents these issues.

IANA Considerations IANA has updated the "Media Types" registry with the registration information in for the media type application/yaml. IANA has updated the "Structured Syntax Suffixes" registry with the registration information in for the structured syntax suffix +yaml.

References Normative References The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. JSON Pointer defines a string syntax for identifying a specific value within a JavaScript Object Notation (JSON) document. This document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice. v3.0.0 In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. Informative References This document describes the JavaScript Object Notation (JSON) text sequence format and associated media type "application/json-seq". A JSON text sequence consists of any number of JSON texts, all encoded in UTF-8, each prefixed by an ASCII Record Separator (0x1E), and each ending with an ASCII Line Feed character (0x0A).

Examples Related to Fragment Identifier Interoperability

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.

Example of YAML Nodes That Are Not Referenceable Based on JSON Data Model %YAML 1.2 --- a-map-cannot: ? {be: expressed} : with a JSON Pointer 0: no numeric mapping keys in JSON

Referencing a Missing Node In this example, the fragment #/0 does not reference an existing node.

Example of a JSON Pointer That Does Not Reference an Existing Node %YAML 1.2 --- 0: "JSON Pointer `#/0` references a string mapping key."

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.

Example of a Cyclic Reference and Alias Nodes %YAML 1.2 --- anchor: &anchor baz: you foo: &foo bar: *anchor bat: *foo

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.

Example of YAML Merge Keys %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

Acknowledgements Thanks to and for being the initial contributors to this specification and to and 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: , , , , and

Authors' Addresses Digital Transformation Department, Italian Government

Italy robipolli@gmail.com

Axway

Switzerland erik.wilde@dret.net

Mozilla

Finland eemeli@gmail.com