CVE-2026-6339
Medium
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
SeverityMedium
CVSS4.3
Exploit probability—
Product
Mattermost Server
PublishedMay 18, 2026
First seenMay 18, 2026
Weakness typeCWE-346CWE-346
How to fix
Update
- mattermost mattermost_server11.4.4, 11.5.2 or later
- github.com/mattermost/mattermost/server/v811.5.2, 11.4.4, 8.0.0-20260327001745-7a339a6438f5 or later
- github.com/mattermost/mattermost-server5.3.2-0.20260327001745-7a339a6438f5, 11.4.4+incompatible, 11.5.2+incompatible or later
Vendor advisories
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| mattermost mattermost_server | 11.4.0 ≤ v < 11.4.4, 11.5.0 ≤ v < 11.5.2 | 11.4.4, 11.5.2 |
| github.com/mattermost/mattermost/server/v8Go | 11.5.0 ≤ v < 11.5.2, 11.4.0 ≤ v < 11.4.4, v < 8.0.0-20260327001745-7a339a6438f5 | 11.5.2, 11.4.4, 8.0.0-20260327001745-7a339a6438f5 |
| github.com/mattermost/mattermost-serverGo | v < 5.3.2-0.20260327001745-7a339a6438f5, 11.4.0+incompatible ≤ v < 11.4.4+incompatible, 11.5.0+incompatible ≤ v < 11.5.2+incompatible | 5.3.2-0.20260327001745-7a339a6438f5, 11.4.4+incompatible, 11.5.2+incompatible |
| github.com/mattermost/mattermost-server/v5Go | No range specified | — |
| github.com/mattermost/mattermost-server/v6Go | No range specified | — |
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