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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

PUBLISHED Reserved 2026-04-07 | Published 2026-05-22 | Updated 2026-05-22 | Assigner Mattermost




HIGH: 7.5CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Problem types

CWE-789: Memory Allocation with Excessive Size Value

Product status

Default status
unaffected

11.6.0 (semver)
affected

11.5.0 (semver)
affected

11.4.0 (semver)
affected

10.11.0 (semver)
affected

11.7.0
unaffected

11.6.1
unaffected

11.5.4
unaffected

11.4.5
unaffected

10.11.15
unaffected

Credits

ZephrFish finder

References

mattermost.com/security-updates (MMSA-2026-00647) vendor-advisory

cve.org (CVE-2026-5740)

nvd.nist.gov (CVE-2026-5740)

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