To authorize an agent on a user’s behalf, keep the user as the subject, identify the agent as the actor, limit the token to the intended resource and operations, and have the resource server check every request. OAuth token exchange, Rich Authorization Requests and token introspection can support this design, but none supplies a universal policy or makes the authorization decision for your API.
How can an agent act on a user’s behalf?
Model the user and the agent as distinct identities. In a delegated request, the user is the subject—the party on whose behalf access is requested—and the agent is the actor to whom some rights are delegated. Keeping both identities available lets authorization rules and audit records distinguish the user’s authority from the agent’s actions.
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RFC 8693, OAuth 2.0 Token Exchange, describes a flow in which a service exchanges tokens to call a backend on a user’s behalf. It defines the subject_token for the party on whose behalf a token is requested and the optional actor_token for the party to whom access rights are delegated. Whether an authorization server issues a token that carries the resulting subject and actor context depends on that server’s policy and configuration; token exchange does not require every deployment to issue the same kind of composite token.
| Identity model | Meaning for the request | When it fits |
|---|---|---|
| Delegation | The user remains the subject, and the agent is represented as an actor with delegated rights. | Use when the system needs to know both whose authority is involved and which agent performed the action. |
| Impersonation | The actor is treated as indistinguishable from the represented user within the relevant rights context. | Use only when policy intentionally gives the actor that user-equivalent identity treatment. |
RFC 8693 treats delegation and impersonation as different semantics, not interchangeable labels. Choose deliberately: if the agent must remain visible as the actor, an impersonation model obscures an identity distinction that delegation preserves.
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How do I limit an agent to specific actions?
Define authorization in terms of the resource and the operation the agent needs, rather than relying on a broad permission that could cover unrelated activity. For each API, establish precise resource identifiers and a controlled vocabulary of operations, then make the resource server enforce the granted rights against the requested action.
OAuth scopes can express authorization at a broader level. For APIs that need more specific authorization data, RFC 9396, OAuth 2.0 Rich Authorization Requests, defines the authorization_details request parameter: a JSON structure through which a client can express fine-grained requirements, such as access to a particular resource or an operation with transaction details. The meaning of its fields belongs to the API or APIs using them; RFC 9396 does not define a single field schema that every API must share.
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| Mechanism | What it can express | Design consideration |
|---|---|---|
scope |
Authorization using the scope vocabulary supported by the client, authorization server and API. | Choose scopes whose breadth matches the API’s permission model. |
authorization_details |
Structured, fine-grained authorization requirements, with API-specific field meanings. | Document the API’s schema and enforce the granted details; do not assume another API interprets the same fields the same way. |
RFC 9700, the IETF’s January 2025 OAuth 2.0 security best current practice, recommends associating access tokens with their relevant resources and actions. It also says resource servers should verify on every request that the token is intended for the requested resource and action. Both scope and authorization_details can help clients and authorization servers determine the resources or actions involved, but the resource server still has to enforce the applicable policy.
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What does token introspection tell a resource server?
RFC 7662, OAuth 2.0 Token Introspection, defines a way for a protected resource to ask an authorization server about an OAuth token’s current state and metadata. An introspection response can indicate whether the token is active and provide context such as its access rights, who authorized it and which client it was issued to. RFC 9396 also registers authorization_details for introspection responses: when the authorization server includes those details, they appear as a top-level response member.
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“Active” is determined by the authorization server. RFC 7662 gives a commonly used interpretation: the token was issued by that server, has not expired or been revoked, and is valid for the protected resource making the query. A positive result is a statement about token status and metadata—not permission for every operation. The resource server must compare the token’s relevant rights and context with the particular resource and action requested.
Introspection is a protected endpoint because it accepts tokens and returns information used in access decisions. RFC 7662 requires the authorization server to require authorization to access the endpoint and, when TLS is used, to support TLS 1.2 and check the server certificate. Those requirements protect the query and its response; they do not replace the resource server’s operation-level check.
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How should authorization work on each request?
Use the following sequence to connect delegated identity, token information and API enforcement. The standards provide protocol mechanisms; your authorization server and API still need an explicit policy that defines which actor may perform which operation on which resource.
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- Identify the parties. Decide which user is the subject and which software agent is the actor. Select delegation when both identities must remain distinguishable in the authorization context.
- Define the permission boundary. Specify the resource identifiers and operation names the agent may use. Choose scopes, API-specific
authorization_details, or both according to the granularity the API requires. - Protect the authorization request. If rich authorization details pass through a user agent, account for the possibility that they may be modified or swapped. RFC 9396 says clients must protect them against tampering and swapping when integrity matters, and identifies signed request objects or a request URI referring to a request object as possible mechanisms.
- Apply issuance policy. If using token exchange, configure the authorization server to decide whether it will issue a token for the requested delegation and what authorization context it will carry. A client’s requested rights do not, by themselves, establish what the server grants.
- Check the grant at the resource server. For each incoming request, establish that the token is valid for that resource and compare its granted rights with the specific operation and target resource. Use introspection when the deployment needs the authorization server to provide current token status or metadata; introspection is an input to this check, not its outcome.
- Enforce the granted details. Make the decision using the authorization actually granted and conveyed to the resource server, not just the original client request. Preserve subject and actor context where available so policy and audit handling can tell whose authority was used and which agent acted.
How do I choose between introspection and token-contained information?
Introspection lets a resource server query the authorization server for token status and metadata, regardless of whether metadata is also carried in the token. Whether to use that query or rely on locally verifiable token content depends on the deployment. The cited specifications do not establish a universal choice, performance figures or a caching strategy; those are implementation decisions that must account for the system’s requirements.
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Whichever approach supplies token context, keep the final authorization decision at the resource server and bind it to the request’s resource and operation. A token can be valid yet lack permission for the requested action.
What these OAuth standards do—and do not—standardize
Together, RFC 8693, RFC 9396, RFC 7662 and RFC 9700 describe mechanisms and security guidance for token exchange, fine-grained authorization requests, token introspection and request-by-request authorization checks. They do not define a universal agent policy, a shared schema for every API’s authorization details, or a guarantee that an authorization server will issue a particular delegated token. Those choices belong to the deployment and the API. The specifications are international IETF documents, not jurisdiction-specific legal requirements.
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