CVE-2026-6339
MEDIUM4.3EPSS 0.02%Mattermost doesn't validate the X-Requested-With header on the burn-on-read reveal endpoint
Published: 5/18/2026Modified: 6/1/2026
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
Affected packages (2)
- Go/github.com/mattermost/mattermost-serverfrom 0, < 5.3.2-0.20260327001745-7a339a6438f5
- Go/github.com/mattermost/mattermost/server/v8>= 11.5.0, < 11.5.2
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | MEDIUM4.3 | CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L |