CVE-2026-27656

CVE-2026-27656 is a medium-severity security vulnerability in github.com/mattermost/mattermost-server (go), affecting versions >= 11.4.0-rc1, < 11.4.1. It is fixed in 11.4.1, 11.3.2, 11.2.4, 10.11.12, 8.0.0-20260217110922-b7d4a1f1f59b.

Summary

Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow. Mattermost Advisory ID: MMSA-2026-00590

Impact

CVE-2026-27656 has a CVSS score of 5.7 (Medium). The vector is network-reachable, high 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, 11.2.4, 10.11.12, 8.0.0-20260217110922-b7d4a1f1f59b); upgrading removes the vulnerable code path.

Affected versions

github.com/mattermost/mattermost-server (>= 11.4.0-rc1, < 11.4.1) github.com/mattermost/mattermost-server (>= 11.3.0-rc1, < 11.3.2) github.com/mattermost/mattermost-server (>= 11.2.0-rc1, < 11.2.4) github.com/mattermost/mattermost-server (>= 10.11.0-rc1, < 10.11.2) github.com/mattermost/mattermost/server/v8 (>= 8.0.0-20260105080200-d27a2195068d, < 8.0.0-20260217110922-b7d4a1f1f59b)

Security releases

github.com/mattermost/mattermost-server → 11.4.1 (go) github.com/mattermost/mattermost-server → 11.3.2 (go) github.com/mattermost/mattermost-server → 11.2.4 (go) github.com/mattermost/mattermost-server → 10.11.12 (go) github.com/mattermost/mattermost/server/v8 → 8.0.0-20260217110922-b7d4a1f1f59b (go)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

See it in your environment

Remediation advice

Upgrade the following packages to resolve this vulnerability:

github.com/mattermost/mattermost-server to 11.4.1 or later; github.com/mattermost/mattermost-server to 11.3.2 or later; github.com/mattermost/mattermost-server to 11.2.4 or later; github.com/mattermost/mattermost-server to 10.11.12 or later; github.com/mattermost/mattermost/server/v8 to 8.0.0-20260217110922-b7d4a1f1f59b or later

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

  1. What is CVE-2026-27656? CVE-2026-27656 is a medium-severity security vulnerability in github.com/mattermost/mattermost-server (go), affecting versions >= 11.4.0-rc1, < 11.4.1. It is fixed in 11.4.1, 11.3.2, 11.2.4, 10.11.12, 8.0.0-20260217110922-b7d4a1f1f59b.
  2. How severe is CVE-2026-27656? CVE-2026-27656 has a CVSS score of 5.7 (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 packages are affected by CVE-2026-27656?
    • github.com/mattermost/mattermost-server (go) (versions >= 11.4.0-rc1, < 11.4.1)
    • github.com/mattermost/mattermost/server/v8 (go) (versions >= 8.0.0-20260105080200-d27a2195068d, < 8.0.0-20260217110922-b7d4a1f1f59b)
  4. Is there a fix for CVE-2026-27656? Yes. CVE-2026-27656 is fixed in 11.4.1, 11.3.2, 11.2.4, 10.11.12, 8.0.0-20260217110922-b7d4a1f1f59b. Upgrade to this version or later.
  5. Is CVE-2026-27656 exploitable, and should I be worried? Whether CVE-2026-27656 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-2026-27656 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-2026-27656?
    • Upgrade github.com/mattermost/mattermost-server to 11.4.1 or later
    • Upgrade github.com/mattermost/mattermost-server to 11.3.2 or later
    • Upgrade github.com/mattermost/mattermost-server to 11.2.4 or later
    • Upgrade github.com/mattermost/mattermost-server to 10.11.12 or later
    • Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260217110922-b7d4a1f1f59b or later

Other vulnerabilities in github.com/mattermost/mattermost-server

CVE-2026-4915CVE-2026-28735CVE-2026-5308CVE-2026-3473CVE-2026-5740

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