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CVE-2026-4646 Mattermost CVE debrief

CVE-2026-4646 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 vulnerability allows an authenticated attacker to crash the plugin process via a crafted HTTP request to the PR details endpoint. The Mattermost Advisory ID for this vulnerability is MMSA-2026-00638. This issue has a CVSS score of 4.3 and is classified as MEDIUM severity. The vulnerability is caused by insufficient validation of user-supplied input in API request handlers.

Vendor
Mattermost
Product
Mattermost Server
CVSS
MEDIUM 4.3
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 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 should be aware of this vulnerability and take necessary actions to mitigate it. This includes applying patches, implementing compensating controls, and verifying the integrity of the plugin process and API request handlers.

Technical summary

The vulnerability exists due to insufficient validation of user-supplied input in API request handlers. An authenticated attacker can exploit this vulnerability by sending a crafted HTTP request to the PR details endpoint, which can cause the plugin process to crash. The CVSS score for this vulnerability is 4.3, and the severity is classified as MEDIUM. This 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, and the Mattermost Advisory ID for this vulnerability is MMSA-2026-00638.

Defensive priority

Medium priority should be given to patching or mitigating this vulnerability, as it can be exploited by authenticated attackers to cause a denial-of-service (DoS) condition.

Recommended defensive actions

  • Apply the patches provided by Mattermost to update the affected versions to 11.6.1, 11.5.4, 11.4.5, or 10.11.15.
  • Implement compensating controls, such as monitoring and exception tracking, to detect and respond to potential exploitation attempts.
  • Verify the integrity of the plugin process and API request handlers to prevent similar vulnerabilities in the future.
  • 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.
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.

Evidence notes

The CVE record was published on 2026-05-22T11:16:22.863Z and was last modified on 2026-07-23T16:10:00.137Z. The NVD entry is currently Analyzed. The vulnerability details are based on the information provided by the NVD and Mattermost. The source detail is limited, and defenders should verify the affected scope and vendor guidance.

Official resources

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-05-22T11:16:22.863Z and has not been modified since then. The NVD entry is currently Analyzed.