CVE-2026-6339: Mattermost doesn't validate the X-Requested-With header on the burn-on-read reveal endpoint

May 18, 2026 (updated June 1, 2026)

Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636

References

github.com/advisories/GHSA-xvcx-mgpc-5xh3
github.com/mattermost/mattermost/commit/7a339a6438f5a4a5feba6b8de887f17a1378b207
mattermost.com/security-updates
nvd.nist.gov/vuln/detail/CVE-2026-6339

Code Behaviors & Features

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