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Mattermost CVE-2026-6339

| EUVD-2026-30749 MEDIUM
Origin Validation Error (CWE-346)
2026-05-18 Mattermost GHSA-xvcx-mgpc-5xh3
Information Disclosure Mattermost
4.3
CVSS 3.1
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CVSS VectorNVD

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

Lifecycle Timeline

1
Analysis Generated
May 18, 2026 - 09:32 vuln.today

DescriptionNVD

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

AnalysisAI

Authenticated Mattermost channel members can forcibly reveal burn-on-read messages without recipient consent by exploiting missing X-Requested-With header validation on the reveal endpoint through crafted Markdown image tags. This bypasses the intended ephemeral messaging security control in Mattermost versions 11.4.x through 11.4.3 and 11.5.x through 11.5.1. …

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CVE-2026-6339 vulnerability details – vuln.today
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