What is CVE-2026-0999?

CVE-2026-0999 is a medium-severity vulnerability in Mattermost, classified under Incorrect Implementation of Authentication Algorithm. CVSS score: 5.4/10. Published 2026-02-16.

How severe is CVE-2026-0999?

Medium severity. CVSS v3 base score is 5.4 out of 10.

Vulnerability in Mattermost

CVE-2026-0999

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

EPSS: 0.002 (6.9th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N.

Affected products

Mattermost — versions 11.1.0, 10.11.0, 11.2.0
Mattermost Mattermost_server

Weakness classification (CWE)

CWE-303 — Incorrect Implementation of Authentication Algorithm

References

responsibledisclosure@mattermost.com (vendor-advisory, Vendor Advisory)

Frequently asked questions

What is CVE-2026-0999?: CVE-2026-0999 is a medium-severity vulnerability in Mattermost, classified under Incorrect Implementation of Authentication Algorithm. CVSS score: 5.4/10. Published 2026-02-16.
How severe is CVE-2026-0999?: Medium severity. CVSS v3 base score is 5.4 out of 10.