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

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-22 · updated 2026-07-23
Weakness: CWE-789

Description

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 products

Mattermost: from 10.11.0 before 10.11.15; from 11.4.0 before 11.4.5; from 11.5.0 before 11.5.4; from 11.6.0 before 11.6.1 — fixed in 10.11.15, 11.4.5, 11.5.4, 11.6.1

How to fix

Upgrade to a fixed version: Mattermost 10.11.15, 11.4.5, 11.5.4, 11.6.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

Get alerted when CVE-2026-5740 or the next vulnerability affects your stack