High severity · published May 22, 2026
CVE-2026-5740
Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server
7.5
CVSS
0.3%
EPSS
No
On KEV
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
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.3% · higher than 27% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
11.6.0 through 11.6.0; 11.5.0 through 11.5.3; 11.4.0 through 11.4.4; 10.11.0 through 10.11.14 · source: vendor
MS
Mattermost Server · Mattermost
10.11.0 before 10.11.15; 11.4.0 before 11.4.5; 11.5.0 before 11.5.4; 11.6.0 before 11.6.1 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-5740
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-5740
Vendor Advisory
https://mattermost.com/security-updates
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