How to Fix CORS Issues in Spring Gateway Security

Cross-Origin Resource Sharing (CORS) is a critical aspect of modern web applications, especially when your frontend and backend are hosted on different origins. In this blog post, we’ll explore how to configure CORS in a Spring Gateway with security and ensure it works seamlessly.

Common CORS Issue Scenario

You might encounter the following error when making cross-origin requests from a frontend application:

Access to XMLHttpRequest at 'https://api.example.com/resource' from origin 'https://frontend.example.com' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

While testing APIs in Postman works fine, the browser enforces CORS policies, leading to this error when integrating with a frontend.

Typical Causes

Missing or incorrect CORS configuration.
Conflicts between Spring Security and global CORS settings.
Preflight OPTIONS requests not handled properly.

Let’s walk through the solution.

Step-by-Step Solution

1. Enable CORS in Spring Gateway Security

When using Spring Security, it’s essential to integrate CORS configuration directly within your SecurityWebFilterChain to ensure preflight requests are handled correctly.

Here’s a concise solution:

@Bean
public SecurityWebFilterChain securityFilterChain(ServerHttpSecurity httpSecurity) {
    return httpSecurity
        .cors(cors -> cors.configurationSource(request -> {
            CorsConfiguration config = new CorsConfiguration();
            config.addAllowedOrigin("*"); // Replace '*' with specific origins in production
            config.addAllowedMethod("*");
            config.addAllowedHeader("*");
            config.setAllowCredentials(true); // Optional, if credentials are used
            return config;
        }))
        .csrf(ServerHttpSecurity.CsrfSpec::disable)
        .authorizeExchange(authorize -> authorize
            .pathMatchers(
                "/public-endpoints/**", "/swagger-ui/**", "/v3/api-docs/**"
            ).permitAll()
            .anyExchange().authenticated()
        )
        .oauth2ResourceServer(oauth2 -> oauth2.jwt(Customizer.withDefaults()))
        .build();
}

Key Highlights

Inline CORS Configuration: Tying the CorsConfigurationSource directly to httpSecurity.cors() ensures Spring Security handles CORS requests properly.
Allow Credentials: Enable this only if you’re dealing with cookies or authorization headers that require credentials.

2. Using Global CORS Configuration

Spring Gateway provides a global CORS configuration option in application.yml that can also handle CORS for all routes. However, this might conflict with Spring Security settings if not done correctly.

`application.yml`

server:
    servlet:
        context-path: /
spring:
  cloud:
    gateway:
      globalcors:
        cors-configurations:
          '[/**]':
            allowedOrigins: "*" # Replace with specific origins for production
            allowedMethods: "*"
            allowedHeaders: "*"

While this approach is simpler, ensure that the SecurityWebFilterChain does not disable CORS (.cors(ServerHttpSecurity.CorsSpec::disable)) to avoid conflicts.

3. Debugging and Testing CORS

If you’re still facing issues, follow these debugging steps:

Check the Preflight Request

A CORS preflight is an OPTIONS request sent by the browser to verify CORS permissions. Test it using curl:

curl -X OPTIONS \
  -H "Origin: https://frontend.example.com" \
  -H "Access-Control-Request-Method: GET" \
  https://api.example.com/resource -I

Ensure the response includes headers like:

Access-Control-Allow-Origin: https://frontend.example.com
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS
Access-Control-Allow-Headers: Content-Type, Authorization

Inspect Gateway Logs

Enable debug logs for Spring Gateway to see how requests are routed and whether CORS headers are being set:

logging:
  level:
    org.springframework.cloud.gateway: DEBUG

Verify with Browser DevTools

Use browser developer tools to inspect network requests. Look for the OPTIONS request and verify the response headers.

4. Avoiding Common Pitfalls

Do Not Use * for Credentials: If config.setAllowCredentials(true) is set, avoid using * for allowedOrigins. Specify exact origins instead.

Example:
```
 config.setAllowedOrigins(List.of("https://frontend.example.com"));
```
Downstream Services: If the gateway forwards requests to other services, ensure those services also handle CORS properly.
Version Compatibility: Ensure you’re using compatible versions of Spring Boot, Spring Cloud Gateway, and Spring Security.

Conclusion

Fixing CORS issues in a Spring Gateway with security involves careful coordination between Spring Security and the gateway’s global CORS configuration. The recommended approach is to configure CORS directly within the SecurityWebFilterChain to ensure proper handling of preflight requests.

By following the steps outlined above, you can eliminate CORS-related errors and enable seamless cross-origin communication for your applications.

Happy coding! 🚀