CVE-2025-11777: Cross-team channel membership access

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-11-13 · updated 2026-10-19
Weakness: CWE-863

Description

Mattermost versions 10.11.x <= 10.11.3, 10.5.x <= 10.5.11 fail to properly validate team membership permissions in the Add Channel Member API which allows users from one team to access user metadata and channel membership information from other teams via the API endpoint

Affected products

Mattermost: from 10.5.0 before 10.5.12; from 10.11.0 before 10.11.4 — fixed in 10.11.4, 10.5.12

How to fix

Upgrade to a fixed version: Mattermost 10.11.4, 10.5.12. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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