CVE-2026-10103: Mattermost Server

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

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to verify post ownership in the shared channel inbound sync handler, which allows an authenticated remote cluster to modify or delete posts authored by local users or other remotes via crafted sync messages referencing arbitrary post IDs in channels shared with that remote.. Mattermost Advisory ID: MMSA-2026-00689

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.20 (fixed in 10.11.20); from 11.6.0, before 11.6.5 (fixed in 11.6.5); from 11.7.0, before 11.7.3 (fixed in 11.7.3)

Published 2026-07-13. Last modified 2026-07-14.