Описание
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:
- Returns
Falseimmediately if lengths differ - 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
- Attacker intercepts
authorization_codevia Custom URI Scheme Hijacking - PKCE blocks token request — attacker lacks
code_verifier - Attacker sends repeated requests to
/tokenendpoint measuring response times - Using timing oracle, attacker recovers
code_verifiercharacter by character - 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
Ссылки
- https://github.com/oauthlib/oauthlib/security/advisories/GHSA-xpv3-w29h-x7cv
- https://nvd.nist.gov/vuln/detail/CVE-2026-49265
- https://github.com/oauthlib/oauthlib/pull/963
- https://github.com/oauthlib/oauthlib/commit/40b0ab56da3682c2484a4b78bbff309f8025d950
- https://github.com/pypa/advisory-database/tree/main/vulns/oauthlib/PYSEC-2026-4114.yaml
Пакеты
oauthlib
>= 3.0.0, < 4.0.0
4.0.0
Связанные уязвимости
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.
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.
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.
OAuthLib: Timing Attack Vulnerability in PKCE code_verifier Comparison
OAuthLib is an implementation of the OAuth request-signing logic for P ...