CVE-2026-10085 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 restrict the group_constrained channel flag to public and private channels that support group synchronization, which allows an ordinary group or direct message member to remove all participants from the conversation via the channel patch API.. Mattermost Advisory ID: MMSA-2026-00688
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-2026-10085 affects Mattermost Server. In simple terms, an attacker may be able to make the service slow or stop if the affected part is exposed or receives 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 now, limit access until the fix is installed.
- Find every Mattermost Server install and check its version.
- Update to Mattermost 11.8.0, 11.7.3, 11.6.5, or 10.11.20 or later where applicable.
- If you cannot update today, block public access to the risky feature or service.
- Review logs for strange access, crashes, deleted files, or failed requests.
- Rotate secrets if you think the issue may have been used.
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Verify the fix
- Confirm the installed version is fixed or the mitigation is active.
- Test the affected feature with a normal user account.
- Confirm untrusted users cannot reach the risky path or service.
- Run a fresh Fixnx scan and keep the result with your change record.
Related categories
Trusted references
FAQ
What is affected by CVE-2026-10085?
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.