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

Source: https://junglewise.ai/threats/cve-2026-6333-mattermost-server-ssrf-via-host-header-in-slash-commands  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-18. Updated: 2026-06-01  
License: CC BY 4.0 with a link back

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)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server), [github.com/mattermost/mattermost-server/v5 (Go)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server-v5), [github.com/mattermost/mattermost-server/v6 (Go)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server-v6), [github.com/mattermost/mattermost/server/v8 (Go)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server-v8), [Mattermost Server](https://junglewise.ai/threats/technologies/server-public). Vendors: [Go](https://junglewise.ai/threats/vendors/go), [Mattermost](https://junglewise.ai/threats/vendors/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

  ```sh
  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

  ```sh
  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

- [https://mattermost.com/security-updates](https://mattermost.com/security-updates)
- [https://github.com/mattermost/mattermost/commit/e738016c592045e14bf926eafaeda6f8521def6d](https://github.com/mattermost/mattermost/commit/e738016c592045e14bf926eafaeda6f8521def6d)

## Related threats

- [Mattermost CSRF via Agents plugin mention reminder validation](https://junglewise.ai/threats/cve-2026-93851-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10)
- [Mattermost denial of service via PDF text extraction memory exhaustion](https://junglewise.ai/threats/cve-2026-93850-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11)
- [Mattermost denial of service in WebSocket msgpack decoder](https://junglewise.ai/threats/cve-2026-93849-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11)
- [Mattermost information disclosure in post deletion events](https://junglewise.ai/threats/cve-2026-19621-mattermost-versions-11-10-x-11-10-0-11-7-x-11-7-9-11-9-x-11-9-1)
- [Mattermost authentication token leak in server logs](https://junglewise.ai/threats/cve-2026-19600-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11)