Oauthlib: Timing Attack Vulnerability in PKCE code_verifier Comparison (CWE-208)

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-49265: 0.24% estimated exploitation activity in the next 30 days; percentile 15.48 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A timing side-channel vulnerability exists in the PKCE (RFC 7636) implementation of the Authorization Code Grant flow. The codechallengemethodplain 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/granttypes/authorizationcode.py - Functions: codechallengemethodplain, codechallengemethods256 - Vulnerability Type: CWE-208 (Observable Timing Discrepancy) Technical Details Python's == operator uses short-circuit evaluation when comparing strings: 1. Returns False immediately if lengths differ 2. 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…

Published by the source

29 Sep 2026, 17:56 UTC

Last modified by the source

13 Oct 2026, 16:00 UTC

Source last checked

26 Oct 2026, 13:55 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-xpv3-w29h-x7cv

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-xpv3-w29h-x7cv

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-xpv3-w29h-x7cv

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# Patch Next advisory brief: CVE-2026-49265

Advisory: Oauthlib: Timing Attack Vulnerability in PKCE code_verifier Comparison (CWE-208)
IDs: CVE-2026-49265, GHSA-xpv3-w29h-x7cv
Record: https://patchnext.com/advisories/cve-2026-49265 (revision 3, source checked 26 Oct 2026, 13:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: oauthlib 4.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI oauthlib: affected >= 3.0.0, < 4.0.0; fixed in 4.0.0

## Suggested inspection (read-only)
- `python -m pip show oauthlib`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on oauthlib.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-xpv3-w29h-x7cv (GitHub-reviewed advisory GHSA-xpv3-w29h-x7cv lists affected ranges: PyPI oauthlib >=3.0.0 <4.0.0)
- Verified: https://osv.dev/vulnerability/GHSA-xpv3-w29h-x7cv (OSV record GHSA-xpv3-w29h-x7cv lists affected ranges: PyPI oauthlib >=3.0.0 <4.0.0 (fixed))

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