CVE-2026-5139: GitLab Plugin Allows Non-Admin Users to Modify Default Instance Configuration

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-22 · updated 2026-06-23
Weakness: CWE-862

Description

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

Affected products

Mattermost: from 10.11.0 before 10.11.18; from 11.5.0 before 11.5.6; from 11.6.0 before 11.6.3; from 11.7.0 before 11.7.1 — fixed in 10.11.18, 11.5.6, 11.6.3, 11.7.1

How to fix

Upgrade to a fixed version: Mattermost 10.11.18, 11.5.6, 11.6.3, 11.7.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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