Mattermost doesn't validate the X-Requested-With header on the burn-on-read reveal endpoint

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-6339: 0.11% estimated exploitation activity in the next 30 days; percentile 1.40 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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

Published by the source

18 May 2026, 09:31 UTC

Last modified by the source

1 Jun 2026, 15:22 UTC

Source last checked

26 Oct 2026, 15:23 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-xvcx-mgpc-5xh3

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# Patch Next advisory brief: CVE-2026-6339

Advisory: Mattermost doesn't validate the X-Requested-With header on the burn-on-read reveal endpoint
IDs: CVE-2026-6339, GHSA-xvcx-mgpc-5xh3
Record: https://patchnext.com/advisories/cve-2026-6339 (revision 1, source checked 26 Oct 2026, 15:23 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/mattermost/mattermost/server/v8 11.5.2, 11.4.4, 8.0.0-20260327001745-7a339a6438f5; github.com/mattermost/mattermost-server 5.3.2-0.20260327001745-7a339a6438f5. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/mattermost/mattermost/server/v8: affected >= 11.5.0, < 11.5.2; >= 11.4.0, < 11.4.4; < 8.0.0-20260327001745-7a339a6438f5; fixed in 11.4.4, 11.5.2, 8.0.0-20260327001745-7a339a6438f5
- Go github.com/mattermost/mattermost-server: affected < 5.3.2-0.20260327001745-7a339a6438f5; fixed in 5.3.2-0.20260327001745-7a339a6438f5

## Suggested inspection (read-only)
- Go github.com/mattermost/mattermost/server/v8: check the installed version with your package manager.
- Go github.com/mattermost/mattermost-server: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-xvcx-mgpc-5xh3 (GitHub-reviewed advisory GHSA-xvcx-mgpc-5xh3 lists affected ranges: Go github.com/mattermost/mattermost/server/v8 >=11.5.0 =11.4.0 =0 =0 <5.3.2-0.20260327001745-7a339a6438f5)

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