CVE-2026-5163: Missing authorization check in AI message rewrite endpoint allows access to private thread content

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-18 · updated 2026-10-14
Weakness: CWE-862

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

Affected products

Mattermost: from 11.5.0 before 11.5.2 — fixed in 11.5.2

How to fix

Upgrade to a fixed version: Mattermost 11.5.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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