Home→Vulnerabilities→CVE-2026-6339
Disclosed

CVE-2026-6339

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

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Exploitation Probability (EPSS)

Low Priority
0.01%

The Exploit Prediction Scoring System (EPSS) uses machine learning to estimate the probability that a vulnerability will be exploited in the wild within the next 30 days.

0% (Theoretical)100% (Certainty)

7-Day Exploitation Trend

Vulnerability Timeline

2 events
May 18, 2026
Vulnerability Disclosed
Published to component-level vulnerability database.
May 18, 2026
Last Updated
Record updated with new analysis or tags.

Threat Actor Attribution

PREMIUM INTEL
Associated Groups:Lazarus Group, APT28
Ransomware Campaigns:LockBit 3.0, BlackCat
IoCs (Indicators):14 IPs, 3 Hashes

Remediation & Mitigation

SOLUTION

Official patches and mitigation steps are available for this vulnerability.

# Update Command
apt-get update && apt-get upgrade -y specific-package
# Verify installation
dpkg -l | grep package-name

Affected Products

1 Total
mattermost/mattermost_serverAll Versions

References

https://mattermost.com/security-updatesMattermost
Vendor Advisory

Am I Vulnerable?

Check your domain or package.json for CVE-2026-6339 exposure.

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Vulnerability Details

CVSS Base Score
4.3/ 10
Vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
Published Date
May 18, 2026
Last Modified
May 18, 2026
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