What is the CVSS score of CVE-2026-6339?

CVE-2026-6339 has a CVSS 3.1 base score of 4.3 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L. EPSS exploitation probability: 0.0%.

Is there a patch available for CVE-2026-6339?

Yes, a patch is available for CVE-2026-6339. Update the affected software to the latest version immediately.

Mattermost CVE-2026-6339

Q: Which versions of Mattermost are affected by CVE-2026-6339?

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…. A patch is available.

EUVD-2026-30749 MEDIUM

Origin Validation Error (CWE-346)

2026-05-18 Mattermost

GHSA-xvcx-mgpc-5xh3

Mattermost Information Disclosure

4.3

CVSS 3.1 · NVD

Severity by source

NVD PRIMARY

4.3 MEDIUM

AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

Primary rating from NVD · only source for this CVE.

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

Analysis Generated

May 18, 2026 - 09:32 vuln.today

DescriptionCVE.org

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. The CVSS vector indicates network-accessible exploitation by low-privileged authenticated users with low attack complexity. Exploitation status: no public exploit identified at time of analysis. EPSS data not provided.

Technical ContextAI

Burn-on-read is a Mattermost feature providing ephemeral messaging where messages are automatically deleted after being viewed by recipients. The vulnerability stems from inadequate Origin header validation (CWE-346: Origin Validation Error) on the burn-on-read reveal API endpoint. The endpoint fails to validate the X-Requested-With header, a common CSRF defense mechanism. Attackers exploit this by embedding malicious Markdown image tags in channel messages. When the Markdown renderer processes the image tag, it triggers an HTTP request to the reveal endpoint without proper origin checks, causing the burn-on-read message to be revealed server-side. The affected CPE (cpe:2.3:a:mattermost:mattermost) confirms this impacts the core Mattermost server application across specified version ranges, not plugins or mobile clients.

RemediationAI

Upgrade to Mattermost server versions newer than 11.5.1 or 11.4.3 as the vendor has released fixes per advisory MMSA-2026-00636. Specific patched versions not published in available data - consult https://mattermost.com/security-updates for exact fixed release numbers. As interim mitigation if immediate patching is infeasible, consider disabling burn-on-read messaging feature entirely until upgrade completes, though this eliminates the feature's business value. Implement strict channel membership controls to limit authenticated user access to sensitive burn-on-read channels, reducing insider threat surface. Configure web application firewalls to enforce X-Requested-With header presence on API endpoints if architectural compatibility permits, though this may break legitimate integrations. Monitor server logs for unusual access patterns to burn-on-read reveal endpoints, particularly high-frequency requests from single authenticated sessions. Note that compensating controls cannot fully prevent exploitation by legitimate authenticated channel members.