CVE-2026-6339

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

CVSS v3.1 Score

4.3

MEDIUM

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

EPSS — Exploit Prediction

0.0002

Probability of exploitation

0.04%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-346 CWE-346

Affected Products

Vendor	Product	Versions
mattermost	mattermost_server	11.4.0 — 11.4.4
mattermost	mattermost_server	11.5.0 — 11.5.2

References

Advisories & Patches

https://mattermost.com/security-updates

Frequently Asked Questions

What is 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 It has a CVSS v3.1 base score of 4.3 (MEDIUM).

How severe is CVE-2026-6339? +

CVE-2026-6339 has a CVSS v3.1 score of 4.3 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0002, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-6339? +

CVE-2026-6339 affects products from mattermost, specifically: mattermost_server. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-6339? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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