Mattermost: Login Method Bypass via UID in v10.11.x-11.2.1 (SSO Bypass)
CVE-2026-0999 Published on February 16, 2026
Authentication bypass via userID login when email and username login are disabled
Mattermost versions 11.1.x <= 11.1.2, 10.11.x <= 10.11.9, 11.2.x <= 11.2.1 fail to properly validate login method restrictions which allows an authenticated user to bypass SSO-only login requirements via userID-based authentication. Mattermost Advisory ID: MMSA-2025-00548
Vulnerability Analysis
CVE-2026-0999 can be exploited with network access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have a small impact on confidentiality and integrity, and no impact on availability.
Weakness Type
Incorrect Implementation of Authentication Algorithm
The requirements for the software dictate the use of an established authentication algorithm, but the implementation of the algorithm is incorrect. This incorrect implementation may allow authentication to be bypassed.
Products Associated with CVE-2026-0999
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Affected Versions
Mattermost:- Version 11.1.0, <= 11.1.2 is affected.
- Version 10.11.0, <= 10.11.9 is affected.
- Version 11.2.0, <= 11.2.1 is affected.
- Version 11.3.0 is unaffected.
- Version 11.1.3 is unaffected.
- Version 10.11.10 is unaffected.
- Version 11.2.2 is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.