CVE-2026-27659 CSRF vulnerability in UpdateAccessControlPolicyActiveStatus endpoint

What is CVE-2026-27659?

CVE-2026-27659 is a medium-severity vulnerability in Mattermost Mattermost Server, published on 25 Mar 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

Official description

Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to properly validate CSRF tokens in the /api/v4/access_control_policies/{policy_id}/activate endpoint, which allows an attacker to trick an admin into changing access control policy active status via a crafted request.. Mattermost Advisory ID: MMSA-2026-00578

Weakness: CWE-352

Which versions are affected by CVE-2026-27659?

14 open source packages ship an affected version, including github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server and github.com/mattermost/mattermost-server/v5. It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2026-27659?

Upgrade github.com/mattermost/mattermost/server/v8 to a fixed version for your release line: 10.11.11, 11.2.3, 11.3.2 or 11.4.1.

Is CVE-2026-27659 being exploited?

There is no known exploitation of CVE-2026-27659 in the wild and no public exploit code.

Other vulnerabilities in Mattermost Mattermost Server

• CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers Critical 9.9
• CVE-2025-12421 Account Takeover via Code Exchange Endpoint Critical 9.9
• CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User Critical 9.9
• CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards Critical 9.9
• CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive Critical 9.9

All Mattermost Mattermost Server vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-rmhw-c3xr-m3xx, GO-2026-5629.