Oauthlib: Timing Attack Vulnerability in PKCE code_verifier Comparison (CWE-208)

Summary

A timing side-channel vulnerability exists in the PKCE (RFC 7636) implementation of the Authorization Code Grant flow. The code_challenge_method_plain function uses Python's standard == operator for string comparison instead of a constant-time comparison function, potentially allowing timing-based attacks.

Affected Component

• File: oauthlib/oauth2/rfc6749/grant_types/authorization_code.py
• Functions: code_challenge_method_plain, code_challenge_method_s256
• Vulnerability Type: CWE-208 (Observable Timing Discrepancy)

Technical Details

Python's == operator uses short-circuit evaluation when comparing strings:

1. Returns False immediately if lengths differ
2. Compares characters left-to-right, stopping at first mismatch

This means comparison time varies linearly with the length of the common prefix between the attacker-supplied verifier and the stored challenge, creating a measurable timing oracle.

Proof of Concept

Tested locally against oauthlib source (network jitter eliminated to isolate pure Python execution time):

| Input | Result | Time (10M iterations) | |---|---|---| | Wrong first char (B + A*49) | Fast reject | 0.34106s | | 49 chars correct (A*49 + B) | Deep compare | 0.37847s | | Difference | | 0.03741s |

The ~37ms delta over 10M iterations corresponds to nanosecond-level differences per call, which are statistically exploitable under controlled conditions.

Attack Scenario

1. Attacker intercepts authorization_code via Custom URI Scheme Hijacking
2. PKCE blocks token request — attacker lacks code_verifier
3. Attacker sends repeated requests to /token endpoint measuring response times
4. Using timing oracle, attacker recovers code_verifier character by character
5. Attacker obtains Access Token → Account Takeover

Note: Practical exploitability is limited due to the single-use nature of authorization codes and real-world network noise. However, the vulnerable pattern should be corrected as a defense-in-depth measure.

Recommended Fix

Replace == with hmac.compare_digest() for constant-time comparison:

cr: Elvin Latifli