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Medium, CVSS 5.4amended

Mattermost doesn't enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler

CVE-2026-5139GHSA-2g8v-grq3-hq2g

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (github.com/mattermost/mattermost-server 11.7.1, 11.6.3, 11.5.6, 10.11.18), 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

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
5.4 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L
EPSS
  • CVE-2026-5139: 0.29% estimated exploitation activity in the next 30 days; percentile 21.33 (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 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler, which allows any authenticated user to overwrite the global default GitLab instance configuration via the {{/gitlab connect <instance-name}} slash command.. Mattermost Advisory ID: MMSA-2026-00644

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-2g8v-grq3-hq2g

  2. Revision 1 · created ·

    Published from osv record GO-2026-6480

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-2026-5139

Advisory: Mattermost doesn't enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler
IDs: CVE-2026-5139, GHSA-2g8v-grq3-hq2g
Record: https://patchnext.com/advisories/cve-2026-5139 (revision 2, source checked 26 Oct 2026, 15:29 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.4)
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 11.7.1, 11.6.3, 11.5.6, 10.11.18. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/mattermost/mattermost-server: affected >= 11.7.0, < 11.7.1; >= 11.6.0, < 11.6.3; >= 11.5.0, < 11.5.6; >= 10.11.0, < 10.11.18; fixed in 11.6.3, 11.7.1, 10.11.18, 11.5.6

## 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://osv.dev/vulnerability/GO-2026-6480 (OSV record GO-2026-6480 lists affected ranges: Go github.com/mattermost/mattermost-server >=10.11.0+incompatible =11.5.0+incompatible =11.6.0+incompatible =11.7.0+incompatible <11.7.1+incompatible (fixed))
- Verified: https://github.com/advisories/GHSA-2g8v-grq3-hq2g (GitHub-reviewed advisory GHSA-2g8v-grq3-hq2g lists affected ranges: Go github.com/mattermost/mattermost-server >=11.7.0 =11.6.0 =11.5.0 =10.11.0 <10.11.18)

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