Oauthlib versions >=3.0.0 <4.0.0 contain a timing side-channel vulnerability (CWE-208) in the PKCE code_verifier comparison within the Authorization Code Grant flow. The functions code_challenge_method_plain and code_challenge_method_s256 use Python's '==' operator for string comparison, which short-circuits on the first mismatch and leaks timing information proportional to the length of the matching prefix. This timing discrepancy can be exploited by an attacker who intercepts an authorization code and repeatedly sends token requests, measuring response times to recover the code_verifier character by character. The vulnerability is mitigated by replacing '==' with hmac.compare_digest(), a constant-time comparison function. The CVSS v3.1 score is 6.8 (medium severity) with network attack vector, high attack complexity, no privileges required, user interaction required, and high confidentiality and integrity impact.

Successful exploitation allows an attacker to recover the PKCE code_verifier via timing analysis, potentially leading to access token theft and account takeover. However, practical exploitation is constrained by the single-use nature of authorization codes and real-world network noise, limiting the feasibility of the attack in typical environments.

A patch is available that replaces the vulnerable string comparison with a constant-time comparison using hmac.compare_digest(). Users of oauthlib versions >=3.0.0 <4.0.0 should upgrade to a fixed version. No additional mitigations are indicated as the vulnerability is addressed by this fix.