CVE-2025-9076: Mattermost Server exposes sensitive user credentials during shared channel membership synchronization

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: 2025-09-15 · updated 2026-10-19
Weakness: CWE-862

Description

Mattermost versions 10.10.x <= 10.10.1 fail to properly sanitize user data during shared channel membership synchronization, which allows malicious or compromised remote clusters to access sensitive user information via unsanitized user objects. This vulnerability affects Mattermost Server instances with shared channels enabled.

Affected products

Mattermost: from 10.10.0 before 10.10.2 — fixed in 10.10.2

How to fix

Upgrade to a fixed version: Mattermost 10.10.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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