How do you secure the OAuth 2.0 authorization code flow?

Question

ICSM Computer · Answer

Securing the OAuth 2.0 Authorization Code Flow is critical because it involves handling sensitive tokens. Here's how to secure it effectively: 1. Use PKCE (Proof Key for Code Exchange) Required for public clients like SPAs and mobile appsStrongly recommended for all clients, even confidential ones How PKCE Works: Client generates a code verifier and a code challenge.During authorization request: Sends code_challenge + code_challenge_method (usually SHA256). During token request: Sends original code_verifier. Server validates the challenge. Benefits: Prevents authorization code interception. 2. Use HTTPS Only OAuth 2.0 MUST use TLS/SSL to prevent MITM attacks. Never transmit tokens over plain HTTP.Reject any non-HTTPS redirect URIs. 3. Register and Validate Redirect URIs Prevent open redirect attacks by enforcing strict URI matching. Always pre-register allowed redirect URIs. Enforce exact match (no wildcards like *). 4. Use Short-lived Authorization Codes Limit validity of the code to 60–120 seconds.Prevent code reuse. 5. Use State Parameter to Prevent CSRF Include a random state in the authorization request and validate it when the response comes back. Example: When user returns, verify the state matches.Helps prevent Cross-Site Request Forgery (CSRF) attacks. 6. Use Scopes Wisely Limit token capabilities Only request what's needed: openid profile email etc.Enforce least privilege on access tokens. 7. Store Tokens Securely Client Type Store in Web App Encrypted server-side session store SPA In-memory (not localStorage) with refresh tokens using refresh token rotation Mobile App Secure storage (e.g., Keychain, Android Keystore) 8. Enable Refresh Token Rotation Issue a new refresh token every time it’s used.Invalidate the old one.Prevents replay attacks if the refresh token is stolen. 9. Log and Monitor Track token usage and anomalies.Set up alerts for suspicious activity. Summary Checklist Security Measure Mandatory? Applies To Use HTTPS Yes All clients Implement PKCE Yes Mobile, SPA (public) Validate state param Yes All clients Register redirect URIs Yes All clients Use short-lived auth codes Yes All clients Store tokens securely Yes All clients Use scopes & least privilege Yes All clients Enable refresh token rotation Yes When using refresh

Client Type	Store in
Web App	Encrypted server-side session store
SPA	In-memory (not localStorage) with refresh tokens using refresh token rotation
Mobile App	Secure storage (e.g., Keychain, Android Keystore)

Security Measure	Mandatory?	Applies To
Use HTTPS	Yes	All clients
Implement PKCE	Yes	Mobile, SPA (public)
Validate state param	Yes	All clients
Register redirect URIs	Yes	All clients
Use short-lived auth codes	Yes	All clients
Store tokens securely	Yes	All clients
Use scopes & least privilege	Yes	All clients
Enable refresh token rotation	Yes	When using refresh

interview

How do you secure the OAuth 2.0 authorization code flow?

Can you answer this question?

1 Answers

1. Use PKCE (Proof Key for Code Exchange)

How PKCE Works:

Benefits:

2. Use HTTPS Only

3. Register and Validate Redirect URIs

4. Use Short-lived Authorization Codes

5. Use State Parameter to Prevent CSRF

Example:

6. Use Scopes Wisely

7. Store Tokens Securely

8. Enable Refresh Token Rotation

9. Log and Monitor

Summary Checklist