CVE-2026-3495: Mattermost Server

Medium severity, CVSS 4.8. EPSS: 0.2% chance of exploitation in the next 30 days.

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to escape some variables that could contain malicious content during error page composition which allows an attacker with access to edit some site configuration to execute some malicious code via injecting some JS as part of those values.. Mattermost Advisory ID: MMSA-2026-00622

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.14 (fixed in 10.11.14); from 11.5.0, before 11.5.2 (fixed in 11.5.2)

Published 2026-05-18. Last modified 2026-06-17.