CVE-2026-9816: Mattermost <=11.7.6 Admin Escalation via Missing BoardMember.Scheme Validation
CVE-2026-9816 Published on August 17, 2026
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 and POST /api/v2/teams/{teamID}/archive/import.. Mattermost Advisory ID: MMSA-2026-00685
Vulnerability Analysis
CVE-2026-9816 is exploitable with network access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have a small impact on confidentiality, a high impact on integrity and availability.
Weakness Type
What is an AuthZ Vulnerability?
The software performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. This allows attackers to bypass intended access restrictions.
CVE-2026-9816 has been classified to as an AuthZ vulnerability or weakness.
Products Associated with CVE-2026-9816
Want to know whenever a new CVE is published for MatterMost? stack.watch will email you.
Affected Versions
Mattermost:- Version 11.7.0, <= 11.7.6 is affected.
- Version 10.11.0, <= 10.11.21 is affected.
- Version 11.8.0, <= 11.8.3 is affected.
- Version 11.9.0 is unaffected.
- Version 11.7.7 is unaffected.
- Version 10.11.22 is unaffected.
- Version 11.8.4 is unaffected.