CVE-2026-5740: mattermost server — Fix

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

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CVSS v3.1 Score

7.5

HIGH

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

EPSS — Exploit Prediction

0.0045

Probability of exploitation

0.35%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-789 CWE-789

Affected Products

Vendor	Product	Versions
mattermost	mattermost_server	10.11.0 — 10.11.15
mattermost	mattermost_server	11.4.0 — 11.4.5
mattermost	mattermost_server	11.5.0 — 11.5.4
mattermost	mattermost_server	11.6.0 — 11.6.1

References

Advisories & Patches

https://mattermost.com/security-updates

Frequently Asked Questions

What is CVE-2026-5740? +

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 It has a CVSS v3.1 base score of 7.5 (HIGH).

How severe is CVE-2026-5740? +

CVE-2026-5740 has a CVSS v3.1 score of 7.5 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0045, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-5740? +

CVE-2026-5740 affects products from mattermost, specifically: mattermost_server. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-5740? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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