CVE-2026-28759 MEDIUM

CVE-2026-28759: Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels

Vendor Mattermost
Product Mattermost
Weakness CWE-863 · Incorrect authorization
Published May 18, 2026
Last update May 18, 2026
View on NVD All CVEs

CVSS base score

4.3/10
Attack vector Network
Attack complexity Low
Privileges required Low
User interaction None
Confidentiality None
Integrity Low

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

What the vulnerability does

01Description

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576

Key dates

02Disclosure timeline

May 18, 2026 CVE published
May 18, 2026 Record updated