CVE-2025-53971: Channel and Team Membership APIs inadvertently allow loss of Member privileges.

Severity: low (CVSS 3.8 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: 2025-08-21 · updated 2026-06-17
Weakness: CWE-863

Description

Mattermost versions 10.5.x <= 10.5.8, 9.11.x <= 9.11.17 fail to properly validate authorization for team scheme role modifications which allows Team Admins to demote Team Members to Guests via the PUT /api/v4/teams/team-id/members/user-id/schemeRoles API endpoint.

Affected products

Mattermost: from 9.11.0 before 9.11.18; from 10.5.0 before 10.5.9 — fixed in 10.5.9, 9.11.18

How to fix

Upgrade to a fixed version: Mattermost 10.5.9, 9.11.18. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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