CVE-2026-49265

What it is

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.

At a glance

Exploitability

Can be exploited over the network without logging in, but needs a user to act. High risk of data exposure, data tampering.

Exploit odds

0.2% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2026-05-28 14:33 UTC ReservedThe CVE id was reserved by its CNA
2. 2026-10-13 16:17 UTC PublishedDisclosed publicly on CVE.org and NVD
3. 2026-10-16 20:04 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 0.2% in the next 30 days (EPSS).
• Severity: 6.8 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2026-10-13 16:17 UTC

Last modified

2026-10-16 20:04 UTC

Exploit probability

0.2% · EPSS, next 30 days

Weakness

CWE-208

Record status

Deferred

Attack profile CVSS 3.1 · 6.8

Attack vector

Network

Complexity

High

Privileges needed

None

User interaction

Required

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

None

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2026-49265.md and follow it. Check whether this project is affected by CVE-2026-49265. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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