CVE-2026-10556: Mattermost Server

Medium severity, CVSS 5.3. EPSS: 0.2% chance of exploitation in the next 30 days.

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate null entries in Microsoft Graph webhook notification payloads, which allows an unauthenticated attacker to crash the Microsoft Calendar plugin process and deny calendar integration service to all users on the instance via a crafted {{POST}} request to the public webhook endpoint.. Mattermost Advisory ID: MMSA-2026-00693

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.23 (fixed in 10.11.23); from 11.7.0, before 11.7.8 (fixed in 11.7.8); from 11.8.0, before 11.8.5 (fixed in 11.8.5); from 11.9.0, before 11.9.1 (fixed in 11.9.1)

Published 2026-09-14. Last modified 2026-10-07.