Message Reveal Vulnerability in Mattermost by Mattermost Inc.
CVE-2026-6339

4.3MEDIUM

Key Information:

Vendor

Mattermost
Status
Mattermost
Vendor
CVE Published:
18 May 2026

What is CVE-2026-6339?

Mattermost versions 11.5.x (up to 11.5.1) and 11.4.x (up to 11.4.3) are affected by a vulnerability where the X-Requested-With header is not properly validated on the burn-on-read reveal endpoint. This inadequacy allows an authenticated member of a channel to exploit this issue, enabling the unauthorized revelation of burn-on-read messages without the recipient's consent. This can be achieved through a specially crafted Markdown image tag, posing a significant risk to user privacy.

Affected Version(s)

Mattermost 11.5.0 <= 11.5.1

Mattermost 11.4.0 <= 11.4.3

Mattermost 11.6.0

References

https://mattermost.com/security-updates
vendor-advisory

CVSS V3.1

Score:
4.3
Severity:
MEDIUM
Confidentiality:
None
Integrity:
None
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
Low
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

game0v3r
.