CVE-2026-6689: Mattermost Server

Medium severity, CVSS 4.3. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 Fail to enforce PermissionInviteUser when setting AllowOpenInvite or AllowedDomains during team creation (the check was only applied on update/patch), which allows an authenticated user holding PermissionCreateTeam but not PermissionInviteUser on the resulting team to configure invite-controlled team settings (make the team publicly joinable via open invite and/or constrain membership via allowed domains) that they are not permitted to set on an existing team via POST /api/v4/teams with allow_open_invite: true and/or a non-empty allowed_domains in the request body.. Mattermost Advisory ID: MMSA-2026-00655

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.17 (fixed in 10.11.17); from 11.5.0, before 11.5.5 (fixed in 11.5.5); from 11.6.0, before 11.6.2 (fixed in 11.6.2)

Published 2026-06-12. Last modified 2026-06-18.