CVE-2026-5740CWE-789

Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

High · published May 22, 2026

CVSS v3.1
7.5
EPSS
1%
Percentile
46.6
In the wild
Unconfirmed
What it is

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

The record
Technical detail
CVSS v3.1
7.5 · HIGH
Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS v4.0
Not supplied
EPSS
0.00573 · 46.6th percentile
Weakness
CWE-789 · Memory Allocation with Excessive Size Value
Published
2026-05-22T10:22Z
EPSS history
Timeline
  • 22 MAY 10:22Z
    Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server
    cvelistv5