CVE-2026-27659

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

A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones. Typical impact: state-changing actions performed as the victim without their consent.

CVE-2026-27659 has a CVSS score of 4.6 (Medium). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (11.4.1, 11.3.2, 11.2.3, 10.11.11); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-27659? CVE-2026-27659 is a medium-severity cross-site request forgery (CSRF) vulnerability in github.com/mattermost/mattermost/server/v8 (go), affecting versions >= 11.4.0-rc1, < 11.4.1. It is fixed in 11.4.1, 11.3.2, 11.2.3, 10.11.11. A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones.
2. How severe is CVE-2026-27659? CVE-2026-27659 has a CVSS score of 4.6 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/mattermost/mattermost/server/v8 are affected by CVE-2026-27659? github.com/mattermost/mattermost/server/v8 (go) versions >= 11.4.0-rc1, < 11.4.1 is affected.
4. Is there a fix for CVE-2026-27659? Yes. CVE-2026-27659 is fixed in 11.4.1, 11.3.2, 11.2.3, 10.11.11. Upgrade to this version or later.
5. Is CVE-2026-27659 exploitable, and should I be worried? Whether CVE-2026-27659 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-27659 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-27659?
   • Upgrade github.com/mattermost/mattermost/server/v8 to 11.4.1 or later
   • Upgrade github.com/mattermost/mattermost/server/v8 to 11.3.2 or later
   • Upgrade github.com/mattermost/mattermost/server/v8 to 11.2.3 or later
   • Upgrade github.com/mattermost/mattermost/server/v8 to 10.11.11 or later
   • Upgrade github.com/mattermost/mattermost/server/v8 to 10.11.11 or later