CVE-2025-12689: DoS in Calls plugin via malformed UTF-8 in WebSocket request

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-12-17 · updated 2026-10-13
Weakness: CWE-1287

Description

Mattermost versions 11.0.x <= 11.0.4, 10.12.x <= 10.12.2, 10.11.x <= 10.11.6 fail to check WebSocket request field for proper UTF-8 format, which allows attacker to crash Calls plug-in via sending malformed request.

Affected products

Mattermost: from 10.11.0 before 10.11.7; from 10.12.0 before 10.12.3; from 11.0.0 before 11.0.5 — fixed in 10.11.7, 10.12.3, 11.0.5

How to fix

Upgrade to a fixed version: Mattermost 10.11.7, 10.12.3, 11.0.5. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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