CVE-2026-5740 — Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

Description (CNA)

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

Affected (CNA)

Vendor / product	Versions
Mattermost / Mattermost	affected 11.6.0 through 11.6.0; affected 11.5.0 through 11.5.3; affected 11.4.0 through 11.4.4; affected 10.11.0 through 10.11.14; unaffected 11.7.0; unaffected 11.6.1; unaffected 11.5.4; unaffected 11.4.5; unaffected 10.11.15

CWE

CWE-789

References

https://mattermost.com/security-updates

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.