CVE-2026-5740

HIGH

Mattermost doesn't properly validate msgpack-encoded WebSocket frames before memory allocation

Published May 26, 2026Updated July 7, 2026Source: osv

Summary

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

Remediation

Upgrade to the fixed version using your package manager.

Update github.com/mattermost/mattermost-server to 10.11.15 or later

go get github.com/mattermost/mattermost-server@v10.11.15

Update github.com/mattermost/mattermost/server/v8 to 8.0.0-20260410202636-17939826efa2 or later

go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260410202636-17939826efa2

Update github.com/mattermost/mattermost-server to 11.6.1 or later

go get github.com/mattermost/mattermost-server@v11.6.1

Update github.com/mattermost/mattermost-server to 11.5.4 or later

go get github.com/mattermost/mattermost-server@v11.5.4

Update github.com/mattermost/mattermost-server to 11.4.5 or later

go get github.com/mattermost/mattermost-server@v11.4.5

After upgrading, run your dependency scanner again to confirm the vulnerability is resolved.

Affected Packages (5)

Package	Ecosystem	Affected	Fixed In
github.com/mattermost/mattermost-server	go	All versions	10.11.15
github.com/mattermost/mattermost/server/v8	go	All versions	8.0.0-20260410202636-17939826efa2
github.com/mattermost/mattermost-server	go	11.6.0	11.6.1
github.com/mattermost/mattermost-server	go	All versions	11.5.4
github.com/mattermost/mattermost-server	go	All versions	11.4.5

Vulnerability Classification

Common Weakness Enumeration (CWE) identifiers for this vulnerability type.

CWE-789
View on MITRE

CVSS Score Breakdown

What the CVSS (Common Vulnerability Scoring System) 7.5 score means for each attack dimension.

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

Frequently Asked Questions

What is CVE-2026-5740?: Mattermost doesn't properly validate msgpack-encoded WebSocket frames before memory allocation This vulnerability has been assigned a severity rating of HIGH (CVSS score: 7.5/10).
How do I check if my project is affected by CVE-2026-5740?: CVE-2026-5740 affects github.com/mattermost/mattermost-server and github.com/mattermost/mattermost/server/v8. Use GeekWala's free vulnerability scanner to check your dependencies against CVE-2026-5740 and 200,000+ other known vulnerabilities.

Severity & Exploitability

CVSS Score

7.5

High exploitability or significant impact. Prioritize remediation within days.

Also Known As

GHSA-w9m8-p4cc-4qj9

GO-2026-5838

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