CVE-2026-4054: Mattermost Server

Medium severity, CVSS 6.5. EPSS: 0.4% chance of exploitation in the next 30 days.

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 Fail to validate the response body of proxied images, which allows a remote attacker to enact client-side DoS via an SVG file served from an attacker-controlled origin under a non-SVG Content-Type header (e.g. image/png) embedded in an og:image meta tag or Markdown image link.. Mattermost Advisory ID: MMSA-2026-00630

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.14 (fixed in 10.11.14); from 11.4.0, before 11.4.4 (fixed in 11.4.4); from 11.5.0, before 11.5.2 (fixed in 11.5.2)

Published 2026-05-15. Last modified 2026-06-17.