]> 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. In addition, no interoperable guidance is offered to clients, to remediate failures by resource servers due to insufficient authorization details. This document addresses this interoperability challenge, allowing clients to dynamically discover metadata instead of relying on out-of-band agreements, as well as standardizes failure signaling including interoperable remediation when insufficient authorization details are the cause of failure. 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 as the cause of the error.
• A new OAuth 2.0 WWW-Authenticate response parameter, authorization_remediation, providing actionable authorization details objects, to be used directly for remediation in a follow-up OAuth request.
• Authorization server considerations for when RAR authorization details objects should perhaps be omitted from JWT access tokens and provided instead through token instrospection.

Providing clients with actionable authorization details objects enables:

• Interoperability benefit as clients can simply and directly proceed to remediate, without first learning how to construct valid authorization details objects.
• Support for ephemeral, interaction-specific attributes included by the resource server, such as a risk profile or an internal interaction identifier, guiding authorization servers on the 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 Client remediates using actionable authorization details objects provided by resource server: | Resource | (A) User +---| | | Server | Starts| | |<----------------------| | Flow +-->| Client | (C) 401 Unauthorized +--------------------+ | | WWW-Authenticate: Bearer | | error="insufficient_authorization", | | error_description=[human readable message], | | authorization_remediation=[required | | 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 401 with a WWW-Authenticate header with error code insufficient_authorization and in authorization_remediation the required authorization details objects.
• (D) The client uses the provided 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.

Remediation of failures due to insufficient authorization This document defines:

• The authentication error code insufficient_authorization for the WWW-Authenticate header. Resource servers SHOULD return insufficient_authorization when access is denied due to missing or insufficient authorization details.
• The authorization_remediation error parameter, which contains a base64url-encoded JSON object guiding the client on remediating the error. Its attributes are:
  • authorization_details: RECOMMENDED. Array of actionable authorization details objects, matching the format specified in RAR for the authorization_details request parameter, built using the failed resource request. Their inclusion in successful new OAuth grant SHALL satisfy the resource's requirements and remediate the failure.
  • authorization_reference: RECOMMENDED. An opaque string generated by the resource server to enable the client to select an existing access token associated with equivalent authorization details, without requiring the client to understand the semantics of the authorization details object:
    • Resource server SHOULD generate the authorization_reference by canonicalizing and hashing the authorization_details object or an equivalent stable representation, so that the same or semantically equivalent authorization details produce the same authorization_reference value.
    • The value MUST NOT reveal any sensitive or private information.
    • Clients MUST treat this value as opaque and MUST NOT attempt to interpret or derive meaning from it.
    • Returning stable authorization_reference values enables clients to reliably match existing tokens to incoming authorization_remediation responses, to avoid requesting new tokens when a matching token is already in their possession.
    • The resource server SHALL NOT include this attribute when tokens issued for the provided authorization_details are intended for single-use only.

Notes:

• The error_description parameter MAY be included to provide a human-readable description.
• The provided authorization_details are intended to be interoperable with all OAuth specifications and usable in any grant flow supporting RAR.
• Deployments where resource servers have out-of-band agreements with clients to provide other types of payloads for authorization failure remediation, MAY define and use different attributes of authorization_remediation as they see fit.

Example HTTP response from a direct debit resource: The decoded authorization_remediation contents in this example are: Example HTTP response from a payment initiation resource: The decoded authorization_remediation contents in this example are:

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.

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.

RAR objects in JWT access tokens Pursuant with RAR section 9, 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. There may however be cases, where due to various considerations such as token size or information privacy, including approved RAR objects in JWT access tokens would be advised against. It is RECOMMENDED that when an authorization server issues JWT access tokens, it should consider the size, sensitivity, and privacy implications of including the authorization_details attribute. Where appropriate, the authorization server SHOULD omit this attribute from JWT tokens and instead provide the approved RAR objects to resource servers via the token introspection endpoint. This endpoint SHOULD use appropriate client authentication methods to prevent unauthorized access, in case of token leakage.

Processing Rules

Client Processing Rules General: Client MAY attempt calling resource server, either on first attempt or as a remediation step, using any valid tokens which were obtained following a remediation challenge from same resource server origin, which included an authorization_reference, as such tokens are not limited for single-use. Existing tokens whose authority is inclusive may permit resource calls requiring lower authority, despite their authorization_reference value differs from value obtained in other remediation challenges. For example, a recurring direct debit token permitting up to 100$ can authorize a 80$ debit, although the remediation challenges returned when attempting 80$ or 100$ debits with insufficient authority, will differ in their authorization_details and authorization_reference values. Therefore attempting a 80$ debit with an existing token permitting 100$ debits may succeed. Handling an HTTP 401 failure response: When a client receives an HTTP 401 response with WWW-Authenticate error code insufficient_authorization and an authorization_remediation parameter, it SHOULD process it as follows.

Step 1 - Parse the remediation response The client decodes the base64url-encoded authorization_remediation JSON object and extracts:

• authorization_details (RECOMMENDED): the actionable RAR objects.
• authorization_reference (OPTIONAL): an opaque string for token-bag lookup.

Step 2 - Attempt token reuse via authorization_reference (if present)

1. If the authorization_remediation contains an authorization_reference attribute, the client SHOULD search its in-session tokens for a token previously associated with that reference value and the same resource server origin.
2. Matching is a simple string comparison — the client MUST NOT attempt to compute, parse, or derive meaning from the reference value.
3. If a matching, non-expired token is found, the client MAY retry the failing request with that token. If the retry also fails with insufficient_authorization, the client MUST NOT retry again with the same token for the same reference and SHOULD proceed to Step 3.
4. If no matching token is found, the client proceeds to Step 3.

Step 3 - Obtain a new token via OAuth + RAR The client proceeds to this step if: (a) no authorization_reference was present, (b) no matching token in client's possession was found, (c) a matched token was rejected by the resource server (Step 2, item 4), or (d) the client elects to skip an existing token lookup and use authorization_details directly.

1. The client uses the authorization_details from the authorization_remediation response in a new OAuth authorization request per . The client MAY use any grant type or extension that supports RAR (such as PAR , JAR , etc).
2. Upon successful token issuance, if the triggering resource server response included an authorization_reference, the client SHOULD persist the newly obtained token associated with that reference value and the resource server origin in its in-session token storage. This token-to-reference association enables future lookups in Step 2 when the same authorization_reference is encountered again.
3. The client retries the failing request with the newly obtained token.

Step 4 - Handle continued failure If after obtaining a new token and retrying, the resource server still returns insufficient_authorization:

• If the new response contains a different authorization_reference, the client MAY attempt remediation again (subject to implementation-defined retry limits).
• If the new response contains the same authorization_reference, the client MUST NOT loop — it SHOULD treat the failure as non-remediable and report an error to the user or calling application.

Additional guidance

• Clients MAY ignore authorization_reference entirely if they do not implement token caching or reuse. In that case, each insufficient_authorization response triggers a fresh authorization request using the provided authorization_details.
• If the client's current authorization server does not support the required authorization details types (as indicated by its metadata), the client MAY use Protected Resource Metadata to discover alternative authorization servers for the resource.
• The token storage MUST be scoped per end-user session. Concurrent users operating through the same client instance MUST maintain separate token storage instances.

Resource Server Processing Rules When a resource server receives a request with an OAuth token:

Step 1 - Validate the access token Verify token validity following or if JWT profiled. If the token is invalid for reasons other than insufficient authorization details, return the appropriate existing error code (e.g., invalid_token).

Step 2 - Verify authorization details (if present) Determine whether the token carries sufficient authorization details for the requested operation. Authorization details MAY be obtained from the JWT access token payload or via token introspection .

Step 3 - If authorization details are missing or insufficient The resource server responds with an error per the bearer token error framework Section 3. The specific error code depends on the nature of the failure: - If the token is valid but lacks sufficient scope, the resource server returns insufficient_scope per Section 3.1. - If the token is valid but lacks sufficient authentication context (e.g., ACR/AMR level), the resource server returns insufficient_user_authentication per . - If the token is valid but lacks sufficient authorization details, the RS returns insufficient_authorization per Section 4 of this document, with an authorization_remediation parameter as defined in Section 4.1. The authorization_remediation parameter carries the actionable authorization_details and optional authorization_reference as specified in Section 4. The resource server constructs these per the rules defined below.

Limitations and Considerations for authorization_reference Implementers should be aware of the following limitations:

Token reuse is opportunistic, not guaranteed A matching authorization_reference with client's existing tokens does NOT guarantee the token will be accepted by the resource server. The token may have been issued under conditions that no longer apply:

• The resource owner may have revoked consent since the token was issued.
• Contextual risk may have changed (e.g., geolocation, device posture), causing the resource server to require stronger authorization ceremonies.
• The authorization server may have issued the token with a subset of the requested authorization details (per Section 7).

Clients MUST handle the case where a reused token is rejected despite matching the authorization_reference (see Section 7.1, Step 2, item 4).

authorization_reference does not replace authorization_details The authorization_reference is an optimization for token selection. It is NOT a substitute for authorization_details:

• authorization_details is RECOMMENDED in every authorization_remediation response providing actionable RAR objects and is what the client uses when initiating a new authorization request.
• authorization_reference is RECOMMENDED and enables the client to avoid unnecessary authorization flows when it already possesses a suitable token.

Clients that do not implement token caching MAY safely ignore authorization_reference with no loss of interoperability.

Loop prevention If the resource server consistently returns the same authorization_reference and rejects tokens obtained via the associated authorization_details, the client may enter an infinite loop. To prevent this:

• Clients MUST implement a maximum retry count (RECOMMENDED: 1 retry with a cached token, then 1 fresh authorization attempt, then fail).
• If a freshly obtained token (from a new authorization flow using the resource server provided authorization_details) is immediately rejected by the same resource server with the same authorization_reference, the client MUST stop and report the error.

No cross-resource-server portability The authorization_reference value is scoped to the producing resource server. It MUST NOT be used for token selection when interacting with a different resource server, even if the two servers enforce similar authorization details types.

Analogy to scope-based token selection The authorization_reference mechanism is analogous to how clients select tokens based on OAuth scopes in traditional deployments. Just as a client maintains a mapping of {scope → token} and selects the appropriate token for each resource server call, this mechanism extends that pattern to RAR:

Traditional (scope-based)	RAR + authorization_reference
Resource server returns insufficient_scope with required scope value	Resource server returns insufficient_authorization with authorization_remediation
Client checks if it has a token with matching scope	Client checks if it has a token with matching authorization_reference
Simple string comparison on scope values	Simple string comparison on reference values
If not found, request new token with required scope	If not found, request new token with provided authorization_details

The key advantage: clients do not need to understand, parse, or compare complex JSON authorization_details objects — the resource server has already reduced the comparison to an opaque string.

Security Considerations

Confidentiality of resource server provided authorization_details Resource servers when 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	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

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)

Document History -06

• Expanded processing rules to be more explicit, using Jeff Lombardo's feedback.
• Explained how potential reuse of existing tokens is possible despite authorization_reference mismatches.

-05

• Removed required authorization details types.
• Changed from HTTP 403 to 401.
• Moved resource servers response from body to WWW-Authenticate header.
• Renamed authorization_hint to authorization_reference and clarified its usage.
• Clarified authorization server broader considerations on omitting RAR from JWT access tokens.
• Clarified document's interoperability with any OAuth rfc and any grant that supports RAR.

-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_reference 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, Justin Richer, Jeff Lombardo, Judith Kahrer, Pieter Kasselman.