Token Introspection

The primary function of token introspection is to query the authorization server to confirm whether an access token is currently active, expired, revoked, or otherwise invalid. This moves authorization logic from the client-side token to a server-side check, enabling real-time policy enforcement.

• Prevents Token Replay: Even a validly signed token can be rejected if the authorization server has revoked it.
• Centralized Control: Token lifecycle (revocation, expiration) is managed at the authorization server, not inferred from the token itself.
• Essential for Zero-Trust: Aligns with the 'never trust, always verify' principle by validating every token for every request.

Beyond a simple active/inactive response, the introspection endpoint returns a JSON object containing the token's metadata. This provides the protected resource (e.g., API gateway) with the contextual data needed for fine-grained authorization.

• Standard Claims: Returns standardized fields like scope, client_id, username, and exp (expiration).
• Extended Claims: Authorization servers can include custom claims specific to the application's domain.
• Informs Policy Decisions: The gateway uses this metadata (e.g., user roles, scopes) to make context-aware authorization decisions before routing the request to a backend service.

Token introspection is initiated by the protected resource (the API gateway), not the client application. This separates the concern of token validation from the client, enhancing security and simplifying client logic.

1. Client presents token: Sends an access token in the Authorization header to the API gateway.
2. Gateway calls introspection endpoint: The gateway makes a POST request to the authorization server's /introspect endpoint with the token.
3. Server responds with metadata: Returns a JSON response indicating active: true/false and associated claims.
4. Gateway enforces policy: Uses the introspection response to permit or deny the API request.

This flow ensures the client cannot bypass validation by presenting a self-constructed or expired token.

The introspection endpoint itself is a protected resource. The API gateway must authenticate itself to the authorization server to call it, typically using client credentials (Client ID and Secret) or a mutual TLS (mTLS) certificate. This prevents unauthorized parties from querying the token status.

• Confidential Client: The gateway acts as a confidential OAuth 2.0 client.
• Prevents Information Leakage: An attacker cannot use the introspection endpoint to probe for valid tokens without first compromising the gateway's credentials.
• Mandatory for Security: RFC 7662 requires the introspection endpoint to support authentication, making the query itself a trusted transaction.

Token introspection enables a stateful authorization model, contrasting with stateless JSON Web Token (JWT) validation. This choice involves a key trade-off between architectural simplicity and real-time control.

Introspection is chosen when real-time revocation and centralized policy control are critical security requirements.

Within a Zero-Trust API Gateway, token introspection is the mechanism that feeds the Policy Decision Point (PDP). The gateway uses the returned token metadata as attributes for a context-aware authorization decision.

• Attribute Source: Introspection provides user roles, token scopes, and client_id as inputs to Attribute-Based Access Control (ABAC) or Role-Based Access Control (RBAC) policies.
• Dynamic Policy Evaluation: A dynamic policy engine can evaluate rules like: PERMIT if token.active == true AND token.scope includes 'api:read' AND resource.sensitivity == 'low'.
• Audit Trail: The introspection request and response are logged, creating a verifiable audit trail that links the API call to a specific, validated authorization context.

The component in a zero-trust architecture that intercepts and enforces access decisions. In the context of API security, the PEP is often the API Gateway or a sidecar proxy.

• Intercepts every API request from an AI agent.
• Extracts the access token and calls the token introspection endpoint (or a Policy Decision Point).
• Enforces the authorization decision by allowing or denying the request based on the introspection response (e.g., active: true, valid scopes).

The system component that evaluates access requests against security policies and renders an authorization decision. The authorization server hosting the token introspection endpoint often acts as the PDP.

• Receives a token introspection request from a PEP.
• Evaluates the token's validity, expiration, and associated metadata (scopes, client ID).
• Returns a structured response (per RFC 7662) informing the PEP if the token is active and what permissions it grants.

A strong authentication protocol that verifies both client and server identities using X.509 certificates before establishing a connection. It provides a foundational layer of trust for machine-to-machine communication.

• Used to secure the connection between the API Gateway (PEP) and the Authorization Server (PDP) hosting the introspection endpoint, preventing man-in-the-middle attacks.
• Can be used in conjunction with OAuth. The client certificate can be bound to the OAuth access token (RFC 8705), and the introspection endpoint can validate this binding.