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

Project Subscriptions

Vendors Products
Mattermost Subscribe
Mattermost Subscribe
Mattermost Server Subscribe
Advisories
Source ID Title
Github GHSA Github GHSA GHSA-8r89-8w26-cq32 Mattermost doesn't verify channel membership when processing AI-assisted message rewrites
Fixes

Solution

Update Mattermost to versions 11.6.0, 11.5.2 or higher.


Workaround

No workaround given by the vendor.

References
History

Mon, 18 May 2026 19:15:00 +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 15:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

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


Mon, 18 May 2026 10:45:00 +0000

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

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

Type Values Removed Values Added
Description 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
Title Missing authorization check in AI message rewrite endpoint allows access to private thread content
Weaknesses CWE-862
References
Metrics cvssV3_1

{'score': 6.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N'}


Projects

Sign in to view the affected projects.

cve-icon MITRE

Status: PUBLISHED

Assigner: Mattermost

Published:

Updated: 2026-05-18T14:40:03.490Z

Reserved: 2026-03-30T14:35:03.836Z

Link: CVE-2026-5163

cve-icon Vulnrichment

Updated: 2026-05-18T14:39:56.738Z

cve-icon NVD

Status : Analyzed

Published: 2026-05-18T09:16:23.273

Modified: 2026-05-18T19:12:19.827

Link: CVE-2026-5163

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-05-18T11:00:11Z

Weaknesses