Mattermost doesn't validate whether users were correctly owned by the correct Connected Workspace

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

2.7 · CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:N

EPSS

• CVE-2026-27769: 0.17% estimated exploitation activity in the next 30 days; percentile 6.00 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Mattermost versions 10.11.x <= 10.11.12 fail to validate whether users were correctly owned by the correct Connected Workspace which allows a malicious remote server connected using the Conntexted Workspaces feature to change the displayed status of local users via the Connected Workspaces API. Mattermost Advisory ID: MMSA-2026-00603.

Published by the source

17 Apr 2026, 15:31 UTC

Last modified by the source

28 Apr 2026, 23:02 UTC

Source last checked

26 Oct 2026, 15:19 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-mxxh-fmjq-j6x4

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

# Patch Next advisory brief: CVE-2026-27769

Advisory: Mattermost doesn't validate whether users were correctly owned by the correct Connected Workspace
IDs: CVE-2026-27769, GHSA-mxxh-fmjq-j6x4
Record: https://patchnext.com/advisories/cve-2026-27769 (revision 1, source checked 26 Oct 2026, 15:19 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.7)
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 8.0.0-20260316060126-bc1a2b34b1f9. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/mattermost/mattermost-server: affected >= 8.0.0-20250721062209-4952acea88ce, < 8.0.0-20260316060126-bc1a2b34b1f9; fixed in 8.0.0-20260316060126-bc1a2b34b1f9

## Suggested inspection (read-only)
- 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.
- 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-mxxh-fmjq-j6x4 (GitHub-reviewed advisory GHSA-mxxh-fmjq-j6x4 lists affected ranges: Go github.com/mattermost/mattermost-server >=8.0.0-20250721062209-4952acea88ce <8.0.0-20260316060126-bc1a2b34b1f9)

Commands are suggestions: review each one before running it.