CVE-2026-4915: Mattermost Server

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

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to filter nil elements from outgoing webhook attachment payloads before processing, which allows an authenticated user to cause a denial of service (server process termination) via a crafted webhook callback response containing a null attachment entry.. Mattermost Advisory ID: MMSA-2026-00641

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.15 (fixed in 10.11.15); from 11.4.0, before 11.4.5 (fixed in 11.4.5); from 11.5.0, before 11.5.4 (fixed in 11.5.4); from 11.6.0, before 11.6.1 (fixed in 11.6.1)

Published 2026-05-25. Last modified 2026-07-23.