CVE-2026-5740

Q: How do I fix CVE-2026-5740?

Upgrade mattermost server to 10.11.15 or later.

Q: Is CVE-2026-5740 being actively exploited?

Not that we know of. CVE-2026-5740 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.57% is the estimated probability that it will be exploited in the next 30 days. That is higher than 46% of all scored CVEs.

Q: How severe is CVE-2026-5740?

CVE-2026-5740 has a CVSS v3 base score of 7.5, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

Q: What does CVE-2026-5740 affect?

Published advisories record a fix for mattermost server (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

HIGH

7.5

CVSS v3

NVD

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

CWE-789

How to fix

Remediation Available

mattermost serverNVD

Affected:>= 10.11.0, < 10.11.15Fixed in:10.11.15CVE-2026-5740derived from NVD

Affected:>= 11.5.0, < 11.5.4Fixed in:11.5.4CVE-2026-5740derived from NVD

Affected:>= 11.4.0, < 11.4.5Fixed in:11.4.5CVE-2026-5740derived from NVD

Affected:>= 11.6.0, < 11.6.1Fixed in:11.6.1CVE-2026-5740derived from NVD

TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityNone

IntegrityNone

AvailabilityHigh

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

Exploit Intelligence

0.57%probability of exploitation in 30 days

46thpercentile

Moderate risk: more likely to be exploited than 46% of all known CVEs.

References

Vendor Advisory1

mattermost.com

Related Vulnerabilities

Other CWE-789 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2022-30522	High	7.5	90%	-	Fix
CVE-2025-27533	High	7.5	9.0%	-	Fix
CVE-2017-7651	High	7.5	5.1%	-	Fix
CVE-2026-49975	High	7.5	4.2%	-	Fix
CVE-2021-31811	Medium	5.5	3.6%	-	-
CVE-2021-27906	Medium	5.5	3.3%	-	Fix

Common questions

How do I fix CVE-2026-5740?

Upgrade mattermost server to 10.11.15 or later.

Is CVE-2026-5740 being actively exploited?

Not that we know of. CVE-2026-5740 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.57% is the estimated probability that it will be exploited in the next 30 days. That is higher than 46% of all scored CVEs.

How severe is CVE-2026-5740?

CVE-2026-5740 has a CVSS v3 base score of 7.5, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

What does CVE-2026-5740 affect?

Published advisories record a fix for mattermost server (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

Embed a live status badge for CVE-2026-5740

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-07-23.