CVE-2026-3495

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to escape some variables that could contain malicious content during error page composition which allows an attacker with access to edit some site configuration to execute some malicious code via injecting some JS as part of those values.. Mattermost Advisory ID: MMSA-2026-00622

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-3495 has a CVSS score of 3.8 (Low). The vector is network-reachable, high 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 (10.11.14, 11.5.2, 8.0.0-20260310115442-5a1ea95044d, 5.3.2-0.20260310115442-5a1ea95044dc); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-3495? CVE-2026-3495 is a low-severity cross-site scripting (XSS) vulnerability in github.com/mattermost/mattermost/server/v8 (go), affecting versions >= 10.11.0, < 10.11.14. It is fixed in 10.11.14, 11.5.2, 8.0.0-20260310115442-5a1ea95044d, 5.3.2-0.20260310115442-5a1ea95044dc. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-3495? CVE-2026-3495 has a CVSS score of 3.8 (Low). 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-2026-3495?
   • github.com/mattermost/mattermost/server/v8 (go) (versions >= 10.11.0, < 10.11.14)
   • github.com/mattermost/mattermost-server (go) (versions < 5.3.2-0.20260310115442-5a1ea95044dc)
4. Is there a fix for CVE-2026-3495? Yes. CVE-2026-3495 is fixed in 10.11.14, 11.5.2, 8.0.0-20260310115442-5a1ea95044d, 5.3.2-0.20260310115442-5a1ea95044dc. Upgrade to this version or later.
5. Is CVE-2026-3495 exploitable, and should I be worried? Whether CVE-2026-3495 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-2026-3495 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-2026-3495?
   • Upgrade github.com/mattermost/mattermost/server/v8 to 10.11.14 or later
   • Upgrade github.com/mattermost/mattermost/server/v8 to 11.5.2 or later
   • Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260310115442-5a1ea95044d or later
   • Upgrade github.com/mattermost/mattermost-server to 5.3.2-0.20260310115442-5a1ea95044dc or later