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Description

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

PUBLISHED Reserved 2026-04-15 | Published 2026-05-18 | Updated 2026-05-18 | Assigner Mattermost




MEDIUM: 4.3CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

Problem types

CWE-346: Origin Validation Error

Product status

Default status
unaffected

11.5.0 (semver)
affected

11.4.0 (semver)
affected

11.6.0
unaffected

11.5.2
unaffected

11.4.4
unaffected

Credits

game0v3r finder

References

mattermost.com/security-updates (MMSA-2026-00636) vendor-advisory

cve.org (CVE-2026-6339)

nvd.nist.gov (CVE-2026-6339)

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