OAuthLib: Timing Attack Vulnerability in PKCE code_verifier Comparison

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-49265
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-13 16:17:20 UTC
Updated2026-10-16 20:04:29 UTC
DescriptionOAuthLib 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.

Risk And Classification

Primary CVSS: v3.1 6.8 MEDIUM from [email protected]

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N

EPSS: 0.003750000 probability, percentile 0.298590000 (date 2026-10-16)

Problem Types: CWE-208 | CWE-208 CWE-208: Observable Timing Discrepancy

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary6.8MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N
3.1CNADECLARED6.8MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
High
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Oauthlib Oauthlib affected >= 3.0.0, < 4.0.0 Not specified

References

ReferenceSourceLinkTags
github.com/oauthlib/oauthlib/security/advisories/GHSA-xpv3-w29h-x7cv [email protected] github.com
github.com/oauthlib/oauthlib/pull/963 [email protected] github.com
github.com/pypa/advisory-database/tree/main/vulns/oauthlib/PYSEC-2026-41... [email protected] github.com
github.com/oauthlib/oauthlib/releases/tag/v4.0.0 [email protected] github.com
github.com/oauthlib/oauthlib/commit/74664d3fe037a354e180e305135c6bab1747... [email protected] github.com
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