Protecting Your Spring Boot Application with OAuth2 Resource Server and Auth0

Security is non-negotiable in modern application development. As APIs become the backbone of digital services, protecting them with robust authentication and authorization mechanisms is critical. OAuth2 has become the de facto standard for securing APIs, and when combined with an identity provider like Auth0, it provides a scalable and secure solution for managing access.

In this guide, we’ll walk through how to configure a Spring Boot application as an OAuth2 Resource Server using Auth0 as the authorization server. By the end of this article, your application will be protected against unauthorized access, leveraging JWT (JSON Web Tokens) for secure API calls.

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Why OAuth2 Resource Server?

Before diving into implementation, let’s establish why OAuth2 Resource Server is the preferred approach for securing your Spring Boot API:

• Decouples authentication from the application: The authentication logic is handled by an external identity provider (Auth0), reducing security risks.
• Token-based authentication: Stateless and scalable authorization using JWTs ensures performance and security.
• Industry-standard security: OAuth2 is widely adopted and integrates well with modern security best practices.

Now, let’s get our hands dirty and configure our Spring Boot API with Auth0 as the OAuth2 provider.

Step 1: Set Up an Auth0 Account and API

1. Sign up on Auth0: If you don’t have an Auth0 account yet, sign up here.
2. Create an API:
   • Navigate to Applications > APIs.
   • Click Create API.
   • Enter a name (e.g., Spring Boot API).
   • Set the Identifier (e.g., https://myapi.com).
   • Choose RS256 as the Signing Algorithm.
   • Save the API.

Step 2: Configure Spring Boot as an OAuth2 Resource Server

Add Dependencies

Ensure your pom.xml includes the necessary dependencies:

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

Configure Security Properties

In application.yml, set up the OAuth2 configuration:

spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          issuer-uri: <https://your-auth0-domain/>

Replace your-auth0-domain with your actual Auth0 domain.

Step 3: Create Endpoints

Now, let’s create our controller:

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/orders")
public class SecuredController {

    @GetMapping
    public ResponseEntity<List<OrderDTO>> findAll() {
        return ResponseEntity.ok(List.of(
            new OrderDTO(1, "Order 1"),
            new OrderDTO(2, "Order 2"),
            new OrderDTO(3, "Order 3")
        ));
    }
}

Step 4: Testing the Integration

1. Get a JWT Token

Use Auth0’s Test API section to generate a JWT token.

2. Call the API with a Token

Use curl or Postman to test:

curl -H "Authorization: Bearer YOUR_ACCESS_TOKEN" <http://localhost:8080/api/protected>

If the token is valid, the API will return a response. Otherwise, it will return a 401 Unauthorized.

Conclusion

Integrating OAuth2 Resource Server with Auth0 in a Spring Boot application ensures robust security while keeping your application stateless and scalable. With this setup, you:

• Offload authentication to a trusted provider (Auth0)
• Secure API endpoints with JWT authentication
• Maintain a seamless, secure user experience

By implementing these best practices, you ensure that your Spring Boot API remains protected and resilient against unauthorized access.