Mattermost <11.8.4 Unchecked PMBR Allows Channel Hijack
CVE-2026-9859 Published on August 17, 2026
Mattermost Boards plugin didn’t enforce role-based authorization on board channel link allowing board editors to expose boards to arbitrary channels
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 Advisory ID: MMSA-2026-00686
Vulnerability Analysis
CVE-2026-9859 can be exploited 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 high impact on confidentiality, with no 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-2026-9859 has been classified to as an AuthZ vulnerability or weakness.
Products Associated with CVE-2026-9859
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.