How to Fix 'Cannot Convert Access Token to JSON' Error When Using Spring OAuth2 with JWT in Separate Auth and Resource Servers?

How can I resolve the 'Cannot convert access token to JSON' error when using Spring OAuth2 with JWT in a setup with separate authentication and resource servers?

// Example of token configuration in a Spring Boot application configuration class
@Bean
public JwtDecoder jwtDecoder() {
    return NimbusJwtDecoder.withJwkSetUri(jwkSetUri).build();
}

When implementing Spring Security with OAuth2 using JWT (JSON Web Tokens) in a microservices architecture where authentication and resource servers are separated, you may encounter a common error: 'Cannot convert access token to JSON'. This issue usually arises due to misconfiguration or discrepancies in token handling between the two servers.

// Configuring JWT on the Resource Server
@EnableResourceServer
public class ResourceServerConfig extends ResourceServerConfigurerAdapter {
    @Override
    public void configure(HttpSecurity http) throws Exception {
        http
            .authorizeRequests()
            .antMatchers("/api/public").permitAll()
            .anyRequest().authenticated();
    }
}

Causes

• The resource server is not correctly configured to validate the JWT access tokens from the authentication server.
• The opaque token being sent to the resource server is not a valid JWT, resulting in a deserialization error.
• Missing dependencies for JWT handling in the Spring context.

Solutions

• Ensure that both authentication and resource servers are properly configured to use the same JWT signing algorithm and public keys.
• Implement the correct JWT decoder in the resource server that matches the way tokens are issued by the auth server.
• Validate that the resource server is aware of the issuer and audience parameters defined in the JWT tokens.

Mistake: Not including the correct 'aud' (audience) claim in the JWT token.

Solution: Make sure to set the 'aud' claim when generating the token and validate it in your resource server.

Mistake: Omitting dependency for JWT support in the Maven/Gradle configuration.

Solution: Include 'spring-security-oauth2-resource-server' and 'spring-security-oauth2-jose' dependencies in your project.

Mistake: Incorrect URL configuration for the public keys or JWK.

Solution: Check the JWK Set URI endpoint and ensure it returns valid keys for JWT signature verification.