Description
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
Published: 2026-05-18
Score: 4.3 Medium
EPSS: < 1% Very Low
KEV: No
Impact: n/a
Action: n/a
AI Analysis

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Remediation

Vendor Solution

Update Mattermost to versions 11.6.0, 11.5.2, 10.11.14, 11.4.4 or higher.

Tracking

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Advisories
Source ID Title
Github GHSA Github GHSA GHSA-8h9w-w78c-vvr3 Mattermost does not verify remote cluster channel access when processing shared channel membership removals
References
History

Mon, 18 May 2026 19:30:00 +0000

Type Values Removed Values Added
First Time appeared Mattermost mattermost Server
CPEs cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*
Vendors & Products Mattermost mattermost Server

Mon, 18 May 2026 16:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Mon, 18 May 2026 09:45:00 +0000

Type Values Removed Values Added
First Time appeared Mattermost
Mattermost mattermost
Vendors & Products Mattermost
Mattermost mattermost

Mon, 18 May 2026 08:00:00 +0000

Type Values Removed Values Added
Description 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
Title Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels
Weaknesses CWE-863
References
Metrics cvssV3_1

{'score': 4.3, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N'}


Subscriptions

Mattermost Mattermost Mattermost Server
cve-icon MITRE

Status: PUBLISHED

Assigner: Mattermost

Published:

Updated: 2026-05-18T14:36:08.107Z

Reserved: 2026-03-10T13:45:40.017Z

Link: CVE-2026-28759

cve-icon Vulnrichment

Updated: 2026-05-18T14:35:59.925Z

cve-icon NVD

Status : Analyzed

Published: 2026-05-18T08:16:13.573

Modified: 2026-06-17T10:28:59.777

Link: CVE-2026-28759

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-05-18T10:00:13Z

Weaknesses