CVE-2026-5755

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.2, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate the TIFF IFD offset in the image header before allocating memory, which allows authenticated users with file upload or posting permissions to cause a denial of service (server OOM) via uploading a crafted TIFF file or posting a URL that serves one.. Mattermost Advisory ID: MMSA-2026-00648

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-5755 has a CVSS score of 6.5 (Medium). The vector is network-reachable, low 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 (11.6.1, 11.5.4, 11.4.5, 10.11.15); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-5755? CVE-2026-5755 is a medium-severity uncontrolled resource consumption vulnerability in github.com/mattermost/mattermost-server (go), affecting versions = 11.6.0. It is fixed in 11.6.1, 11.5.4, 11.4.5, 10.11.15. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-5755? CVE-2026-5755 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 versions of github.com/mattermost/mattermost-server are affected by CVE-2026-5755? github.com/mattermost/mattermost-server (go) versions = 11.6.0 is affected.
4. Is there a fix for CVE-2026-5755? Yes. CVE-2026-5755 is fixed in 11.6.1, 11.5.4, 11.4.5, 10.11.15. Upgrade to this version or later.
5. Is CVE-2026-5755 exploitable, and should I be worried? Whether CVE-2026-5755 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-2026-5755 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-2026-5755?
   • Upgrade github.com/mattermost/mattermost-server to 11.6.1 or later
   • Upgrade github.com/mattermost/mattermost-server to 11.5.4 or later
   • Upgrade github.com/mattermost/mattermost-server to 11.4.5 or later
   • Upgrade github.com/mattermost/mattermost-server to 10.11.15 or later