Vulnerability in Mattermost

[ Description ]Official description

Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to limit the length of the post ID array accepted by the bulk reactions endpoint which allows an authenticated user to cause excessive database load via a crafted request to {{POST /api/v4/posts/ids/reactions}}.. Mattermost Advisory ID: MMSA-2026-00771

[ Fixes ]Fixes and patches

• Security bulletin: Mattermost MMSA-2026-00775Vendor patch or bulletin · CERT-FR · mattermost.com/security-updates, cited by CERT-FR CERTFR-2026-AVI-1212

[ Advisories ]Government CERT advisories

• CERT-FR23 Sep 2026CERTFR-2026-AVI-1212

  Multiple vulnerabilities in Mattermost Server Translated from French

  Multiple vulnerabilities have been discovered in Mattermost Server. They allow an attacker to cause remote denial of service and a breach of data confidentiality.

  CVE-2026-96260CVE-2026-95666CVE-2026-96259

[ References ]References

• mattermost.com/security-updatesvendor-advisory