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 apps
Strongly 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:
GET /authorize?response_type=codeandstate=randomString123
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 and least privilege
Yes
All clients
Enable refresh token rotation
Yes
When using refresh
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How do you secure the OAuth 2.0 authorization code flow?

ICSM Computer

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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

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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