CVE-2026-3590

Mattermost versions 10.11.x <= 10.11.12, 11.5.x <= 11.5.0, 11.4.x <= 11.4.2, 11.3.x <= 11.3.2 fail to enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent requests. Mattermost Advisory ID: MMSA-2026-00624.

CVE-2026-3590 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. 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 (8.0.0-20250723052842-4cb8d8940332, 10.11.13, 11.5.0, 11.4.3, 11.3.3); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-3590? CVE-2026-3590 is a medium-severity security vulnerability in github.com/mattermost/mattermost/server/v8 (go), affecting versions >= 8.0.0-20250721062209-4952acea88ce, < 8.0.0-20250723052842-4cb8d8940332. It is fixed in 8.0.0-20250723052842-4cb8d8940332, 10.11.13, 11.5.0, 11.4.3, 11.3.3.
2. How severe is CVE-2026-3590? CVE-2026-3590 has a CVSS score of 6.5 (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-2026-3590?
   • github.com/mattermost/mattermost/server/v8 (go) (versions >= 8.0.0-20250721062209-4952acea88ce, < 8.0.0-20250723052842-4cb8d8940332)
   • github.com/mattermost/mattermost-server (go) (versions >= 10.11.0-rc1, < 10.11.13)
4. Is there a fix for CVE-2026-3590? Yes. CVE-2026-3590 is fixed in 8.0.0-20250723052842-4cb8d8940332, 10.11.13, 11.5.0, 11.4.3, 11.3.3. Upgrade to this version or later.
5. Is CVE-2026-3590 exploitable, and should I be worried? Whether CVE-2026-3590 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-2026-3590 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-2026-3590?
   • Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20250723052842-4cb8d8940332 or later
   • Upgrade github.com/mattermost/mattermost-server to 10.11.13 or later
   • Upgrade github.com/mattermost/mattermost-server to 11.5.0 or later
   • Upgrade github.com/mattermost/mattermost-server to 11.4.3 or later
   • Upgrade github.com/mattermost/mattermost-server to 11.3.3 or later