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Low, CVSS 3.5

Mattermost doesn't validate the Host header when constructing response URLs for custom slash command

CVE-2026-6333GHSA-vqp5-2mrp-qqxg

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (github.com/mattermost/mattermost/server/v8 11.5.2, 10.11.14, 8.0.0-20260325160634-e738016c5920; github.com/mattermost/mattermost-server 5.3.2-0.20260325160634-e738016c5920), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

  • GitHub Advisory Database and maintainer security advisories on GitHub

    GitHub-reviewed advisory GHSA-vqp5-2mrp-qqxg lists affected ranges: Go github.com/mattermost/mattermost/server/v8 >=11.5.0 =10.11.0 =0 =0 <5.3.2-0.20260325160634-e738016c5920

    VerifiedObserved Structured GitHub Advisory Database record (type=reviewed) from the GitHub REST API.

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
3.5 · CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:N/I:L/A:N
EPSS
  • CVE-2026-6333: 0.14% estimated exploitation activity in the next 30 days; percentile 3.11 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from ghsa record GHSA-vqp5-2mrp-qqxg

Agent brief

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-6333

Advisory: Mattermost doesn't validate the Host header when constructing response URLs for custom slash command
IDs: CVE-2026-6333, GHSA-vqp5-2mrp-qqxg
Record: https://patchnext.com/advisories/cve-2026-6333 (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: Low (CVSS 3.5)
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, 10.11.14, 8.0.0-20260325160634-e738016c5920; github.com/mattermost/mattermost-server 5.3.2-0.20260325160634-e738016c5920. 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; >= 10.11.0, < 10.11.14; < 8.0.0-20260325160634-e738016c5920; fixed in 10.11.14, 11.5.2, 8.0.0-20260325160634-e738016c5920
- Go github.com/mattermost/mattermost-server: affected < 5.3.2-0.20260325160634-e738016c5920; fixed in 5.3.2-0.20260325160634-e738016c5920

## 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-vqp5-2mrp-qqxg (GitHub-reviewed advisory GHSA-vqp5-2mrp-qqxg lists affected ranges: Go github.com/mattermost/mattermost/server/v8 >=11.5.0 =10.11.0 =0 =0 <5.3.2-0.20260325160634-e738016c5920)

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