CVE-2026-4635 Mattermost CVE debrief

Who should care

Administrators and users of Mattermost Server, particularly those with versions 10.11.x <= 10.11.14, 11.4.x <= 11.4.4, 11.5.x <= 11.5.3, and 11.6.x <= 11.6.0, should be aware of this vulnerability and take necessary actions to mitigate the risk. This includes updating to a patched version, reviewing server configurations, and monitoring for potential exploitation attempts.

Technical summary

The vulnerability exists due to inadequate handling of persistent notifications and channel archiving in Mattermost Server. Specifically, the server fails to archive channels before removing persistent notifications. This allows an authenticated user to exploit the timing of creating persistent notification messages to crash the server. The issue affects Mattermost Server versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, and 10.11.x <= 10.11.14.

Defensive priority

Medium

Recommended defensive actions

• Update Mattermost Server to a version that addresses this vulnerability
• Implement monitoring to detect potential exploitation attempts
• Review and adjust notification and archiving configurations to minimize risk
• Restrict access to sensitive areas of the server for authenticated users
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The CVE record and NVD detail provide information on the vulnerability, including affected versions and a CVSS score of 6.5 (Medium). The vendor advisory from Mattermost offers guidance on mitigation. Evidence is limited to public sources, and defenders should verify the vulnerability's existence and scope within their environments.

Official resources