{"id":"CVE-2026-5740","aliases":["GHSA-w9m8-p4cc-4qj9","GO-2026-5838"],"url":"https://o3.security/vulnerability/CVE-2026-5740","summary":"Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server","details":"Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and cause a full service outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647","published":"2026-05-22T10:22:01.030Z","modified":"2026-08-12T03:51:42.893349788Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":{"score":0.0034,"percentile":0.27058,"asOf":"2026-08-15"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server","fixedVersion":"11.6.1"},{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server","fixedVersion":"11.5.4"},{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server","fixedVersion":"11.4.5"},{"ecosystem":"Go","name":"github.com/mattermost/mattermost-server","fixedVersion":"10.11.15"},{"ecosystem":"Go","name":"github.com/mattermost/mattermost/server/v8","fixedVersion":"8.0.0-20260410202636-17939826efa2"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/5xxx/CVE-2026-5740.json"},{"type":"ADVISORY","url":"https://mattermost.com/security-updates"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-5740"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:42.893349788Z"}}