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

Published May 22, 2026
·
Updated

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 Software

1 affected component
Mattermost Mattermost Server<=11.6.0, <=11.5.3, <=11.4.4, <=10.11.14

Remediation

Information

Update Mattermost to versions 11.7.0, 11.6.1, 11.5.4, 11.4.5, 10.11.15 or higher.

Event History

May 22, 2026
CVE Published
via MITRE·10:22 AM
Data Sourced
via MITRE·10:22 AM
RemedyDescriptionSeverityWeakness
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Frequently Asked Questions

1

What is the severity of CVE-2026-5740?

The severity of CVE-2026-5740 is high with a score of 7.5.

2

What does CVE-2026-5740 affect?

CVE-2026-5740 affects Mattermost Server versions 11.6.x, 11.5.x, 11.4.x, and 10.11.x.

3

How do I fix CVE-2026-5740?

To fix CVE-2026-5740, update Mattermost to versions 11.7.0, 11.6.1, 11.5.4, 11.4.5, or 10.11.15 or higher.

4

What risk does CVE-2026-5740 pose?

CVE-2026-5740 poses a risk of denial of service due to improper validation of msgpack-encoded WebSocket frames.

5

Can CVE-2026-5740 be exploited remotely?

Yes, CVE-2026-5740 can be exploited by unauthenticated remote attackers.

Company

Resources

Contact

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