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Mattermost CVE-2026-5163

| EUVD-2026-30753 MEDIUM
Missing Authorization (CWE-862)
2026-05-18 Mattermost GHSA-8r89-8w26-cq32
Authentication Bypass Mattermost
6.5
CVSS 3.1
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CVSS VectorNVD

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

Lifecycle Timeline

1
Analysis Generated
May 18, 2026 - 09:32 vuln.today

DescriptionNVD

Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645

AnalysisAI

Authenticated Mattermost users can read private channel threads and direct messages they lack access to by exploiting the AI post rewrite endpoint. Versions 11.5.0 and 11.5.1 fail to verify channel membership before processing AI-assisted message rewrites, enabling privilege escalation from low-privileged authenticated users to access confidential communications. …

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CVE-2026-5163 vulnerability details – vuln.today
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