{"id":"CVE-2026-6339","title":"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 re…","summary":"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 re…","severity":"medium","cvss":4.3,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L","cwe":["CWE-346"],"vendor":"mattermost","product":"mattermost_server","affected":["mattermost_server >= 11.4.0, < 11.4.4","mattermost_server >= 11.5.0, < 11.5.2"],"patched":["mattermost_server 11.5.2"],"published":"2026-05-18","updated":"2026-10-14","sourceUpdated":"2026-10-14T20:10:00.190","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-6339","references":[{"url":"https://mattermost.com/security-updates","label":"responsibledisclosure@mattermost.com"}],"tags":["nvd"],"epss":0.00157,"epssPercentile":0.04349,"ingestedAt":"2026-10-14T20:53:12.300Z","slug":"CVE-2026-6339","body":"## Overview\n\nMattermost 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\n\n## Affected\n\n- `mattermost_server >= 11.4.0, < 11.4.4`\n- `mattermost_server >= 11.5.0, < 11.5.2`\n\n## Remediation\n\nUpgrade past the affected range:\n\n- `mattermost_server 11.5.2`","depth":"sunlit","depthScore":24,"depthScoreParts":{"impact":23.7,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}