OAuth 2.0 Authorization Code Flow vs Client Credentials Flow: What Are the Differences?

sequenceDiagram
    participant User
    participant App as Client App
    participant AuthServer as Authorization Server
    participant Resource as Resource Server

    User->>App: 1. Click Login
    App->>AuthServer: 2. Authorization Request
    AuthServer->>User: 3. Login Page
    User->>AuthServer: 4. Authenticate
    AuthServer->>App: 5. Authorization Code
    App->>AuthServer: 6. Exchange Code for Token
    AuthServer->>App: 7. Access Token + Refresh Token
    App->>Resource: 8. API Request with Token
    Resource->>App: 9. Protected Resource

OAuth 2.0 offers multiple flows designed to accommodate different use cases, ranging from user-driven web apps to backend services operating without direct user interaction. Two commonly used flows in the ecosystem are the Authorization Code Flow and the Client Credentials Flow. Each serves distinct purposes and understanding their differences is critical for building secure and efficient authentication systems.

Understanding the Authorization Code Flow

The Authorization Code Flow is primarily designed for applications that involve user interaction. It allows an application to obtain an authorization code after the user authenticates, which is then exchanged on the server side for an access token. This flow supports features like refresh tokens and scopes and is commonly used in web and mobile applications.

What is the Client Credentials Flow?

The Client Credentials Flow is intended for machine-to-machine (M2M) communication where no user is involved. Here, the client application directly requests an access token from the authorization server by authenticating itself with its client ID and client secret. This flow is ideal for service accounts, APIs, or backend microservices that need to authenticate and authorize themselves to access protected resources.

Key Differences

Security Considerations

Authorization Code Flow leverages an intermediate authorization code and often PKCE to mitigate interception risks, especially for public clients. The Client Credentials Flow depends on secure storage of client credentials, since the token request is made directly without user context.

Example Scenario: Using Client Credentials Flow

Imagine a backend microservice that needs to call another API to fetch data on behalf of the service itself (not a user). It uses Client Credentials Flow to authenticate to the authorization server, obtain an access token, and then call the downstream API securely.

POST /token HTTP/1.1
Host: authorization-server.com
Content-Type: application/x-www-form-urlencoded

grant_type=client_credentials&
client_id=your_client_id&
client_secret=your_client_secret&
scope=read:data

The server responds with an access token usable by the microservice.

Choosing the Right Flow

• Use Authorization Code Flow when your application involves user authentication and you need to act on behalf of a user.
• Use Client Credentials Flow for server-to-server communication without user context.
• When building modern SPAs, prefer Authorization Code Flow with PKCE instead of Implicit Flow for better security.

Thought-Provoking Questions

• How do you securely store client secrets in your backend or microservices?
• Are there scenarios where you might combine both flows in a single application ecosystem?

Conclusion

That’s client credentials flow. Simple, secure, no users involved. Perfect for service-to-service auth.

👉 Related: Understanding the Authorization Code Flow in OAuth 2.0

👉 Related: Understanding the Authorization Code Flow with PKCE in OAuth 2.0