CVE-2026-49265 OAuthLib: Timing Attack Vulnerability in PKCE code_verifier Comparison
Timing side-channel in OAuthLib PKCE verification allows potential recovery of code_verifier, leading to unauthorized access token acquisition and account takeover.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-49265?
CVE-2026-49265 is a medium-severity vulnerability in oauthlib, published on 13 Oct 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
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.
Weakness: CWE-208
Which versions are affected by CVE-2026-49265?
3 open source packages ship an affected version, including oauthlib.
| Package | Affected | Fixed in |
|---|---|---|
oauthlib PyPI | From 3.0.0 | 4.0.0 |
oauthlib PyPI | From 3.0.0 | 4.0.0 |
oauthlib PyPI | From 3.0.0 | 4.0.0 |
How do I fix CVE-2026-49265?
Upgrade oauthlib to 4.0.0 or later.
Is CVE-2026-49265 being exploited?
There is no known exploitation of CVE-2026-49265 in the wild and no public exploit code.
Sources
Also known as AZL-108045, CLEANSTART-2026-NO84858, CLEANSTART-2026-PQ13819, CLEANSTART-2026-ZA90599, DEBIAN-CVE-2026-49265, ECHO-e653-cb4c-55e4 and 27 more.