CVE-2026-6333: Mattermost doesn't validate the Host header when constructing response URLs for custom slash command

May 18, 2026 (updated June 1, 2026)

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate the Host header when constructing response URLs for custom slash commands which allows an authenticated attacker to redirect slash command responses to an attacker-controlled server via a spoofed Host header.. Mattermost Advisory ID: MMSA-2026-00582

References

github.com/advisories/GHSA-vqp5-2mrp-qqxg
github.com/mattermost/mattermost/commit/e738016c592045e14bf926eafaeda6f8521def6d
mattermost.com/security-updates
nvd.nist.gov/vuln/detail/CVE-2026-6333

Code Behaviors & Features

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