CVE-2026-5163 PUBLISHED

Missing authorization check in AI message rewrite endpoint allows access to private thread content

Assigner: Mattermost
Reserved: 30.03.2026 Published: 18.05.2026 Updated: 18.05.2026

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

Metrics

CVSS 3.1

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
CVSS Score: 6.5

Attack Vector	Network	Scope	Unchanged
Attack Complexity	Low	Confidentiality Impact	High
Privileges Required	Low	Integrity Impact	None
User Interaction	None	Availability Impact	None

CVSS 3.1

Product Status

Vendor	Mattermost
Product	Mattermost
Versions	Default: unaffected affected from 11.5.0 to 11.5.1 (incl.) Version 11.6.0 is unaffected Version 11.5.2 is unaffected

Solutions

Update Mattermost to versions 11.6.0, 11.5.2 or higher.

Credits

daw10 finder

References

MMSA-2026-00645

Problem Types

CWE-862: Missing Authorization CWE