Mattermost <12 Allow Attacker Crashes Server via msgpack-WS Frame (CVE-2026-5740)
CVE-2026-5740 Published on May 22, 2026

Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server
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

Vendor Advisory NVD

Vulnerability Analysis

CVE-2026-5740 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.

Attack Vector:
NETWORK
Attack Complexity:
LOW
Privileges Required:
NONE
User Interaction:
NONE
Scope:
UNCHANGED
Confidentiality Impact:
NONE
Integrity Impact:
NONE
Availability Impact:
HIGH

Weakness Type

What is a Stack Exhaustion Vulnerability?

The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated.

CVE-2026-5740 has been classified to as a Stack Exhaustion vulnerability or weakness.


Products Associated with CVE-2026-5740

Want to know whenever a new CVE is published for MatterMost? stack.watch will email you.

MatterMost
 

Affected Versions

Mattermost: