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CVE-2026-4635 Mattermost CVE debrief

CVE-2026-4635 is a medium-severity vulnerability affecting 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. The issue arises from the failure to archive channels before removing persistent notifications, allowing an authenticated user to crash the server by timing the creation of persistent notification messages between server deletion of existing notifications and channel archiving. This vulnerability has a CVSS score of 6.5 and is considered medium severity.

Vendor
Mattermost
Product
Mattermost Server
CVSS
MEDIUM 6.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-22
Original CVE updated
2026-07-23
Advisory published
2026-05-22
Advisory updated
2026-07-23

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.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-4635 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-4635

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-4635 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-4635

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.