CVE-2026-9859 Missing authorization in Mattermost
Authenticated board editors can expose boards to arbitrary channels due to missing role-based authorization checks on the batch endpoint.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-9859?
CVE-2026-9859 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 17 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
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
Weakness: CWE-863
Which versions are affected by CVE-2026-9859?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-9859?
Apply the vendor's patch.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-9859 being exploited?
There is no known exploitation of CVE-2026-9859 in the wild and no public exploit code.
Other vulnerabilities in Mattermost Mattermost Server
- CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers
- CVE-2025-12421 Account Takeover via Code Exchange Endpoint
- CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User
- CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards
- CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive
Sources
Also known as GHSA-27x7-qjjm-4f83, MINI-fqhh-9xh8-qh4v, MINI-x29h-rc66-2p37.