CVE-2026-5740Network attack vector

Unauthenticated server crash via oversized msgpack allocation

Published May 22, 2026 · Updated May 22, 2026

Denial of service in affected Mattermost releases allows remote attackers to crash the server through a crafted binary WebSocket message. The public WebSocket handler allocates memory from a msgpack-encoded size before adequately validating the frame's requested allocation. No authentication is required, and one malicious frame terminates the server process, interrupting service for all users.

CVSS severity7.5
High
EPSS probability0.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in affected Mattermost releases allows remote attackers to crash the server through a crafted binary WebSocket message. The public WebSocket handler allocates memory from a msgpack-encoded size before adequately validating the frame's requested allocation. No authentication is required, and one malicious frame terminates the server process, interrupting service for all users.

The record

CVE
CVE-2026-5740
Published
May 22, 2026
Updated
May 22, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-789, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 22, 2026CVE publishedPublication date reported by the CVE source.
  2. May 22, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=10.11.0 <=10.11.14
  2. Affected versionversion=11.4.0 <=11.4.4
  3. Affected versionversion=11.5.0 <=11.5.3
  4. Affected versionversion=11.6.0 <=11.6.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mattermost · Mattermostversion=10.11.0 <=10.11.14; version=11.4.0 <=11.4.4; version=11.5.0 <=11.5.3; version=11.6.0 <=11.6.0

Published CVSS scores

7.5Mattermost, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-789Memory Allocation with Excessive Size Value
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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