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

No advisories yet.

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