CVE-2026-3473: Mattermost Server

High severity, CVSS 7.1. EPSS: 0.2% 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 validate file ownership and access control, which allows an authenticated user to access and download files belonging to other users or teams via crafted Boards API requests using valid file IDs.. Mattermost Advisory ID: MMSA-2026-00620

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 2026-05-22. Last modified 2026-07-23.