CVE-2026-9816 Insufficient server-side validation of board member role fields permits privilege escalation

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fail to validate BoardMember.Scheme fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin to arbitrary users via POST /api/v2/boards/boardID/members a...

CVE-2026-9816

Mattermost 11.7.x (≤11.7.6), 10.11.x (≤10.11.21), and 11.8.x (≤11.8.3) lack server-side validation of BoardMember.Scheme* fields on insert and archive-import paths, allowing board editors or non-guest team members to grant Board Admin to arbitrary users via POST /api/v2/boards/{boardID}/members a...

CVE-2026-9816

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fail to validate BoardMember.Scheme fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin to arbitrary users via POST /api/v2/boards/boardID/members a...

CVE-2026-9816 Insufficient server-side validation of board member role fields permits privilege escalation

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fail to validate BoardMember.Scheme fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin to arbitrary users via POST /api/v2/boards/boardID/members a...

CVE-2026-16047

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...

CVE-2026-16044

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 Advisory ID:...

CVE-2026-10527

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fails to reconcile SchemeAdmin flags with a user's current role which allows a user demoted to System Guest to retain Board Admin privileges and perform admin-only operations via the Boards REST API or UI.. Mattermost Adviso...

EUVD-2026-60192

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fails to reconcile SchemeAdmin flags with a user's current role which allows a user demoted to System Guest to retain Board Admin privileges and perform admin-only operations via the Boards REST API or UI.. Mattermost Adviso...

CVE-2026-10527 Boards plugin retains Board Admin rights for users demoted to System Guest

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fails to reconcile SchemeAdmin flags with a user's current role which allows a user demoted to System Guest to retain Board Admin privileges and perform admin-only operations via the Boards REST API or UI.. Mattermost Adviso...

CVE-2026-16044 Insufficient validation of guest board admin privileges on archive import

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 Advisory ID:...

CVE-2026-16044 Insufficient validation of guest board admin privileges on archive import

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 Advisory ID:...

CVE-2026-16044

Mattermost versions 11.7.6 and 10.11.21 fail to validate guest user privileges during board archive import, allowing a board member to escalate a guest user to Board Admin via a crafted .boardarchive file. CVSS 5.4 (MEDIUM). No exploitation details or patch version are provided. Upgrade to a fixe...

EUVD-2026-60211

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 Advisory ID:...

CVE-2026-16044

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 Advisory ID:...

CVE-2026-16047 Board channel linking without read channel permission validation

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...

CVE-2026-16047

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...

EUVD-2026-60208

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...

CVE-2026-10527: Incorrect Authorization

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fails to reconcile SchemeAdmin flags with a user's current role which allows a user demoted to System Guest to retain Board Admin privileges and perform admin-only operations via the Boards REST API or UI.. Mattermost Adviso...

PT-2026-76661

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...

CVE-2026-16044: Incorrect Authorization

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 Advisory ID:...