Vulnerability · Published May 18, 2026 · Assigned by Mattermost
CVE-2026-6339Missing request origin validation on burn-on-read reveal endpoint
Medium 4.3
In plain English
A medium-severity security flaw in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.1% (higher than 1% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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 & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | 11.4.4+incompatible, 11.5.2+incompatible | GO-2026-5769 |
| Go | github.com/mattermost/mattermost-server | 5.3.2-0.20260327001745-7a339a6438f5 | GHSA-xvcx-mgpc-5xh3 |
| Go | github.com/mattermost/mattermost-server/v5 | no fix listed | GO-2026-5769 |
| Go | github.com/mattermost/mattermost-server/v6 | no fix listed | GO-2026-5769 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260327001745-7a339a6438f5 | GO-2026-5769 |
| Go | github.com/mattermost/mattermost/server/v8 | 11.4.4 | GHSA-xvcx-mgpc-5xh3 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260327001745-7a339a6438f5 | GHSA-xvcx-mgpc-5xh3 |
| Go | github.com/mattermost/mattermost/server/v8 | 11.5.2 | GHSA-xvcx-mgpc-5xh3 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.5.0 – through 11.5.111.4.0 – through 11.4.3 |
Weakness: CWE-346 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
References
- MMSA-2026-00636 vendor-advisory