Oauth2 Resource Server skill

OAuth2 Resource Server

Dependency

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-oauth2-resource-server</artifactId>
</dependency>

Security Configuration

@Configuration
@EnableWebSecurity
@EnableMethodSecurity
public class ResourceServerConfig {

    @Bean
    public SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
        return http
            .csrf(AbstractHttpConfigurer::disable)
            .sessionManagement(s -> s.sessionCreationPolicy(STATELESS))
            .authorizeHttpRequests(auth -> auth
                .requestMatchers("/actuator/health").permitAll()
                .requestMatchers("/api/v1/admin/**").hasAuthority("SCOPE_admin")
                .anyRequest().authenticated()
            )
            .oauth2ResourceServer(oauth2 -> oauth2
                .jwt(jwt -> jwt.jwtAuthenticationConverter(jwtAuthConverter()))
            )
            .build();
    }

    @Bean
    public JwtAuthenticationConverter jwtAuthConverter() {
        // Keep the defaults: the "scope" OR "scp" claim (Okta/Entra use scp) -> SCOPE_*.
        // Calling setAuthoritiesClaimName(...) replaces them — then hasAuthority("SCOPE_admin")
        // can never pass. Extend the defaults with Keycloak realm roles instead.
        var scopes = new JwtGrantedAuthoritiesConverter();
        var converter = new JwtAuthenticationConverter();
        converter.setJwtGrantedAuthoritiesConverter(jwt -> {
            Collection<GrantedAuthority> authorities = new ArrayList<>(scopes.convert(jwt));
            // Keycloak nests realm roles under realm_access.roles -> ROLE_* (for hasRole)
            Map<String, Object> realmAccess = jwt.getClaimAsMap("realm_access");
            if (realmAccess != null && realmAccess.get("roles") instanceof Collection<?> roles) {
                roles.forEach(role -> authorities.add(new SimpleGrantedAuthority("ROLE_" + role)));
            }
            return authorities;
        });
        return converter;
    }
}

application.yml — Common Providers

Choose the provider configuration matching your issuer; these are alternatives, not duplicate spring: mappings to paste into one YAML document.

# Keycloak
spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          issuer-uri: https://keycloak.example.com/realms/my-realm
          jwk-set-uri: https://keycloak.example.com/realms/my-realm/protocol/openid-connect/certs
          audiences: my-api   # without it, tokens minted for ANY client in the realm are accepted

# Auth0
spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          issuer-uri: https://your-domain.auth0.com/
          audiences: https://your-api.example.com  # custom claim validation

Custom Claim Extraction

@Component
public class JwtClaimExtractor {

    // Provider subjects are opaque strings ("auth0|64f1c2", Okta "00u1abcd..."), not UUIDs.
    public String getSubject(JwtAuthenticationToken token) {
        return token.getToken().getSubject();
    }

    public String getEmail(JwtAuthenticationToken token) {
        return token.getToken().getClaimAsString("email");
    }

    public List<String> getRoles(JwtAuthenticationToken token) {
        // Keycloak nests roles under realm_access.roles
        Map<String, Object> realmAccess = token.getToken().getClaimAsMap("realm_access");
        if (realmAccess == null) return List.of();
        return (List<String>) realmAccess.getOrDefault("roles", List.of());
    }
}

Controller — Accessing Current User

Use the exact (iss, sub) pair as the external identity key, with a database unique constraint on that pair. Email and username are optional, mutable profile attributes, not account lookup or automatic account-linking keys, even when email is verified. The ExternalIdentity template accepts an already verified access token; signature, issuer, audience and expiry validation must happen first. Allow only configured issuers; never discover an arbitrary issuer URL from an untrusted token. For existing subject-only mappings, backfill the known issuer before introducing a second issuer. Account linking requires an explicit authenticated flow proving control of both accounts.

See OpenID claim stability.

@RestController
@RequiredArgsConstructor
public class OrderController {

    @GetMapping("/api/v1/orders/my")
    public ApiResponse<List<OrderResponse>> myOrders(
        @AuthenticationPrincipal Jwt jwt  // inject JWT directly
    ) {
        ExternalIdentity identity = ExternalIdentity.from(jwt);
        return ApiResponse.ok(orderService.findByOwner(identity.issuer(), identity.subject()));
    }

    // Or with JwtAuthenticationToken for full principal
    @GetMapping("/api/v1/profile")
    public ApiResponse<ProfileResponse> profile(JwtAuthenticationToken token) {
        ExternalIdentity identity = ExternalIdentity.from(token.getToken());
        return ApiResponse.ok(userService.findByExternalIdentity(identity.issuer(), identity.subject()));
    }
}

Method Security with Scopes

@PreAuthorize("hasAuthority('SCOPE_orders:read')")
public List<Order> findAll() { ... }

@PreAuthorize("hasRole('ADMIN') or @orderSecurity.isOwner(#orderId, authentication)")
public Order findById(UUID orderId) { ... }

// Custom security bean
@Component("orderSecurity")
public class OrderSecurityService {
    public boolean isOwner(UUID orderId, Authentication auth) {
        Jwt jwt = (Jwt) auth.getPrincipal();
        ExternalIdentity identity = ExternalIdentity.from(jwt);
        return orderRepository.existsByIdAndOwnerIssuerAndOwnerSubject(
            orderId, identity.issuer(), identity.subject());
    }
}

Verification scope

The repository fixture targets Boot 3.5.16 with Java 17/21 and checks signed-token validation and issuer/subject mapping; it does not contact a production identity provider. Check the coverage matrix before treating another minor version or an illustrative snippet as verified.

Gotchas

• Agent finds or merges accounts by email - use the verified issuer/subject pair; email changes must not change ownership.
• Agent calls setAuthoritiesClaimName(...) on JwtGrantedAuthoritiesConverter — it replaces the default scope/scp lookup, so SCOPE_* authorities vanish (and Okta/Entra scp scopes are dropped); extend the defaults instead
• Agent parses jwt.getSubject() with UUID.fromString — Auth0 (auth0|...) and Okta (00u...) subjects aren't UUIDs; the request fails with 500
• Agent omits audiences — the resource server then accepts any token the issuer minted for any client
• Agent uses hasRole("ADMIN") for scope check — scopes use hasAuthority("SCOPE_admin")
• Agent forgets issuer-uri validation — always configure to prevent token forgery
• Agent maps roles wrong for Keycloak — roles are nested under realm_access.roles
• Agent uses getPrincipal() directly — cast to Jwt or use @AuthenticationPrincipal Jwt
• Agent adds userDetailsService bean — not needed for resource servers (stateless JWT)