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
| Feature | Authorization Code Flow | Client Credentials Flow |
|---|---|---|
| User Involvement | Yes, user authentication required | No user involved |
| Token Request Method | Authorization code exchanged via backend server | Direct token request using client credentials |
| Use Case | User-facing apps (web, mobile) | Backend services, APIs, M2M |
| Support for Refresh Token | Yes | Typically no |
| Security Considerations | Requires secure backend for token exchange | Client secret must be securely stored |
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?
- How does token scope management differ between these flows in your use case?
Conclusion
Both Authorization Code Flow and Client Credentials Flow are fundamental OAuth 2.0 flows tailored for different application scenarios. Selecting the appropriate flow based on whether user interaction is needed or not is key to maintaining security and functionality. For modern architectures, mixing these flows where necessary while following security best practices will yield the best results.
👉 Related: Understanding the Authorization Code Flow in OAuth 2.0
👉 Related: Understanding the Authorization Code Flow with PKCE in OAuth 2.0