CVE-2026-0999: Authentication bypass via userID login when email and username login are disabled

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-02-16 · updated 2026-06-17
Weakness: CWE-303

Description

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

Affected products

Mattermost: from 10.11.0 before 10.11.10; from 11.1.0 before 11.1.3; from 11.2.0 before 11.2.2 — fixed in 10.11.10, 11.1.3, 11.2.2

How to fix

Upgrade to a fixed version: Mattermost 10.11.10, 11.1.3, 11.2.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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