CVE-2026-5163: Mattermost doesn't verify channel membership when processing AI-assisted message rewrites

May 18, 2026 (updated June 1, 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

References

github.com/advisories/GHSA-8r89-8w26-cq32
github.com/mattermost/mattermost/commit/f4d1abe7e8f545f1a87f463fa9fe451c731aebf8
mattermost.com/security-updates
nvd.nist.gov/vuln/detail/CVE-2026-5163

Code Behaviors & Features

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