How do you implement client credentials flow in OAuth2?

Table of Contents

• Introduction
• What is OAuth2 Client Credentials Flow?
  • When to Use the Client Credentials Flow?
• Step-by-Step Implementation in Spring Boot
  • Step 1: Add Required Dependencies
  • Step 2: Configure OAuth2 Client in application.properties
    • Example application.properties for OAuth2 Client Credentials:
  • Step 3: Implement OAuth2 Client to Request Tokens
    • Using WebClient to Request an Access Token
  • Step 4: Secure API Calls Using Access Token
  • Step 5: Testing Client Credentials Flow
• Conclusion

Introduction

The OAuth2 Client Credentials Flow is a mechanism in OAuth2 that allows a client (usually a service or application) to authenticate itself with an authorization server and obtain an access token. This flow is used in server-to-server communication, where no user is involved, and the client application is authorized to access its own resources or other protected resources on behalf of the application. This is useful for APIs or services where access doesn't depend on a user's credentials but rather on the client's identity and permissions.

In this guide, we’ll explore how to implement the OAuth2 Client Credentials Flow in a Spring Boot application to enable secure communication between services.

What is OAuth2 Client Credentials Flow?

In the Client Credentials Flow, the client application authenticates itself with an authorization server using its client ID and client secret. The authorization server returns an access token, which the client can use to authenticate API requests. This flow is often used for machine-to-machine authentication (i.e., no user involvement).

When to Use the Client Credentials Flow?

• Server-to-server communication: A service needs to authenticate itself to another service.
• Accessing own resources: The client application needs access to its own resources stored on a server.
• No user context required: The flow doesn’t involve user login, only the client’s credentials.

Step-by-Step Implementation in Spring Boot

To implement OAuth2 Client Credentials Flow in Spring Boot, you need to set up the following:

1. Authorization Server: A service that issues tokens (you can use an external OAuth2 provider like Google, or you can set up your own).
2. Client Application: A Spring Boot application that requests tokens and makes secure requests to APIs.

Step 1: Add Required Dependencies

For Maven, include the following dependencies in your pom.xml file:

For Gradle, add these dependencies in your build.gradle file:

Step 2: Configure OAuth2 Client in application.properties

Now, you need to configure your Spring Boot application to request tokens using the client credentials flow. This is done by adding the necessary OAuth2 configuration in the application.properties or application.yml.

Example application.properties for OAuth2 Client Credentials:

• client-id and client-secret: These are credentials provided by the authorization server when registering your application.
• token-uri: The endpoint on the authorization server to which the client sends requests to get an access token.
• scope: The permissions or actions the client can perform (e.g., read, write).
• authorization-grant-type: Set this to client_credentials to specify the client credentials flow.

Step 3: Implement OAuth2 Client to Request Tokens

In Spring Boot, you can use the OAuth2RestTemplate to make requests to the authorization server for an access token. However, starting from Spring Security 5, the recommended approach is to use WebClient or RestTemplate with the OAuth2 Client support.

Using WebClient to Request an Access Token

Here’s an example of how you can use WebClient to send a request to the OAuth2 authorization server and fetch an access token.

1. Create a **WebClient** Bean for OAuth2 Access:

2. Request the Access Token Using **WebClient**:

This code sends a POST request to the OAuth2 authorization server’s /oauth/token endpoint with the client credentials and retrieves an access token.

Step 4: Secure API Calls Using Access Token

Once you’ve obtained the access token, you can use it to authenticate API requests to your protected resources. For instance:

In this example:

• The OAuth2Service retrieves the access token.
• The ApiService uses the access token in the Authorization header when calling a protected API.

Step 5: Testing Client Credentials Flow

You can test the OAuth2 Client Credentials flow by calling the fetchDataFromApi() method in your application. The method will first retrieve the access token using the client credentials flow, and then use that token to authenticate the API call.

Conclusion

The OAuth2 Client Credentials Flow is a powerful way to authenticate and authorize applications to access resources on behalf of themselves (not users). By using Spring Security and WebClient, you can securely integrate this flow into your Spring Boot application, ensuring that your services can communicate with each other using secure access tokens. This setup is ideal for server-to-server communication or machine-to-machine authentication.