]> Raiffeisen Bank International

yaron.zehavi@rbinternational.com

Security Web Authorization Protocol RAR Step-up oauth OAuth 2.0 Rich Authorization Requests (RAR) standardizes the exchange and processing of authorization details but does not define metadata for describing authorization details types. This document addresses a practical interoperability challenge regarding authorization details type metadata, allowing clients to dynamically discover metadata instead of relying on out-of-band agreements. It also standardizes error signaling for cases where insufficient authorization details are provided, and offers structured remediation mechanisms. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Web Authorization Protocol Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction OAuth 2.0 Rich Authorization Requests (RAR) allows OAuth clients to request detailed and structured authorization, enabling advanced authorization models across domains such as banking and healthcare. However, RAR does not specify how clients discover metadata describing valid authorization details objects. Such metadata and documentation are obtained out-of-band. This document defines:

• A new authorization server endpoint: authorization_details_types_metadata_endpoint, providing authorization details type metadata, including documentation and JSON Schema definitions .
• A new normative OAuth 2.0 WWW-Authenticate Error Code, for resource servers to indicate insufficient_authorization_details as the cause of the error, as well as a RECOMMENDED response body specifying required authorization details types to satisfy the resource's requirements.
• An OPTIONAL response body attribute that MAY accompany the insufficient_authorization_details error, providing an informative and actionable authorization details object. This object can be used directly in a follow-up OAuth request.
• RECOMMENDED handling of large RAR authorization details objects when issuing JWT access tokens, to avoid failures due to token sizes exceeding header size restrictions.

Providing actionable authorization details objects in the resource server's error response enables:

• Simplification for clients who can directly remediate without learning to construct valid authorization details objects.
• Support for ephemeral, interaction-specific attributes from the resource server, such as a risk profile or an internal interaction identifier, guiding authorization servers on required authentication strength and consent flows.

Conventions and Definitions 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.

Protocol Overview There are two main proposed flows:

• Client remediates using metadata of required authorization details types.
• Client remediates using actionable authorization details objects provided by resource server.

Client remediates using metadata of required authorization details types | Resource | (A) User +---| | | Server | Starts| | |<----------------------| | Flow +-->| | (C) 403 Forbidden +---------------------+ | | WWW-Authenticate: Bearer | | error="insufficient_authorization_details" | | + | | HTTP body specifies | | required authorization_details | | : | | +---------------------+ | | RAR Types | Authorization | | | (D) Metadata Request | Server | | |---------------------->|+-------------------+| | | || RAR Types || | |<----------------------|| Metadata Endpoint || | | (E) Metadata Response |+-------------------+| | | : | | | | (F) Construct RAR | | | | Using Metadata | | | | : | | | | (G) Authorization | | | | Request + RAR | | | |---------------------->|+-------------------+| | | || Authorization || | |<----------------------|| Endpoint || | | (H) Authorization Code|| || | | : |+-------------------+| | | : | | | | (I) Token Request |+-------------------+| | |---------------------->|| || | | || Token Endpoint || | |<----------------------|| || | | (J) Access Token |+-------------------+| | | : +---------------------+ | | : | | (K) API Call with | | Access Token +---------------------+ | |---------------------->| | | | | Resource Server | | |<----------------------| | | | (L) 200 OK + Resource +---------------------+ | | +----------+ ]]> Figure: Client remediates using metadata of required authorization details types

• (A) The user starts the flow.
• (B) The client calls an API with an access token.
• (C) Resource server returns HTTP 403 Forbidden, including a WWW-Authenticate header with error code insufficient_authorization_details and in the response body specifies required authorization details types.
• (D-E) The client consumes authorization details types metadata from authorization server's authorization_details_types_metadata_endpoint.
• (F-G) The client constructs valid authorization details objects and makes an authorization request using RAR .
• (H) Authorization server returns authorization code.
• (I-J) The client exchanges authorization code for access token.
• (K) The client makes an API request with the (RAR) access token.
• (L) Resource server validates access token and returns successful response.

Client remediates using actionable authorization details objects provided by resource server | Resource | (A) User +---| | | Server | Starts| | |<----------------------| | Flow +-->| Client | (C) 403 Forbidden +--------------------+ | | WWW-Authenticate: Bearer | | error="insufficient_authorization_details" | | + | | HTTP body provides authorization_details | | : | | : +--------------------+ | | : | Authorization | | | (D) Authorization | Server | | | Request + RAR |+------------------+| | |---------------------->|| || | | || Authorization || | |<----------------------|| Endpoint || | | (E) Authorization Code|| || | | : |+------------------+| | | : | | | | (F) Token Request |+------------------+| | |---------------------->|| || | | || Token Endpoint || | |<----------------------|| || | | (G) Access Token |+------------------+| | | : +--------------------+ | | : | | : | | (H) Retry API Call +--------------------+ | | with Token | | | |---------------------->| Resource | | | | Server | | |<----------------------| | | | (I) 200 OK + Resource +--------------------+ | | +----------+ ]]> Figure: Client remediates using actionable authorization details objects provided by resource server

• (A) The user starts the flow.
• (B) The client calls an API with an access token.
• (C) Resource server returns HTTP 403 Forbidden, including a WWW-Authenticate header with error code insufficient_authorization_details and in the response body includes the required authorization details objects.
• (D) The client uses the obtained authorization details objects in a new OAuth + RAR request.
• (E) Authorization server returns authorization code.
• (F-G) The client exchanges authorization code for access token.
• (H) The client makes an API request with the (RAR) access token.
• (I) Resource server validates access token and returns successful response.

Authorization Details Types Metadata Endpoint The following authorization server metadata parameter is introduced to indicate the server's support for Authorization Details Types Metadata:

"authorization_details_types_metadata_endpoint":

OPTIONAL. The URL of the Authorization Details Types Metadata endpoint.

Authorization Details Types Metadata Endpoint Response The Authorization Details Types Metadata endpoint is called with HTTP GET and responds with Content-Type application/json and a JSON object whose members are authorization details type identifiers. Each member value is an object describing a single authorization details type. Attribute definition:

"version":

OPTIONAL. String identifying the version of the authorization details type definition. The value is informational and does not imply semantic version negotiation.

"description":

OPTIONAL. String containing a description of the authorization details type. Clients MUST NOT rely on this value for authorization or validation decisions.

"documentation_uri":

OPTIONAL. URI referencing external documentation describing the authorization details type.

"schema":

The schema attribute contains a JSON Schema document that describes a single authorization details object. The schema MUST validate exactly one authorization details object and MUST restrict the type attribute to the corresponding authorization details type identifier. This attribute is REQUIRED unless schema_uri is specified. If present, schema_uri MUST NOT be included.

"schema_uri":

The schema_uri attribute is an absolute URI, as defined by RFC 3986 , referencing a JSON Schema document describing a single authorization details object. The referenced schema MUST satisfy the same requirements as the schema attribute. This attribute is REQUIRED unless schema is specified. If this attribute is present, schema MUST NOT be present.

"examples":

OPTIONAL. An array of example authorization details objects. Examples are non-normative.

See Examples for non-normative response example.

Resource Server Error Signaling of insufficient authorization_details This document defines the Bearer authentication error code insufficient_authorization_details for use in the WWW-Authenticate header field as defined by . Resource servers SHOULD return insufficient_authorization_details to signal that access is denied due to missing or insufficient authorization details. The insufficient_authorization_details error is intended in cases where the access token is otherwise valid, but the authorization details associated with the token do not satisfy the resource's requirements. The insufficient_authorization_details error MUST NOT be used when the access token is missing, expired, malformed, or otherwise invalid. Existing Bearer authentication error codes are used for those cases. Example HTTP response:

RECOMMENDED authorization_details_types_required in response body It is RECOMMENDED that resource servers return, alongside the insufficient_authorization_details error, an HTTP body of Content-Type application/json containing the new attribute: authorization_details_types_required. This attribute specifies the required authorization details types satisfying resource's RAR requirements.

"authorization_details_types_required":

RECOMMENDED. A JSON object that conforms to the syntax described in for a required types expression.

Note: It is RECOMMENDED that authorization_details_types_required specifies ALL acceptable authorization details types combinations, from ALL supported authorization servers. Example resource server response with RECOMMENDED authorization_details_types_required:

Required types expression syntax The following JSON syntax defines a required types expression that describes permitted combinations of required authorization_details types. This expression allows selection operators (oneOf, allOf) and boolean composition (and, or) to be combined in a predictable manner. A required types expression is a JSON object that MUST contain exactly one of the following attributes:

• and
• or
• oneOf
• allOf

Attribute definition:

"and":

OPTIONAL. A non-empty JSON array of required types expressions. When and is specified, the expression is satisfied if all contained expressions are satisfied.

"or":

OPTIONAL. A non-empty JSON array of required types expressions. When or is specified, the expression is satisfied if at least one contained expression is satisfied.

"oneOf":

OPTIONAL. A non-empty JSON array of strings identifying authorization_details types. When oneOf is specified, the expression is satisfied if exactly one of the listed types is present.

"allOf":

OPTIONAL. A non-empty JSON array of strings identifying authorization_details types. When allOf is specified, the expression is satisfied if all of the listed types are present.

Required types expression examples

Example expression using "and" operator Specifies that the selection MUST include a and b, and one of c or d.

Example expression using "or" operator Specifies that the selection MUST include either c and d, or one of a or b.

OPTIONAL actionable authorization_details in response body The resource server MAY include, in addition to or instead of the authorization_details_types_required attribute, actionable authorization details objects capable of satisfying the resource's requirements. Note:

• Resource servers SHOULD provide authorization_details objects intended to be submitted to the authorization server that issued the access token used for the current (failing) request. This simplifies client and end-user interaction, by continuing with the same authorization server previously used.

HTTP response body definition:

"authorization_details":

OPTIONAL. Array of authorization details objects, matching the format specified in RAR for the authorization_details request parameter.

"authorization_hint":

RECOMMENDED. An opaque string generated by the resource server that enables the client to select an existing access token associated with equivalent authorization details without understanding the semantics of the authorization details object. Clients MUST treat this value as opaque. The value SHOULD be stable only within the context of the same resource server, client, subject, and authorization server. The value MUST NOT reveal sensitive information. The resource server SHALL NOT return this attribute when tokens resulting from the provided authorization_details are single-use only.

Clients MAY use the provided authorization_details in a subsequent OAuth request to obtain an access token satisfying the resource's requirements. Example resource server response with OPTIONAL authorization_details:

Handling large RAR objects when issuing access tokens RAR section 9 instructs that authorization servers MUST provide approved RAR objects to resource servers for enforcement. The authorization server MAY add the authorization_details attribute to access tokens in JSON Web Token (JWT) format or to token introspection responses. Including large RAR objects in JWT access tokens may cause interoperability loss due to token sizes exceeding header size restrictions. Authorization servers SHOULD support a configurable maximum approved RAR objects size threshold. The size calculation SHOULD be based on the UTF-8 encoded JSON serialization of the approved authorization_details value before JWT signing or encryption. If the size exceeds this threshold, JWT access tokens SHOULD NOT include the authorization_details attribute; instead, the authorization server MUST ensure that the resource server can obtain the approved authorization details by another means, such as token introspection .

Processing Rules

Client Processing Rules

• When receiving an insufficient_authorization_details error, if the response body contains an authorization_hint attribute that matches a valid token in the client's possession, the client SHOULD retry the failing request using the matching token.
• If the response body contains an authorization_details attribute, the client MAY include it in a subsequent OAuth request to obtain a token, which it MAY then use to retry the failing endpoint.
• Otherwise, the client SHOULD use the authorization_details_types_required attribute, if provided in the response body:
  • Use the response body to understand which authorization details types satisfy the resource's requirements.
  • If the authorization server metadata contains authorization_details_types_supported, the client MAY use it to determine whether the authorization server supports one or more types referenced by authorization_details_types_required.
  • Fetch authorization server's authorization_details_types_metadata_endpoint to obtain authorization details types metadata and schemas.
  • Use the embedded schema value or retrieve the JSON Schema document from schema_uri.
• Construct authorization details conforming to the schema and include them in a subsequent OAuth request to obtain an access token with which the client MAY retry calling the failing endpoint.

Resource Server Processing Rules

• Verify access tokens against required authorization details.
• When a resource server receives a JWT access token that does not contain an authorization_details claim, and authorization details are required for the requested resource, the resource server SHOULD attempt to obtain the authorization details using token introspection , if introspection is available.
• If authorization details are missing or insufficient, the resource server SHALL return HTTP 403 with WWW-Authenticate: Bearer error="insufficient_authorization_details".
• It is RECOMMENDED to also provide an HTTP body with the authorization_details_types_required attribute, specifying the required authorization details types to satisfy the resource's requirement. This attribute applies to ALL supported authorization servers.
• Resource server MAY also provide in the HTTP body, OPTIONAL actionable authorization_details objects, intended to be submitted to the authorization server that issued the current access token, which MAY be used directly for remediation in a subsequent OAuth request.

Security Considerations

Cacheability and Intermediaries HTTP 403 responses with response bodies may be cached or replayed in unexpected contexts. Resource servers SHOULD mitigate by including the Cache-Control: no-store response header.

Confidentiality of resource server provided authorization_details Resource server providing actionable authorization_details SHOULD NOT include sensitive data in those objects. This is consistent with RAR authorization_details OAuth request parameter, representing request semantics. Confidentiality-preserving authorization_details types SHOULD NOT include sensitive data. Instead, the end-user SHOULD provide such information when interacting with the authorization server. Alternatively, authorization_details MAY refer to specific end-user resources using opaque reference handles (e.g., "account_1a" instead of using explicit IBAN).

IANA Considerations

OAuth 2.0 WWW-Authenticate Error Code Registry

Error Code	Error Usage Location	Change Controller	Specification Document
insufficient_authorization_details	Resource access error response	IETF	RFC XXXX, Section X

OAuth Authorization Server Metadata Registry This specification registers the following authorization server metadata parameter in the OAuth Authorization Server Metadata registry:

Metadata Name	Metadata Description	Change Controller	Specification Document
authorization_details_types_metadata_endpoint	URL of the Authorization Details Types Metadata endpoint	IETF	RFC XXXX, Section X

Normative References 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] This specification describes how to use bearer tokens in HTTP requests to access OAuth 2.0 protected resources. Any party in possession of a bearer token (a "bearer") can use it to get access to the associated resources (without demonstrating possession of a cryptographic key). To prevent misuse, bearer tokens need to be protected from disclosure in storage and in transport. [STANDARDS-TRACK] This specification defines a method for a protected resource to query an OAuth 2.0 authorization server to determine the active state of an OAuth 2.0 token and to determine meta-information about this token. OAuth 2.0 deployments can use this method to convey information about the authorization context of the token from the authorization server to the protected resource. This specification defines a metadata format that an OAuth 2.0 client can use to obtain the information needed to interact with an OAuth 2.0 authorization server, including its endpoint locations and authorization server capabilities. This document specifies a new parameter authorization_details that is used to carry fine-grained authorization data in OAuth messages. This specification defines a metadata format that an OAuth 2.0 client or authorization server can use to obtain the information needed to interact with an OAuth 2.0 protected resource. IANA 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.

Examples This section provides non-normative examples of how this specification may be used to support specific use cases.

Authorization Server Metadata Examples

Example authorization_details_types_metadata_endpoint response with Payment Initiation

Example authorization_details_types_metadata_endpoint response for the Norwegian Health Sector (HelseID)

Resource Server authorization_details_types_required Response Examples

Example Resource server authorization_details_types_required response from payments resource

Example Resource server authorization_details_types_required response from the Norwegian Health Sector (HelseID)

Payment initiation with RAR error signaling

Client initiates API request Client uses access token obtained at login to call payment initiation API

Resource server signals insufficient_authorization_details with actionable RAR object Resource server requires payment approval and responds with: Note: the resource server has added the ephemeral attributes interaction_id and risk_profile.

Client initiates OAuth flow using the provided authorization_details object After user approves the request, client obtains an access token representing the approved payment

Client re-attempts API request

Resource server authorizes the request

Document History -04

• Moved required authorization details types from resource metadata to resource server's response.
• Adapted resource server processing rules to reflect error signaling and handling of large RAR payloads.

-03

• Added authorization_hint to guide client on token selection and updated client processing rules accordingly
• Added security consideration on confidentiality of RS-provided authorization_details
• Added authorization server considerations for handling large RAR objects in JWT access tokens

-02

• Defined the required types expression
• Added Protected Resource Metadata examples

-01

• Authorization details moved to HTTP body and made OPTIONAL
• Metadata pointer from resource metadata url, full authorization details types metadata on authorization server new endpoint

-00

• Document creation

Acknowledgments The authors would like to thank the following individuals who contributed ideas, feedback, and wording that helped shape the final specification: Rune Grimstad.