CVE-2026-5740

Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

State: Published
Severity: 7.5 High
Published: May 22, 2026
Updated: May 22, 2026
Reserved: Apr 7, 2026

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

Scores

CVSS	Score	Vector	Source
3.1	7.5 High	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H	Mattermost

Weaknesses

CWE-789 Memory Allocation with Excessive Size Value

Affected Products

Product	Versions
MattermostMattermost	11.6.0 through 11.6.0Affected 11.5.0 through 11.5.3Affected 11.4.0 through 11.4.4Affected 10.11.0 through 10.11.14Affected 11.7.0Not affected 11.6.1Not affected 11.5.4Not affected 11.4.5Not affected 10.11.15Not affected

References

MMSA-2026-00647vendor-advisory

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