CVE-2026-6339: Mattermost origin validation error in burn-on-read reveal endpoint

CVE-2026-6339 · Severity: medium · CVSS 4.3 · Published 2026-05-18

Mattermost is a collaboration platform used for secure team communication. A vulnerability in the 'burn-on-read' feature allows an authorized user to force a message to be revealed and subsequently deleted without the intended recipient's consent. This could be used to disrupt communications or prematurely destroy sensitive information by tricking the system into thinking the message has already been read.

Technologies: github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server/v5 (Go), github.com/mattermost/mattermost-server/v6 (Go), github.com/mattermost/mattermost/server/v8 (Go), Mattermost Server. Vendors: Go, Mattermost.

Quick answers

Is CVE-2026-6339 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-6339 being exploited in the wild.
How severe is CVE-2026-6339?
Medium severity, with a CVSS score of 4.3, published 18 May 2026.
Which products and versions are affected?
github.com/mattermost/mattermost-server (Go) SEMVER:introduced=0,fixed=5.3.2-0.20260327001745-7a339a6438f5, SEMVER:introduced=11.4.0+incompatible,fixed=11.4.4+incompatible,introduced=11.5.0+incompatible,fixed=11.5.2+incompatible; github.com/mattermost/mattermost-server/v5 (Go) SEMVER:introduced=0; github.com/mattermost/mattermost-server/v6 (Go) SEMVER:introduced=0; github.com/mattermost/mattermost/server/v8 (Go) SEMVER:introduced=0,fixed=8.0.0-20260327001745-7a339a6438f5, SEMVER:introduced=11.5.0,fixed=11.5.2; and 1 more.
Is there a fix for CVE-2026-6339?
Yes. github.com/mattermost/mattermost-server (Go): upgrade to 11.5.2+incompatible, 5.3.2-0.20260327001745-7a339a6438f5 or later; github.com/mattermost/mattermost/server/v8 (Go): upgrade to 11.5.2, 8.0.0-20260327001745-7a339a6438f5 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-6339 attacked?
It is attacked over the network, in a hosted service.
Am I affected by CVE-2026-6339?
Check whether you run github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server/v5 (Go), github.com/mattermost/mattermost-server/v6 (Go) or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • github.com/mattermost/mattermost-server (Go): upgrade to 11.5.2+incompatible, 5.3.2-0.20260327001745-7a339a6438f5 or later
    go get github.com/mattermost/mattermost-server@v11.5.2+incompatible
  • github.com/mattermost/mattermost/server/v8 (Go): upgrade to 11.5.2, 8.0.0-20260327001745-7a339a6438f5 or later
    go get github.com/mattermost/mattermost/server/v8@v11.5.2

Technical details

An origin validation error (CWE-346) exists in Mattermost Server's 'burn-on-read' (BoR) reveal endpoint. The application fails to verify the presence or validity of the X-Requested-With header, which is typically used to prevent cross-site request forgery and unauthorized automated interactions. An authenticated channel member can exploit this by embedding a crafted Markdown image tag in a post; when a recipient's client attempts to render the image, it triggers the reveal endpoint. This results in the message being 'burned' (deleted) without the recipient actually viewing the content. Patches are available in versions 11.5.2, 11.4.4, and specific backported builds.

Affected products

  • Mattermost Mattermost Server 11.5.0 - 11.5.1, 11.4.0 - 11.4.3, < 8.0.0-20260327001745-7a339a6438f5

Identifiers

  • CVE-2026-6339
  • GHSA-xvcx-mgpc-5xh3
  • GO-2026-5769

Timeline

  • 2026-05-18: disclosed
  • 2026-05-18: advisory
  • 2026-06-01: patched: GitHub advisory reviewed and updated with patch details.

References

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