What is CVE-2026-27656?

CVE-2026-27656 is a medium-severity vulnerability in Mattermost, classified under Incorrect Implementation of Authentication Algorithm. CVSS score: 5.7/10. Published 2026-03-25.

How severe is CVE-2026-27656?

Medium severity. CVSS v3 base score is 5.7 out of 10.

Vulnerability in Mattermost

CVE-2026-27656

Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590

EPSS: 0.002 (7.8th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 5.7 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N.

Affected products

Mattermost — versions 11.4.0, 11.3.0, 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-2026-27656?: CVE-2026-27656 is a medium-severity vulnerability in Mattermost, classified under Incorrect Implementation of Authentication Algorithm. CVSS score: 5.7/10. Published 2026-03-25.
How severe is CVE-2026-27656?: Medium severity. CVSS v3 base score is 5.7 out of 10.