Cyber Resilience

CVE-2026-5740

High

Published: 22 May 2026

Published
22 May 2026
Modified
22 May 2026
KEV Added
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Patch
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CVSS Score v3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS Score 0.0044 35.4th percentile
Risk Priority 15 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-5740 is a high-severity Memory Allocation with Excessive Size Value (CWE-789) vulnerability in Mattermost Mattermost Server. Its CVSS base score is 7.5 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 35.4th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

EU & UK References

Vulnerability details

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…

more

outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647

CWE(s)
CWE-789

Related Threats

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1499.004 Application or System Exploitation Impact
Adversaries may exploit software vulnerabilities that can cause an application or system to crash and deny availability to users.
attack.mitre.org →
Why these techniques?

Unauthenticated remote DoS via crafted WebSocket message to public endpoint directly enables T1190 (public app exploitation) and T1499.004 (app/system exploitation for availability impact).

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

Affected Assets

mattermost
mattermost server
10.11.0 — 10.11.15 · 11.4.0 — 11.4.5 · 11.5.0 — 11.5.4

Mitigating Controls

No mitigating controls mapped yet. The per-CVE control annotator has not reached this CVE.

References