Opaque Token Introspection and Token Exchange
Validate opaque tokens through introspection and propagate identity with token exchange.
Opaque Token Introspection and Token Exchange is a free Spring Boot 4 Complete Guide lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Spring Boot 4 Complete Guide learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Opaque Tokens Need Introspection
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/falseplus claims likesub,scope, andexp.
Opaque tokens trade offline speed for instant revocation: revoke at the AS and the very next introspection returns active: false.
The Introspection Response
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.
{
"active": true,
"sub": "user-123",
"scope": "orders:read orders:write",
"client_id": "web-app",
"username": "alice",
"token_type": "Bearer",
"exp": 1735689600,
"iat": 1735686000
}Configuring Opaque Token Validation
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.
# application.yml
spring:
security:
oauth2:
resourceserver:
opaque-token:
introspection-uri: https://auth.example.com/oauth2/introspect
client-id: resource-server
client-secret: ${INTROSPECT_SECRET}Enabling It in the Security Filter Chain
The properties only take effect once you opt into opaqueToken() on the resource server DSL. This wires the autoconfigured introspector into the bearer-token filter.
Notice the contrast with JWT mode: with opaque tokens there is no local key set — every request triggers a network call to the AS.
@Configuration
@EnableWebSecurity
public class SecurityConfig {
@Bean
SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
http
.authorizeHttpRequests(auth -> auth
.requestMatchers("/api/**").authenticated()
.anyRequest().permitAll())
.oauth2ResourceServer(oauth2 -> oauth2
.opaqueToken(Customizer.withDefaults()));
return http.build();
}
}A Custom OpaqueTokenIntrospector
You can define your own OpaqueTokenIntrospector bean to control the HTTP client, caching, or response parsing. The built-in SpringOpaqueTokenIntrospector uses a RestClient under the hood.
Returning a bean overrides the autoconfigured one, so you keep the same opaqueToken() DSL while customizing behavior.
@Bean
OpaqueTokenIntrospector introspector(
@Value("${spring.security.oauth2.resourceserver.opaque-token.introspection-uri}") String uri,
@Value("${spring.security.oauth2.resourceserver.opaque-token.client-id}") String clientId,
@Value("${spring.security.oauth2.resourceserver.opaque-token.client-secret}") String secret) {
RestClient restClient = RestClient.builder()
.requestInterceptor(new BasicAuthenticationInterceptor(clientId, secret))
.build();
return new SpringOpaqueTokenIntrospector(uri, restClient);
}Mapping Scopes to Authorities
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.
public class CustomAuthoritiesIntrospector implements OpaqueTokenIntrospector {
private final OpaqueTokenIntrospector delegate;
public CustomAuthoritiesIntrospector(OpaqueTokenIntrospector delegate) {
this.delegate = delegate;
}
@Override
public OAuth2AuthenticatedPrincipal introspect(String token) {
OAuth2AuthenticatedPrincipal principal = delegate.introspect(token);
List<GrantedAuthority> authorities = new ArrayList<>(principal.getAuthorities());
String scope = principal.getAttribute("scope");
if (scope != null && scope.contains("orders:write")) {
authorities.add(new SimpleGrantedAuthority("ROLE_ORDER_MANAGER"));
}
return new DefaultOAuth2AuthenticatedPrincipal(
principal.getName(), principal.getAttributes(), authorities);
}
}Caching Introspection Results
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.
import java.time.Instant;
public class IntrospectionCache {
public static long ttlSeconds(long tokenExpEpoch, long maxCacheSeconds) {
long now = Instant.now().getEpochSecond();
long remaining = tokenExpEpoch - now;
if (remaining <= 0) {
return 0; // already expired, do not cache
}
return Math.min(remaining, maxCacheSeconds);
}
public static void main(String[] args) {
long exp = Instant.now().getEpochSecond() + 300; // expires in 5 min
System.out.println("Cache for " + ttlSeconds(exp, 30) + "s");
System.out.println("Cache for " + ttlSeconds(Instant.now().getEpochSecond() - 10, 30) + "s");
}
}The Identity Propagation Problem
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
audtargets exactly the downstream service. - Scopes can be narrowed for least privilege.
The Token Exchange Request
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.audienceorresource— the downstream service the new token is for.
POST /oauth2/token HTTP/1.1
Host: auth.example.com
Authorization: Basic <gateway-client-credentials>
Content-Type: application/x-www-form-urlencoded
grant_type=urn:ietf:params:oauth:grant-type:token-exchange
&subject_token=<incoming-user-token>
&subject_token_type=urn:ietf:params:oauth:token-type:access_token
&requested_token_type=urn:ietf:params:oauth:token-type:access_token
&audience=orders-serviceToken Exchange in Spring Security 6.3+
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.
@Bean
OAuth2AuthorizedClientManager authorizedClientManager(
ClientRegistrationRepository registrations,
OAuth2AuthorizedClientRepository clients) {
OAuth2AuthorizedClientProvider provider =
OAuth2AuthorizedClientProviderBuilder.builder()
.provider(new TokenExchangeOAuth2AuthorizedClientProvider())
.build();
DefaultOAuth2AuthorizedClientManager manager =
new DefaultOAuth2AuthorizedClientManager(registrations, clients);
manager.setAuthorizedClientProvider(provider);
return manager;
}Calling Downstream With the Exchanged Token
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.
@Bean
RestClient ordersRestClient(OAuth2AuthorizedClientManager manager) {
OAuth2ClientHttpRequestInterceptor interceptor =
new OAuth2ClientHttpRequestInterceptor(manager);
interceptor.setClientRegistrationIdResolver(request -> "orders-service");
return RestClient.builder()
.baseUrl("https://orders.internal")
.requestInterceptor(interceptor)
.build();
}Quick Check: Choosing the Right Mechanism
Test your understanding of when to use introspection versus token exchange.
Recap
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-tokenproperties plusoauth2ResourceServer(o -> o.opaqueToken(...)); customize via a customOpaqueTokenIntrospector. - 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
subwhile setting the correctaud. - In Spring, the
TokenExchangeOAuth2AuthorizedClientProviderplus an authorized-client interceptor makes this automatic for downstream calls.
Frequently asked questions
Is the “Opaque Token Introspection and Token Exchange” lesson free?
Yes — the full text of “Opaque Token Introspection and Token Exchange” is free to read here on the web, and the Spring Boot 4 Complete Guide course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Spring Boot 4 Complete Guide course, upgrade to CoddyKit PRO.
What will I learn in “Opaque Token Introspection and Token Exchange”?
Validate opaque tokens through introspection and propagate identity with token exchange. You practise Spring Boot 4 Complete Guide with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Spring Boot 4 Complete Guide?
No prior experience is required. Spring Boot 4 Complete Guide on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Opaque Token Introspection and Token Exchange” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
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All lessons in this course
- Resource Server JWT Validation and Claims
- OAuth2 Client and Authorization Code Flow
- Method Security with SpEL and Custom Voters
- Opaque Token Introspection and Token Exchange