CVE-2026-6339 MEDIUM

CVE-2026-6339: Missing request origin validation on burn-on-read reveal endpoint

Vendor Mattermost
Product Mattermost
Weakness CWE-346 · Origin validation
Published May 18, 2026
Last update May 18, 2026
View on NVD All CVEs

CVSS base score

4.3/10
Attack vector Network
Attack complexity Low
Privileges required Low
User interaction None
Confidentiality None
Integrity None

CVSS vector

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

What the vulnerability does

01Description

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

Key dates

02Disclosure timeline

May 18, 2026 CVE published
May 18, 2026 Record updated