CVE-2026-2456

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 Mattermost fails to limit the size of responses from integration action endpoints, which allows an authenticated attacker to cause server memory exhaustion and denial of service via a malicious integration server that returns an arbitrarily large response when a user clicks an interactive message button. Mattermost Advisory ID: MMSA-2026-00571

CVE-2026-2456 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no 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 (8.0.0-20260127165411-fe3052073dc6, 5.3.2-0.20260127165411-fe3052073dc6, 10.11.11, 11.2.3, 11.3.1); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-2456? CVE-2026-2456 is a medium-severity security vulnerability in github.com/mattermost/mattermost/server/v8 (go), affecting versions < 8.0.0-20260127165411-fe3052073dc6. It is fixed in 8.0.0-20260127165411-fe3052073dc6, 5.3.2-0.20260127165411-fe3052073dc6, 10.11.11, 11.2.3, 11.3.1.
2. How severe is CVE-2026-2456? CVE-2026-2456 has a CVSS score of 5.3 (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-2026-2456?
   • github.com/mattermost/mattermost/server/v8 (go) (versions < 8.0.0-20260127165411-fe3052073dc6)
   • github.com/mattermost/mattermost-server (go) (versions < 5.3.2-0.20260127165411-fe3052073dc6)
4. Is there a fix for CVE-2026-2456? Yes. CVE-2026-2456 is fixed in 8.0.0-20260127165411-fe3052073dc6, 5.3.2-0.20260127165411-fe3052073dc6, 10.11.11, 11.2.3, 11.3.1. Upgrade to this version or later.
5. Is CVE-2026-2456 exploitable, and should I be worried? Whether CVE-2026-2456 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-2026-2456 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-2026-2456?
   • Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260127165411-fe3052073dc6 or later
   • Upgrade github.com/mattermost/mattermost-server to 5.3.2-0.20260127165411-fe3052073dc6 or later
   • Upgrade github.com/mattermost/mattermost-server to 10.11.11 or later
   • Upgrade github.com/mattermost/mattermost-server to 11.2.3 or later
   • Upgrade github.com/mattermost/mattermost-server to 11.3.1 or later