]> Raiffeisen Bank International

yaron.zehavi@rbinternational.com

Security Web Authorization Protocol oauth RAR authorization request delegation brokering CIMD Brokered OAuth redirect authorization requests involve intermediary authorization servers between a downstream client and the upstream authorization server that obtains user consent and issues tokens. Such deployments have security risks because the upstream authorization server sees only the immediate OAuth client and is unaware of the downstream client or intermediary brokers obtaining its response. This document defines an OAuth 2.0 profile for carrying a verifiable, signed authorization request delegation chain as a RAR authorization_details object . Each node in the chain is a JSON object signed by an attesting authorization server using detached JWS , attesting its validated client, hash-linked to the previous node, allowing the upstream authorization server to validate the integrity of the visible delegation path and apply policy before issuing tokens. 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 redirect authorization requests increasingly pass through intermediary authorization servers before reaching the authorization server that obtains user consent and issues tokens. In a brokered redirect authorization flow, a downstream client's authorization request is forwarded through one or more brokers. Each broker is both an authorization server for its downstream party, and an OAuth client of the next authorization server in the path. The terminal upstream authorization server that ultimately processes the authorization request may only have a direct relationship with the immediate broker. Without additional information, it cannot determine which downstream client initiated the request and which broker path carried the request. Such brokered consent flows are discussed as a risk in OAuth Security Topics update . This document addresses this risk for redirect authorization request delegation. It defines a RAR authorization_details object that carries a signed delegation chain in the authorization request. The chain allows each authorization server in the redirect path to attest the client it directly recognizes and to preserve the prior delegation evidence. The resulting chain allows any upstream authorization server to evaluate its integrity and perform authorization, consent, and policy decisions based on:

• The integrity of the delegation chain.
• The immediate broker client,
• The downstream client that initiated the request,
• The ordered request delegation path,
• The authorization servers or brokers that attested each hop,
• The protected resource requested, and

This document defines a RECOMMENDED authorization_details type for representing a verifiable signed delegation chain. Each chain node states:

• Who is attesting the node,
• Who the node is intended for,
• Which client is being attested for this hop,
• Access to which resource is requested,
• Where the node appears in the chain, and
• A cryptographic proof over the node.

Each node is signed by the attesting entity using detached JWS and hash-linked to the previous node. The result is a JSON-structured, tamper-resistant, verifiable signed delegation chain for redirect authorization request processing. This profile is intentionally narrow. It does not define a new grant type, token format, endpoint, token response parameter, or error code. It defines only a proposed RAR authorization_details type and processing rules for redirect authorization requests.

Relation to OpenID Federation OpenID Federation defines mechanisms for establishing trust between entities using signed entity statements, trust chains, metadata, metadata policy, and federation authorities. The authorization request delegation chain defined by this document is similar to OpenID Federation in that both mechanisms can involve signed statements about entities and can support trust decisions across organizational or administrative boundaries. However, the two mechanisms address different layers of the problem. OpenID Federation primarily addresses entity trust and metadata establishment. It can answer questions such as:

• Which entity controls this identifier?
• Which metadata applies to this entity?
• Which trust anchor or federation authority vouches for this entity?
• Which keys should be used to verify statements from this entity?

This document addresses the authrization request's specific delegation path. It can answer questions such as:

• Which downstream client initiated this redirect authorization request?
• Which brokers carried it?
• Which entity attested each hop?
• Was the visible authorization request delegation chain reordered, shortened at the tail, altered in the middle, or modified?
• Is the visible first node acceptable under local policy?

Deployments MAY use OpenID Federation to establish trust in the entities that appear in a delegation chain. For example, iss values in this profile can correspond to federated entity identifiers, and federation metadata can be used to discover keys or validate metadata policy. This document does not replace OpenID Federation. Instead, it can consume or complement federation trust metadata while providing a per-request signed delegation chain suitable for OAuth authorization request processing.

Relation to OAuth Client ID Metadata Document The OAuth Client ID Metadata Document draft (aka: CIMD) defines a mechanism by which an OAuth client can use a URL as its client_id, where the URL references a client metadata document that can be fetched by an authorization server . This document is complementary to that mechanism and can reference CIMD-style client_id values when used. A delegation node can use a CIMD-style client_id by setting client_ns to cimd and client_id to the metadata document URL. For example: In such deployments, CIMD can provide retrievable client metadata, while this profile provides a signed per-request authorization request delegation chain showing how that client was carried through brokers.

Relation to RFC 8693 Token Exchange and the act Claim OAuth 2.0 Token Exchange defines a token exchange grant and includes the act claim for representing an actor in issued tokens. The act claim can indicate that one party is acting on behalf of another party. Nested act claims can represent prior actors. This document is related to the act claim because both mechanisms represent delegation. However, they apply at different phases of an OAuth deployment. The act claim is a token-time representation. It appears in issued tokens or token introspection responses and is consumed after token issuance, typically by resource servers or downstream authorization servers. This document defines an authorization-request-time representation. The delegation chain is carried in a RAR authorization_details object during the redirect authorization request, before the upstream authorization server has issued tokens. The distinction is important for brokered redirect authorization flows. The upstream authorization server needs to know the downstream client and broker path before it can make a correct consent or authorization decision. A token claim such as act can describe delegation after issuance, but it does not by itself provide a redirect authorization request mechanism for presenting signed per-hop delegation evidence to the authorization endpoint before consent and token issuance. This profile also differs from nested act claims in that:

• Each delegation node is signed by the authorization server or broker that attests that hop,
• Each node is hash-linked to the previous node,
• The chain is carried as JSON in RAR authorization_details,
• The chain is intended for authorization endpoint processing, and
• The upstream authorization server can bind consent to the terminal client and broker path before issuing tokens.

An authorization server MAY translate a validated delegation chain into issued-token claims, including act claims, after authorization succeeds. Such token representation is outside the scope of this document.

Relation to the OAuth Actor Profile for Delegation The OAuth Actor Profile for Delegation draft defines a common profile for representing delegated actor relationships using the act claim across JWT assertion grants, JWT access tokens, Transaction Tokens, and Token Exchange inputs. It also defines actor classification through sub_profile and discovery metadata for advertising support . This document is complementary to the OAuth Actor Profile but has a different scope. The OAuth Actor Profile addresses token and assertion interoperability. It helps systems consistently express actor relationships in issued artifacts such as:

• JWT assertion grants,
• JWT access tokens,
• Transaction Tokens, and
• Token Exchange inputs and outputs.

This document addresses redirect authorization request delegation. It helps an upstream authorization server evaluate a brokered authorization request before token issuance by carrying a signed delegation chain in RAR authorization_details. The two mechanisms can be used together. An authorization server can validate an oauth_request_delegation_chain authorization detail during the redirect authorization request and, after successful authorization, issue a token using the act claim profile defined by the OAuth Actor Profile. In that combined model:

• This document provides pre-token authorization request evidence, and
• The OAuth Actor Profile provides post-authorization token representation.

Relation to OAuth Actor-Signed Hop Proofs OAuth Actor-Signed Hop Proofs defines an optional companion profile for delegated OAuth tokens that conform to the OAuth Actor Profile for Delegation. It introduces an actor_proofs claim containing a signed per-hop proof chain, where each visible actor signs its own participation and target binding . This document is similar in that it also uses signed per-hop evidence and hash linking. However, the placement and processing model are different. OAuth Actor-Signed Hop Proofs is token-oriented. It defines claims and mechanisms for delegated tokens and associated token processing. This document is authorization-request-oriented. It carries the signed chain in RAR authorization_details so that the upstream authorization server can evaluate the delegation path during redirect authorization request processing, before consent and token issuance. A deployment could use both mechanisms:

• oauth_request_delegation_chain in the redirect authorization request to support upstream authorization server consent and policy decisions.
• act and actor_proofs in issued tokens to support downstream resource server enforcement and audit.

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.

Authorization Request Delegation Chain:

An ordered JSON array of signed delegation nodes carried in an OAuth authorization request.

Delegation Node:

A JSON object representing one attestation event in the authorization request delegation chain.

Attester:

The entity identified by the node's iss value. The attester signs the node.

Attested Client:

The client identified by the node's client_ns and client_id values.

Terminal Client:

The original or downstream client ultimately represented through the chain.

Broker:

An entity that acts as an authorization server in one relationship and as an OAuth client of another authorization server or broker in another relationship.

Client Namespace:

The client identifier namespace or resolution mode. This document defines as for AS-local client identifiers and cimd for URL-shaped client identifiers resolved using the OAuth Client ID Metadata Document mechanism.

Motivation In a simple OAuth authorization request, the authorization server evaluates the authenticated client, requested resource, requested authorization details, and user consent. In a brokered redirect authorization request, the authorization server may see only the final broker as the OAuth client. For example: broker-a -> broker-b -> broker-c -> as-domain-1 ]]> When as-domain-1 receives the authorization request, it may only directly recognize broker-c. It may not know that the original downstream client was client-123, nor that the request passed through broker-a and broker-b. If the upstream authorization server binds consent only to the immediate broker, then consent granted for one downstream client can be reused for a different downstream client that reaches the upstream authorization server through the same broker. This is the shared consent problem described in . This document allows each intermediate authorization server in the redirect path to add a signed delegation node: broker-b broker-a attests client-123 Hop 2: broker-b -> broker-c broker-b attests broker-a-client Hop 3: broker-c -> as-domain-1 broker-c attests broker-b-client ]]> The requested protected resource remains constant across the chain: The aud value in the delegation chain identifies only the next authorization server. By validating the signed and hash-linked chain, any upstream authorization server can bind consent and policy to the entire redirect delegation path up to itself, rather than only to the immediate client which might not be the terminal client.

Protocol Overview A downstream client initiates an OAuth authorization request which is forwarded to other authorization servers. Each authorization server or broker can create or append a signed node to the delegation chain. | | | | | | attests | | | | | client-123 | | | | |----------------->| | | | | | attests | | | | | broker-a-client | | | | |----------------->| | | | | | attests | | | | | broker-b-client | | | | |------------------->| | | | | | Validate chain | | | | | Apply consent | | | | | and policy ]]> Figure: Brokered authorization request using an OAuth Authorization Request Delegation Chain The upstream authorization server validates the final node from the broker it directly knows, then walks the prior signed nodes to validate the full delegation path and identify the terminal client.

OAuth Broker Client Metadata This document defines client metadata that allows an authorization server, when acting as a client, to identify itself as an OAuth broker. The following client metadata attribute is defined:

client_roles

OPTIONAL. JSON array of strings identifying roles the OAuth client is expected to perform when interacting with the authorization server. The value oauth_broker indicates that the client may act as an intermediary between the authorization server and one or more downstream clients.

A client metadata value of oauth_broker is a statement a client makes about its expected role. It does not by itself establish trust in the client, any downstream client, or any brokered delegation path. An authorization server MAY also classify a client as an OAuth broker using local policy, even when the client_roles metadata member is absent.

Authorization Details Type This profile defines the following proposed authorization_details type: A delegated authorization request MAY include an authorization detail object of this type: The chain member is a JSON array. JSON arrays are ordered by definition . The explicit n value is nevertheless included to make event order unambiguous across storage, transformation, validation, and partial processing.

Delegation Chain Object The delegation chain authorization details object has the following members.

Member	Required	Description
type	Yes	Authorization details type. Value: oauth_request_delegation_chain.
chain	Yes	Ordered JSON array of delegation nodes.

The chain array MUST contain one or more delegation nodes.

Delegation Node A delegation node is a JSON object with the following members.

Member	Required	Description
iss	Yes	Issuer identifier of the attesting entity.
sub	No	Subject being carried through the chain, such as a user.
aud	Yes	Intended authorization server or broker-AS audience of this node. This profile uses aud only for authorization servers and broker-AS entities, not protected resource APIs.
n	Yes	Zero-based chain position.
p_hash	Yes	Hash of the previous signed node. null for the first node.
client_ns	Yes	Client identifier namespace or resolution mode. This profile defines as and cimd.
client_id	Yes	Client attested by this node.
client_name	No	Human-readable display name. Not a security identifier.
resource	No	Resource indicators or protected resource identifiers relevant to the authorization request.
omit_chain	No	Whether a broker is allowed to omit the delegation chain when forwarding to an upstream authorization server that does not support this profile. Values are forbidden and allowed; default is forbidden.
proof	Yes	Cryptographic proof object.

Additional members MAY be included only if their signing-payload representation is defined by this document, a future specification, or a mutually understood extension. A receiver MUST reject unsupported extension members unless local policy explicitly allows them to be ignored. The stable security identifier for an attested client depends on the client_ns value. For client_ns value as, the stable identifier is: For client_ns value cimd, the stable identifier is: For client_ns value as, the applicable AS issuer context is the iss value of the node that attests the client. The client_name value is display-only and MUST NOT be used as a security identifier.

Proof Object The proof object contains a detached JWS compact serialization.

Member	Required	Description
jws	Yes	Detached JWS compact serialization over the delegation node payload.

Example: The proof.jws value is a JWS Compact Serialization with a detached payload. The payload segment is empty, resulting in the following form: The JWS Protected Header MUST contain an alg value and a kid value. For example, the protected header could be: The detached JWS payload is the deterministic serialization of the delegation node excluding the proof member. The key used to verify proof.jws is resolved from the attester identified by iss. A verifier obtains the attester's verification key by resolving: authorization server metadata -> jwks_uri -> JWK selected by kid ]]> Because brokers in this profile are also authorization servers, a broker is expected to publish authorization server metadata and a jwks_uri.

Attesting a Client Attesting a client means: For example: means: The upstream authorization server can then validate prior nodes to discover the terminal downstream client.

Signature Input The wire format of a delegation node is JSON. The node is not a JWT and MUST NOT be processed as a JWT claims set. Each node is signed using JSON Web Signature (JWS) with a detached payload. The detached payload is the deterministic serialization of the delegation node excluding the proof member. The resulting compact detached JWS is carried in the node's proof.jws member. The use of JWS provides standard JOSE header handling, including alg and kid, while preserving a JSON wire format that can be validated using typed schemas before cryptographic verification.

Delegation Chain Signing Payload The detached JWS payload is a UTF-8 string formed by joining name-value lines with line feed \n. The first line is: Then each supported member is serialized in the following fixed order when present: The proof member is excluded. Members not included in this signing-payload definition are not protected by the node signature. Security-relevant extensions therefore MUST define how they are included in the signing payload, or receivers MUST reject them. Array values are serialized by joining each array element's JSON string serialization, in array order, with a comma character. No extra whitespace is inserted. For example: This UTF-8 string is used as the detached JWS payload. The compact detached JWS is produced according to by signing the payload with the algorithm identified by the JWS Protected Header alg value. The JWS Protected Header MUST contain alg and kid. The compact detached JWS serialization stored in proof.jws MUST contain an empty payload segment: Future specifications MAY define alternative signing-payload schemes. Such specifications MUST identify the scheme unambiguously.

Hash Chain Each delegation node is hash-linked to the previous signed node. For node i, define: The following rules apply: 0 chain[i].n = chain[i - 1].n + 1 ]]> This construction cryptographically binds each node to the signed event that precedes it. Validation of the hash chain, sequence numbers, and audience continuity detects modification, insertion, deletion, reordering, and signature substitution within the visible chain.

Creating or Adding to a Delegation Chain This section defines processing rules for an authorization server or broker creating a new delegation chain or adding a node to an existing chain.

Step 1 - Determine the Attester The attester sets iss to its issuer identifier: The iss value MUST identify the entity signing the node. The iss value SHOULD resolve to authorization server metadata containing a jwks_uri .

Step 2 - Determine the Audience The attester sets aud to the intended authorization server or broker-AS recipient of the node. For an intermediate broker: For the final upstream authorization server: The aud value MUST NOT be used to identify the protected resource API. Protected resources are identified using the resource member.

Step 3 - Determine the Attested Client The attester sets: or: For an AS-local client known by the attesting broker or authorization server: For a client using the OAuth Client ID Metadata Document mechanism: The authorization server can use the client metadata document to obtain client metadata according to , while using the delegation chain to validate the transaction-specific authorization request delegation path. The mechanism for resolving metadata from client_ns and client_id is determined by local policy, federation metadata, or client metadata mechanisms.

Step 4 - Set the Position If creating a new chain: If extending an existing chain, the attester sets n to the previous node's n plus one and sets p_hash to event_hash(previous_node).

Step 5 - Add Optional Display or Resource Information The attester MAY include client_name for user interface purposes. The attester MAY include resource to identify intended protected resources. The client_name value MUST NOT be used as a security identifier. Additional claims MAY be added as parties see fit, subject to local policy or future specifications.

Step 6 - Sign the Node The attester constructs the detached JWS payload as described in . The attester creates a JWS Protected Header containing at least: The attester signs the detached JWS payload using the private key corresponding to the public key published in its jwks_uri. The attester places the compact detached JWS in proof.jws: The JWS payload segment MUST be empty in the compact serialization, because the payload is detached and represented by the delegation node JSON object itself.

Step 7 - Forward the Chain The attester includes the updated chain in an authorization_details object with type oauth_request_delegation_chain.

Validating a Delegation Chain An authorization server validating a delegation chain performs the following checks. Validation is anchored at the receiving authorization server and proceeds from the final node of the chain toward the first node. This reflects the trust model: the final node is the node addressed to the receiving authorization server, and each valid signed node commits to the previous node through p_hash. A node's signature is over the node's deterministic detached JWS payload, including its p_hash value and excluding the proof member. Therefore, a valid signature on node i commits to node i - 1 when p_hash is valid.

Step 1 - Schema Validation The authorization server validates that the authorization detail object contains: and that chain is a non-empty JSON array. Each node MUST contain: A receiver MAY reject nodes containing unsupported values or unsupported extension members.

Step 2 - Client Namespace Validation The authorization server verifies that each node contains a supported client_ns value. This profile defines: If the authorization server does not support the client_ns value, it MUST reject the authorization detail object.

Step 3 - Terminal Node Checks Let last be the index of the final node in the chain. The authorization server performs the following checks on the final node before performing signature validation. These checks are fail-fast checks over unauthenticated input: failure is sufficient to reject the request, but success does not authenticate the node or the chain. The authorization server verifies that the final node is intended for it: If the final node's aud value does not identify the receiving authorization server, the authorization server MUST reject the chain. The authorization server also verifies that the final node is plausibly bound to the OAuth client submitting the authorization request. The exact binding is deployment-specific, but the receiving authorization server MUST be able to establish that: identifies, or is authorized to speak for, the authenticated OAuth client submitting the request. For example, if the request is submitted by an authenticated broker client, the authorization server can verify that the registered metadata for that OAuth client identifies: as the broker authorization server or broker issuer for that client. If the authorization server cannot bind the final node's iss to the OAuth client submitting the request, it MUST reject the chain. The checks in this step are fail-fast checks. Until the final node's signature has been verified, the authorization server MUST treat the final node's iss, aud, and other members as unauthenticated input. A successful preflight check does not by itself authenticate the node or the chain. The aud value identifies an authorization server or broker-AS, not a protected resource API. Protected resource identifiers are represented using the resource member. A protected API endpoint MUST NOT appear in aud.

Step 4 - Signature Validation For each node, the authorization server:

1. Reads iss. The iss value is untrusted until the node signature is verified. Before using iss for metadata retrieval, the authorization server MUST apply its normal issuer validation, discovery, allow-list, federation, or local trust policy.
2. Reads proof.jws.
3. Parses proof.jws as a compact detached JWS.
4. Verifies that the compact JWS contains an empty payload segment.
5. Decodes the JWS Protected Header.
6. Verifies that the JWS Protected Header contains alg and kid.
7. Resolves the issuer metadata for iss. The resolved authorization server metadata issuer value MUST match iss.
8. Obtains the issuer's jwks_uri.
9. Fetches the issuer's JWK Set.
10. Selects a key using the JWS Protected Header kid.
11. Constructs the deterministic detached JWS payload for the node by serializing the node excluding the proof member.
12. Verifies the detached JWS signature over that payload according to .

If a signature cannot be verified, the authorization server MUST reject the chain. After signature validation succeeds, the authorization server treats the signed members of each node as authenticated statements by that node's iss. In particular, the terminal preflight checks in are then authenticated because the final node's signature covers the same iss, aud, client_ns, client_id, p_hash, and other signed members.

Step 5 - Backward Chain Validation The authorization server validates the chain from the final node toward the first node. For every i from last down to 1, the authorization server verifies: where event_hash is computed over the previous node's deterministic detached JWS payload and its proof.jws value. If any hash comparison fails, the authorization server MUST reject the chain. If any audience-continuity check fails, the authorization server MUST reject the chain. If any sequence-number check fails, the authorization server MUST reject the chain. The authorization server then verifies the first node: If either check fails, the authorization server MUST reject the chain. The audience-continuity check ensures that every hop intentionally delegated to the next hop in the chain: Thus, for a chain: node[1] -> node[2] ]]> the following MUST hold:

Step 6 - Resource Consistency Validation If multiple nodes contain resource, the authorization server SHOULD verify that the resource value is consistent across the chain, unless local policy explicitly permits resource transformation. If local policy permits resource transformation, the authorization server SHOULD verify that each transformation is allowed for the issuer performing the transformation.

Step 7 - Delegation Relationship Validation The authorization server SHOULD verify that adjacent nodes are semantically consistent. The final node identifies the client that the immediate trusted broker is attesting. Prior nodes reveal what that client was itself carrying. For example, if the final node is: then the authorization server treats https://broker-c.example.com as attesting broker-b-client. The authorization server then validates the prior node signed by broker-b to determine which client broker-b was carrying. The authorization server MAY reject the chain if the attested client relationship is inconsistent with registration metadata, federation metadata, or local policy.

Step 8 - Policy Validation After cryptographic validation, the authorization server applies local policy. Cryptographic validation proves the integrity of the visible chain. It does not prove that no upstream delegation context existed before chain[0]. In particular, a broker can originate a new chain beginning with itself as chain[0]. Such re-origination can produce a cryptographically valid chain. Whether that chain is acceptable is a local policy decision for the receiving authorization server. Policy decisions can consider:

• the OAuth client authenticated to the authorization server,
• the final node's iss, whether it is allowed for the authenticated OAuth client,
• the first node's iss, whether it is allowed to appear as a first-node issuer,
• the full set of brokers in the chain,
• whether each broker is allowed to appear in its position in the chain,
• the client identified by the first visible client attestation,
• the resources identified by resource,
• the user subject identified by sub, if present,
• the full delegation chain hash, and
• deployment-specific expectations about allowed direct and indirect paths.

The final node's iss is used to validate the relationship between the authenticated OAuth client and the broker-AS or AS that produced the final delegation-chain node. The first node's iss is used to evaluate whether the visible chain is allowed to begin with that issuer. This is the policy check that addresses head-truncation or re-origination. Cryptographic validation cannot prove that no upstream nodes existed before chain[0]. The authorization server MAY reject the request if any broker, client, namespace, resource, subject, first-node issuer, or path is not allowed.

Consent Binding In brokered OAuth, an authorization server SHOULD NOT bind consent only to: Instead, when a valid delegation chain is present, the authorization server SHOULD bind consent to: For example: broker-b -> broker-c + https://api-domain-1.example.com + hash(chain) ]]> This prevents consent granted to one downstream client from being silently reused by another downstream client through the same broker.

Authorization Server Considerations An authorization server that supports this profile SHOULD advertise support for the oauth_request_delegation_chain authorization details type using the authorization_details_types_supported authorization server metadata attribute defined by . An authorization server that receives an oauth_request_delegation_chain authorization detail object SHOULD evaluate whether the chain is required for the requested transaction. An authorization server MAY require an oauth_request_delegation_chain authorization detail object when the immediate OAuth client is known or determined to be an OAuth broker. For this purpose, an authorization server MAY determine that the immediate client is an OAuth broker based on:

• The client's registered client_roles metadata containing oauth_broker;
• Local client configuration;
• A trusted software statement;
• Federation metadata or metadata policy;
• Contractual onboarding;
• Transaction context; or
• Any other trusted local policy input.

If the authorization server determines that the immediate client is acting as an OAuth broker, and local policy requires this profile for the request, then the authorization request MUST contain an authorization_details object of type oauth_request_delegation_chain. If the required oauth_request_delegation_chain authorization detail object is absent, the authorization server MUST reject the authorization request using normal OAuth 2.0 authorization endpoint error signaling . If the authorization server can safely redirect to a valid registered redirection URI for the client, the authorization server SHOULD return: For example: If the oauth_request_delegation_chain authorization detail object is present but malformed, semantically invalid, contains an unsupported value, or fails validation, the authorization server SHOULD reject the request according to RAR error processing rules . If the authorization server supports this profile and delegates the authorization request to another upstream authorization server, it SHOULD include the oauth_request_delegation_chain authorization detail object and extend the delegation chain with details about the current hop. An authorization server MAY include the approved authorization_details object in an access token or token introspection response when appropriate. However, this document does not define a token format or require the chain to be propagated to resource servers. Where token size or privacy considerations apply, an authorization server SHOULD consider storing the validated chain server-side and exposing only necessary authorization results to resource servers via token introspection .

Discovering Upstream Support An authorization server acting as broker that intends to forward an authorization request to an upstream authorization server SHOULD determine whether the upstream authorization server supports the oauth_request_delegation_chain authorization details type before sending the authorization request. The broker SHOULD retrieve the upstream authorization server metadata according to . If the upstream authorization server metadata publishes in authorization_details_types_supported support for the oauth_request_delegation_chain RAR type, the broker SHOULD opt into this mechanism by including an authorization_details object of type oauth_request_delegation_chain in the authorization request. For example, an upstream authorization server can advertise support as follows: A broker that discovers upstream support SHOULD either:

• create a new oauth_request_delegation_chain authorization detail object, if no chain is already present; or
• validate and extend the existing chain, if a chain is already present.

A broker MAY still use this profile with an upstream authorization server when support is established by other means, such as bilateral configuration, federation metadata, contractual onboarding, or local policy. If a broker is unable to determine whether the upstream authorization server supports this profile, the broker SHOULD apply local policy. Local policy can include forwarding the request without this authorization detail, aborting the transaction, or using an alternative delegation mechanism.

Upstream Support Failure If a broker receives an authorization request containing an oauth_request_delegation_chain authorization detail object and determines that the next upstream authorization server does not support this profile, the broker MUST NOT silently discard the delegation chain. The broker MUST either reject the transaction, use another trusted mechanism to preserve the delegation context, or forward the request without the chain only when doing so is permitted by local policy and by the signed chain requirement. A delegation node MAY contain the following member:

omit_chain

OPTIONAL. String indicating whether a broker is allowed to forward the authorization request without the delegation chain if the next upstream authorization server does not support this profile. Defined values are forbidden and allowed. If omitted, the default value is forbidden.

If any validated node contains: or omits omit_chain, then a broker that cannot forward the delegation chain to the next upstream authorization server, and cannot preserve equivalent delegation context by another trusted mechanism, MUST reject the transaction. If every validated node contains: then the broker MAY forward the authorization request without the delegation chain, subject to local policy. A broker MUST NOT silently discard a delegation chain. Forwarding without the chain is an explicit degradation of delegation evidence and is permitted only when allowed by the validated chain and by local policy.

Security Considerations

Chain Integrity The hash chain, per-node detached JWS signatures, sequence numbers, and audience continuity checks are intended to detect modification, insertion, deletion, reordering, and signature substitution within the visible delegation chain. A receiver MUST reject a chain if any required validation check described in fails. A valid signature proves only that the identified issuer signed the node. It does not imply that the issuer is trusted for the requested delegation.

Truncation and Re-origination Truncation has three relevant cases. Tail truncation is detected by the terminal audience check: a shortened chain will not end in a node whose aud identifies the receiving authorization server. Middle removal is detected by the hash-chain and audience-continuity checks: removing an intermediate node breaks the successor's p_hash and the adjacent issuer/audience relationship. Head truncation, or re-origination, is different. A broker can create a fresh chain beginning with itself as chain[0]. Cryptographic validation proves the integrity of the visible chain, but cannot prove that no upstream context existed before chain[0]. Authorization servers MUST handle re-origination through local policy, including whether chain[0].iss is allowed to appear as the first visible issuer for the requested client, resource, and deployment context.

Replay This profile does not define expiration, nonce, or replay-cache claims in the base structure. Deployments that require replay protection MAY add such claims as deployment-specific extensions and validate them according to local policy.

Display Names client_name is intended only for display. Authorization servers MUST NOT use client_name as a security identifier. The stable security identifier depends on client_ns, client_id, and the applicable issuer or namespace context.

Trust in Attesters A valid chain establishes integrity and provenance of the visible attestations. It does not establish that the attesters, clients, resources, subjects, or path are acceptable. Authorization servers MUST apply local trust policy before accepting a delegation chain.

Trust in Broker Client Metadata The client_roles client metadata member can indicate that a client is expected to act as an OAuth broker. An authorization server MUST NOT treat self-asserted client_roles metadata as proof that the client is trustworthy, authorized to broker authorization requests, or authorized to represent downstream clients. An authorization server MUST rely on client_roles for security decisions only when the metadata was established through a trusted mechanism, such as administrative registration, trusted dynamic client registration, a trusted software statement, federation metadata, or local trust policy.

Metadata Resolution This profile assumes that attesters publish verification keys through authorization server metadata and jwks_uri. If metadata cannot be resolved, is not trusted, or does not contain the key identified by kid, the receiver MUST reject the affected node.

Immediate Client Authentication The delegation chain does not replace OAuth client authentication. An authorization server MUST still authenticate the immediate OAuth client according to its normal OAuth processing rules. The authorization server MUST verify that the authenticated immediate client is consistent with the final delegation node.

Privacy A delegation chain can reveal intermediaries, downstream clients, resources, and possibly subjects. Deployments SHOULD minimize included data and avoid including unnecessary personally identifiable information.

IANA Considerations

OAuth Dynamic Client Registration Metadata Registration This document requests registration of the following value in the IANA "OAuth Dynamic Client Registration Metadata" registry established by .

Client Metadata Name:

client_roles

Client Metadata Description:

JSON array of strings identifying roles the OAuth client is expected to perform when interacting with the authorization server. The value oauth_broker indicates that the client may act as an intermediary between the authorization server and one or more downstream clients, applications, agents, relying parties, resource servers, or trust domains.

Change Controller:

IETF

Specification Document:

This document.

OAuth Authorization Details Type This document defines the oauth_request_delegation_chain authorization details type. A future standards-track version of this document may request registration of the oauth_request_delegation_chain authorization details type in the applicable IANA registry.

References Normative References The OAuth 2.0 authorization framework enables a third-party application to obtain limited access to an HTTP service, either on behalf of a resource owner by orchestrating an approval interaction between the resource owner and the HTTP service, or by allowing the third-party application to obtain access on its own behalf. This specification replaces and obsoletes the OAuth 1.0 protocol described in RFC 5849. [STANDARDS-TRACK] JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification. A JSON Web Key (JWK) is a JavaScript Object Notation (JSON) data structure that represents a cryptographic key. This specification also defines a JWK Set JSON data structure that represents a set of JWKs. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and IANA registries established by that specification. This specification registers cryptographic algorithms and identifiers to be used with the JSON Web Signature (JWS), JSON Web Encryption (JWE), and JSON Web Key (JWK) specifications. It defines several IANA registries for these identifiers. JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. This specification defines mechanisms for dynamically registering OAuth 2.0 clients with authorization servers. Registration requests send a set of desired client metadata values to the authorization server. The resulting registration responses return a client identifier to use at the authorization server and the client metadata values registered for the client. The client can then use this registration information to communicate with the authorization server using the OAuth 2.0 protocol. This specification also defines a set of common client metadata fields and values for clients to use during registration. 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. 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. 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. 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. Informative References This specification defines a protocol for an HTTP- and JSON-based Security Token Service (STS) by defining how to request and obtain security tokens from OAuth 2.0 authorization servers, including security tokens employing impersonation and delegation. OpenID Foundation n.d. n.d. n.d. n.d. n.d.

Brokered OAuth Example Without CIMD This example uses AS-local client identifiers. broker-a -> broker-b -> broker-c -> as-domain-1 ]]> Each AS or broker attests the client it directly recognizes:

• broker-a attests client-123 to broker-b.
• broker-b attests broker-a-client to broker-c.
• broker-c attests broker-b-client to as-domain-1.

The aud value always identifies the next authorization server or broker-AS. The protected API is represented only by the resource member.

Full Authorization Details Object

Interpretation Node 0 says: Node 1 says: Node 2 says: The upstream AS validates the final trusted hop first: broker-b-client ]]> Then walks the prior signed nodes: broker-a-client broker-a -> client-123 ]]> The terminal client is therefore: The resource is constant across the chain:

Brokered OAuth Example With CIMD This example uses CIMD-style URL-shaped client identifiers. broker-a -> broker-b -> broker-c -> as-domain-1 ]]> Each AS or broker attests the client it directly recognizes:

• broker-a attests the CIMD-identified terminal client to broker-b.
• broker-b attests the CIMD-identified broker-a client to broker-c.
• broker-c attests the CIMD-identified broker-b client to as-domain-1.

The aud value always identifies the next authorization server or broker-AS. The protected API is represented only by the resource member.

Full Authorization Details Object

Interpretation Node 0 says: Terminal client: Node 1 says: Node 2 says: The upstream AS sees the immediate trusted path as: broker-b -> broker-a -> client-123 ]]> And the resource remains:

Example Signing Payload For this node: the detached JWS payload is: The JWS Protected Header is: The proof.jws value is the compact detached JWS over the UTF-8 bytes of the detached JWS payload.

Document History -01

• Added client_roles OAuth Dynamic Client Registration metadata with oauth_broker as a client role value.
• Added IANA request for the client_roles client metadata name.
• Added broker processing rules for discovering upstream support for the oauth_request_delegation_chain authorization details type.
• Added authorization server processing rules for rejecting brokered authorization requests that omit a required oauth_request_delegation_chain authorization detail.
• Added mitigation of chain truncation and tampering.

-00

• Initial version.
• Defined oauth_request_delegation_chain authorization details type.
• Defined signed authorization request delegation nodes using iss, aud, client_ns, and client_id.
• Defined client_ns values as and cimd.
• Defined detached JWS proof processing using proof.jws.
• Defined p_hash hash-chain processing.
• Added processing rules for creating, extending, and validating delegation chains.
• Added brokered OAuth examples with and without CIMD.

Acknowledgments The author would like to thank the participants in the OAuth Working Group discussions on brokered OAuth, Rich Authorization Requests, client metadata, actor delegation, and authorization request security.