CVE-2026-9708: Mattermost Server

Medium severity, CVSS 4.9. EPSS: 0.4% 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 validate that an assigned incoming webhook user has access to the target team or channel, which allows a requester with webhook management permissions to create posts or direct messages attributed to another user via crafted incoming webhook configuration and payloads.. Mattermost Advisory ID: MMSA-2026-00683

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 2026-07-13.