CVE-2026-9571

Mattermost versions 10.11.x <= 10.11.20 and 11.7.x <= 11.7.5 fail to remove thread membership records when a user is removed from or leaves a team, allowing a previously removed user who is later re-invited to view private channel thread root post content and metadata via the team threads API.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to enforce PermissionManageBoardRoles on the channelId field of the batch endpoint, which allows an authenticated board editor to relink any board they can edit to an arbitrary channel via a crafted PATCH request.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, and 11.8.x <= 11.8.3 fail to validate BoardMember.Scheme* fields server-side on insert and archive-import paths, allowing a board editor or non-guest team member to grant board admin to arbitrary users.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, and 11.8.x <= 11.8.3 fail to validate WebSocket command field types, allowing an authenticated user to crash the plugin process and deny service to all Boards users via a custom_focalboard_SUBSCRIBE_TEAM message with a non-string teamId.

Mattermost versions 11.8.x <= 11.8.2, 11.7.x <= 11.7.6, 10.11.x <= 10.11.21 fail to restrict channel member role assignment to channel-scoped roles, which allows a channel administrator to gain additional channel permissions via the channel member roles API.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate that users have read access to a channel before linking a board to it, which allows an authenticated attacker to discover the membership of private channels on the same team via creating, patching, importing, or bulk-creating boards with an arbitrary channelId.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21 fail to enforce run-state validation on write operations for finished playbook runs, which allows a run participant to modify status, checklists, retrospective content, ownership, and participants on completed runs via REST and GraphQL API requests.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21 fail to restrict OAuth deauthorization and personal access token management endpoints to direct user sessions, which allowed an OAuth app with a delegated user token to revoke the user's authorizations or tokens for other integrations via account-management endpoints.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21 fail to prevent guest users from receiving Board Admin privileges during board archive import, which allows a board member to escalate a guest user to Board Admin via importing a crafted .boardarchive file.

Mattermost versions 11.7.x <= 11.7.6, 11.8.x <= 11.8.3 fail to re-validate that each target channel still belongs to the requesting admin's team when calling the policy unassign endpoint. This allows an authenticated team administrator to remove ABAC policy assignments from channels outside their team after a channel has been moved to another team.