WebSocket Message Parsing Flaw in Mattermost Server by Mattermost
CVE-2026-5740

7.5HIGH

Key Information:

Vendor

Mattermost
Status
Mattermost
Vendor
CVE Published:
22 May 2026

What is CVE-2026-5740?

The Mattermost Server contains a vulnerability in the way it validates msgpack-encoded WebSocket frames. Certain versions of the software fail to perform adequate checks before allocating memory, which permits unauthenticated remote attackers to manipulate server processes. By sending a specifically crafted binary WebSocket message to the public endpoint, attackers can trigger a server crash, potentially resulting in a complete outage for all users.

Affected Version(s)

Mattermost 11.6.0

Mattermost 11.5.0 <= 11.5.3

Mattermost 11.4.0 <= 11.4.4

References

https://mattermost.com/security-updates
vendor-advisory

CVSS V3.1

Score:
7.5
Severity:
HIGH
Confidentiality:
None
Integrity:
None
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

ZephrFish
.