CVE-2026-8823 User Manager can demote bot accounts to guest without bot-management permission

Mattermost versions 11.7.x = 11.7.0, 10.11.x = 10.11.17 fail to validate bot targets when demoting users to guests which allows a lower-privileged administrator to degrade arbitrary bot accounts via the standard demote-user API.. Mattermost Advisory ID: MMSA-2026-00669...

Improper Authorization

github.com/mattermost/mattermost-server is vulnerable to Improper Authorization. The vulnerability is due to insufficient validation of authorization for team scheme role modifications, which allows an attacker Team Admins to demote Team Members to Guests via the affected API endpoint...

CVE-2025-53971 Channel and Team Membership APIs inadvertently allow loss of Member privileges.

Mattermost versions 10.5.x = 10.5.8, 9.11.x = 9.11.17 fail to properly validate authorization for team scheme role modifications which allows Team Admins to demote Team Members to Guests via the PUT /api/v4/teams/team-id/members/user-id/schemeRoles API endpoint...