CVE-2026-27656: Mattermost Server

Medium severity, CVSS 6.1. EPSS: 0.3% chance of exploitation in the next 30 days.

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

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.12 (fixed in 10.11.12); from 11.2.0, before 11.2.4 (fixed in 11.2.4); from 11.3.0, before 11.3.2 (fixed in 11.3.2); from 11.4.0, before 11.4.1 (fixed in 11.4.1)

Published 2026-03-25. Last modified 2026-06-17.