CVE-2026-6850 in Mattermost Server
Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser.. Mattermost Advisory ID: MMSA-2026-00658
Quick answer
Mattermost Server should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.
Who is affected
Affected versions
- Mattermost 11.7.x through 11.7.2
- Mattermost 11.6.x through 11.6.4
- Mattermost 10.11.x through 10.11.19
Fixed versions
- Mattermost 11.8.0, 11.7.3, 11.6.5, or 10.11.20 or later where applicable
How to fix it
CVE-2026-6850 affects Mattermost Server. In simple terms, an attacker may be able to misuse the affected system if it is exposed or if a user opens unsafe input. Update to Mattermost 11.8.0, 11.7.3, 11.6.5, or 10.11.20 or later where applicable. If you cannot update right away, limit access until the fix is installed.
- Find every place where Mattermost Server is installed or used.
- Check the installed version against the affected versions on this page.
- Update to Mattermost 11.8.0, 11.7.3, 11.6.5, or 10.11.20 or later where applicable.
- If an update is not possible today, turn off the risky feature or restrict it to trusted users only.
- Review recent logs for strange access, failed logins, changed files, or unexpected admin actions.
- Rotate passwords, API keys, and tokens if you think the system may have been abused.
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Verify the fix
- Confirm Mattermost Server now runs a fixed version or has the vendor mitigation in place.
- Test the affected feature again and make sure normal users can still work.
- Check that public or untrusted access is blocked where it should be blocked.
- Review logs again after the fix and confirm the same suspicious activity does not continue.
- Run a new Fixnx scan or your normal security check and save the result.
Related categories
Trusted references
FAQ
What is affected by CVE-2026-6850?
Mattermost Server versions listed as affected should be reviewed: Mattermost 11.7.x through 11.7.2, Mattermost 11.6.x through 11.6.4, Mattermost 10.11.x through 10.11.19.
What should I fix first?
Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.
How do I confirm the fix worked?
Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.
How are Fixnx security risk categories chosen?
Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.