Vulnerability · Published Oct 13, 2026 · Assigned by GitHub_M
CVE-2026-49265OAuthLib: Timing Attack Vulnerability in PKCE code_verifier Comparison
Medium 6.8
In plain English
A medium-severity security flaw in oauthlib from oauthlib. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 15% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | python-oauthlib | no fix listed | DEBIAN-CVE-2026-49265 |
| Debian:13 | python-oauthlib | no fix listed | DEBIAN-CVE-2026-49265 |
| Debian:14 | python-oauthlib | no fix listed | DEBIAN-CVE-2026-49265 |
| PyPI | oauthlib | 4.0.0 | GHSA-xpv3-w29h-x7cv |
| PyPI | oauthlib | 4.0.0 | PYSEC-2026-4114 |
| Ubuntu:22.04:LTS | python-oauthlib | no fix listed | UBUNTU-CVE-2026-49265 |
| Ubuntu:24.04:LTS | python-oauthlib | no fix listed | UBUNTU-CVE-2026-49265 |
| Ubuntu:26.04:LTS | python-oauthlib | no fix listed | UBUNTU-CVE-2026-49265 |
| Ubuntu:26.10 | python-oauthlib | no fix listed | UBUNTU-CVE-2026-49265 |
Affected products (as reported)
Weakness: CWE-208 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N
References
- https://github.com/oauthlib/oauthlib/security/advisories/GHSA-xpv3-w29h-x7cv x_refsource_CONFIRM
- https://github.com/oauthlib/oauthlib/pull/963 x_refsource_MISC
- https://github.com/oauthlib/oauthlib/commit/74664d3fe037a354e180e305135c6bab1747a6b0 x_refsource_MISC
- https://github.com/oauthlib/oauthlib/releases/tag/v4.0.0 x_refsource_MISC
- https://github.com/pypa/advisory-database/tree/main/vulns/oauthlib/PYSEC-2026-4114.yaml x_refsource_MISC