CVE-2026-6339

🔶 medium

Summary

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 Score

4.3

Medium

EPSS Score

0.0

Exploit Probability

Published Date

2026-05-18

First Seen: 2026-05-19

📊 Relative Risk Intelligence

This CVE is Lower Risk - more severe than 5.4% of all 321,566 vulnerabilities in our database.

#304,103

Below average severity

Severity Percentile

🎯 CISA SSVC Assessment Updated: May 18, 2026

🔍 Exploitation Status

None

No known exploits

⚙️ Automatable

Requires human interaction

💥 Technical Impact

Partial

Limited system impact

🏆 Discovered By

game0v3r

SSVC data provided by CISA

Last Modified 2026-05-18

Source NVD 🔗

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

CWE IDs (Weakness Types)

CWE-346

📦 Affected Products 2

Vendor	Product	Status	Affected Versions
mattermost	mattermost_server	Affected	> 11.4.0 < 11.4.4
mattermost	mattermost_server	Affected	> 11.5.0 < 11.5.2

🔗 References 1

https://mattermost.com/security-updates

Vendor Advisory

🔗 Related CVEs 6

CVE ID	Severity	CVSS	Summary	Published
CVE-2026-28759	🔶 medium	4.3	Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has a...	2026-05-18
CVE-2026-2325	🔶 medium	4.3	Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to limit the size of the request body o...	2026-05-18
CVE-2026-3495	ℹ️ low	3.8	Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to escape some variables that could contain malicious con...	2026-05-18
CVE-2026-3637	🔶 medium	4.3	Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to check the create_post channel permis...	2026-05-18
CVE-2026-4273	ℹ️ low	3.7	Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate that the RefreshedToken differs from the orig...	2026-05-18
CVE-2026-6340	🔶 medium	4.3	Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate 7zip archive structure befo...	2026-05-18

These CVEs affect the same products