CVE-2026-49265

CVE-2026-49265 · OAuthLib: Timing Attack Vulnerability in PKCE code_verifier Comparison

This page contains facts from public vulnerability sources. Check the original advisory before updating.

OAuthLib is an implementation of the OAuth request-signing logic for Python 3.6+. From 3.0.0 until 4.0.0, OAuthLib's PKCE checks in oauthlib/oauth2/rfc6749/grant_types/authorization_code.py compare attacker-supplied code_verifier values with stored challenges using ordinary string equality in code_challenge_method_plain and code_challenge_method_s256. Because the comparison can return after a length difference or the first mismatching character, repeated token endpoint measurements can reveal common-prefix timing information that could potentially allow recovery of the code_verifier, unauthorized access-token acquisition, and account takeover. Practical exploitation requires an intercepted, single-use authorization code and enough measurements to overcome real-world network noise; the local timing benchmark does not establish practical remote account takeover, so the weakness remains a defense-in-depth concern. This issue is fixed in version 4.0.0.

CVSS 6.8 · Medium · EPSS 0.2% · Public exploit · Fix available

Published

2026-10-13T15:47:11.854Z

Updated

2026-10-13T16:19:39.092Z

Stack: system · python

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