CVE-2026-6339: Mattermost Server

Medium severity, CVSS 4.3. EPSS: 0.2% chance of exploitation in the next 30 days.

Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636

Affected products

• Mattermost Mattermost Server: 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 2026-05-18. Last modified 2026-06-17.