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Mattermost CVE-2026-28759

| EUVD-2026-30738 MEDIUM
Incorrect Authorization (CWE-863)
2026-05-18 Mattermost GHSA-8h9w-w78c-vvr3
Authentication Bypass Mattermost
4.3
CVSS 3.1
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CVSS VectorNVD

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
None

Lifecycle Timeline

1
Analysis Generated
May 18, 2026 - 08:16 vuln.today

DescriptionNVD

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

AnalysisAI

Authorization bypass in Mattermost shared channel synchronization allows authenticated remote cluster administrators to remove arbitrary users from any channel, including private channels outside the attacker's authorization scope. Affects versions 11.5.x through 11.5.1, 10.11.x through 10.11.13, and 11.4.x through 11.4.3. …

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CVE-2026-28759 vulnerability details – vuln.today
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