Introspektion opaker Tokens und Tokenaustausch

A JWT carries its claims inside the token, so a resource server can validate it offline by checking the signature. An opaque token is just a random reference string — it contains no claims and no signature you can verify.

• To learn who the token belongs to and whether it is still valid, the resource server must ask the authorization server.
• That call is the OAuth2 Token Introspection endpoint, defined by RFC 7662.
• The authorization server replies with active: true/false plus claims like sub, scope, and exp.

Opaque tokens trade offline speed for instant revocation: revoke at the AS and the very next introspection returns active: false.

Per RFC 7662, the introspection endpoint accepts the token as a form parameter and returns a JSON document. The single mandatory field is active.

A typical successful response looks like the JSON below. If the token is unknown, expired, or revoked, the server returns simply {"active": false} and Spring rejects the request.

In a Spring Boot 4 resource server you enable opaque-token validation with a single properties block. Spring Security autoconfigures an OpaqueTokenIntrospector from these values.

• introspection-uri — the AS endpoint that implements RFC 7662.
• client-id / client-secret — the resource server's own credentials to authenticate the introspection call.

The properties only take effect once you opt into opaqueToken() on the resource server DSL. This wires the autoconfigured introspector into the bearer-token filter.

Notice the contrast with JWT mode: with opaque tokens there is no local key set — every request triggers a network call to the AS.

You can define your own OpaqueTokenIntrospector bean to control the HTTP client, caching, or response parsing. The built-in SpringOpaqueTokenIntrospector uses a RestClient under the hood.

Returning a bean overrides the autoconfigured one, so you keep the same opaqueToken() DSL while customizing behavior.

By default the introspected scope string becomes a set of SCOPE_ authorities. To customize that mapping — for example adding role authorities — wrap the default introspector and transform its principal.

The OAuth2AuthenticatedPrincipal exposes the raw introspection claims via getAttributes(), which you remap into your own authorities.

Because every request hits the AS, opaque tokens can become a latency and load bottleneck. A safe optimization is to cache the introspection result for a short window — never longer than the token's remaining lifetime.

• Cache on the token value as the key.
• Honor exp: an entry must expire at or before the token's own expiry.
• Keep the TTL small (seconds to a minute) so revocation stays nearly immediate.

This small helper shows the core TTL math you would apply inside a caching introspector.

Now suppose your API gateway received a token for the user, and it must call a downstream service. Forwarding the original token leaks audience and over-grants scope. Re-using the gateway's own client credentials loses the user's identity entirely.

OAuth2 Token Exchange (RFC 8693) solves this: the gateway presents the incoming token to the AS and asks for a new token scoped to the downstream service, while preserving the original subject.

• Identity is propagated, not impersonated blindly.
• The new token's aud targets exactly the downstream service.
• Scopes can be narrowed for least privilege.

A token exchange is a POST to the AS token endpoint with grant_type=urn:ietf:params:oauth:grant-type:token-exchange. The key parameters are the subject_token (the incoming token) and its subject_token_type.

• subject_token — the token representing the user.
• requested_token_type — usually an access token.
• audience or resource — the downstream service the new token is for.

Spring Security ships a TokenExchangeOAuth2AuthorizedClientProvider so a resource server can exchange the current token before calling downstream. Register the provider and a matching client registration with the token-exchange grant type.

The client registration uses authorization-grant-type set to the RFC 8693 URN.

With the manager in place, configure a RestClient (or WebClient) that attaches the exchanged token automatically. The interceptor resolves the authorized client named orders-service, triggering the exchange when needed.

The downstream service then validates a token whose sub is still the original user and whose aud is itself — clean, least-privilege identity propagation.

You learned how to validate reference tokens and propagate identity across services:

• Opaque tokens carry no claims, so the resource server calls the AS introspection endpoint (RFC 7662); the only required field is active.
• Enable it with the opaque-token properties plus oauth2ResourceServer(o -> o.opaqueToken(...)); customize via a custom OpaqueTokenIntrospector.
• Map scopes to authorities and cache results briefly — never beyond the token's exp — to limit AS load while keeping revocation fast.
• Token exchange (RFC 8693) swaps an incoming token for a downstream-scoped one, preserving the original sub while setting the correct aud.
• In Spring, the TokenExchangeOAuth2AuthorizedClientProvider plus an authorized-client interceptor makes this automatic for downstream calls.