CVE-2026-6333: Mattermost Server SSRF via Host header in slash commands

CVE-2026-6333 · Severity: low · CVSS 3.5 · Published 2026-05-18

Mattermost, a collaboration and messaging platform, is affected by a vulnerability in how it handles custom slash commands. An authenticated user could manipulate network headers to redirect command responses to a server they control. This could lead to the redirection of internal communications or automated responses to unauthorized external destinations.

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-2026-6333 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-6333 being exploited in the wild.
How severe is CVE-2026-6333?
Low severity, with a CVSS score of 3.5, published 18 May 2026.
Which products and versions are affected?
github.com/mattermost/mattermost-server (Go) SEMVER:introduced=0,fixed=5.3.2-0.20260325160634-e738016c5920, SEMVER:introduced=10.11.0+incompatible,fixed=10.11.14+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=8.0.0-20260325160634-e738016c5920, SEMVER:introduced=11.5.0,fixed=11.5.2; and 1 more.
Is there a fix for CVE-2026-6333?
Yes. github.com/mattermost/mattermost-server (Go): upgrade to 11.5.2+incompatible, 5.3.2-0.20260325160634-e738016c5920 or later; github.com/mattermost/mattermost/server/v8 (Go): upgrade to 11.5.2, 8.0.0-20260325160634-e738016c5920 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-6333 attacked?
It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-6333?
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, 5.3.2-0.20260325160634-e738016c5920 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, 8.0.0-20260325160634-e738016c5920 or later
    go get github.com/mattermost/mattermost/server/v8@v11.5.2

Technical details

Mattermost versions 11.5.x (<= 11.5.1) and 10.11.x (<= 10.11.13) are vulnerable to a Server-Side Request Forgery (SSRF) variant. The application fails to validate the 'Host' header when generating response URLs for custom slash commands. An authenticated attacker can provide a spoofed Host header in their request, causing the server to construct and send slash command responses to an attacker-controlled destination. The fix involves requiring and validating the 'SiteURL' for external-facing slash commands rather than relying on user-supplied headers.

Affected products

  • Mattermost Mattermost Server >= 11.5.0, < 11.5.2; >= 10.11.0, < 10.11.14; < 8.0.0-20260325160634-e738016c5920

Identifiers

  • CVE-2026-6333
  • GHSA-vqp5-2mrp-qqxg
  • GO-2026-5671

Timeline

  • 2026-03-25: patched: Fix committed to repository
  • 2026-05-18: disclosed: Initial advisory and CVE published

References

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