CVE-2026-5163
Mattermost doesn't verify channel membership when processing AI-assisted message rewrites
6.5
MEDIUM
CVSS 3.1
EPSS 0.04%
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
How to fix CVE-2026-5163
To remediate CVE-2026-5163, upgrade the affected package to a fixed version below.
- —upgrade to 5.3.2-0.20260401090745-f4d1abe7e8f5 or later
- —upgrade to 11.5.2 or later
Is CVE-2026-5163 being exploited?
Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.
Affected packages (2)
- from 0, < 5.3.2-0.20260401090745-f4d1abe7e8f5
- >= 11.5.0, < 11.5.2
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | MEDIUM6.5 | CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N |