CVE-2025-10545: Guest user can add unauthorized team users to private channels

Severity: medium (CVSS 4.3 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: 2025-10-16 · updated 2026-10-19
Weakness: CWE-863

Description

Mattermost versions 10.5.x <= 10.5.10, 10.11.x <= 10.11.2 fail to properly validate guest user permissions when adding channel members which allows guest users to add any team members to their private channels via the `/api/v4/channels/{channel_id}/members` endpoint

Affected products

Mattermost: from 10.5.0 before 10.5.11; from 10.11.0 before 10.11.3 — fixed in 10.11.3, 10.5.11

How to fix

Upgrade to a fixed version: Mattermost 10.11.3, 10.5.11. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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