CVE-2026-6339

Name: CVE-2026-6339
Creator: NVD / NIST
Published: 2026-05-18T09:16:23.573+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 4.3/10EPSS 0.11%

Last modified Jun 17, 2026

CVE-2026-6339 is a medium-severity vulnerability rated 4.3/10 on the CVSS scale. 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. EPSS estimates a 0.11% chance of exploitation in the next 30 days.

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

Metrics

CVSS 3.1

4.3/10

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

EPSS Probability

0.11%

1.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-346

Affected Software

Vendor	Product	Versions
Mattermost	Mattermost Server	>= 11.4.0, < 11.4.4
Mattermost	Mattermost Server	>= 11.5.0, < 11.5.2

References

https://mattermost.com/security-updatesVendor Advisory

Timeline

Published: May 18, 2026
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2026-6339?

How severe is CVE-2026-6339?

CVE-2026-6339 has a CVSS score of 4.3/10 (MEDIUM severity). The EPSS model estimates a 0.11% probability of exploitation in the next 30 days.

How do I fix CVE-2026-6339?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2026-6339?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST