Vulnerability · Published Jun 22, 2026 · Assigned by Mattermost

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

Medium 5.4

In plain English

A medium-severity missing authorization in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 21% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.4Medium

Exploit odds (30 days)0.3%top 78.6% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official 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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Go	github.com/mattermost/mattermost-server	11.7.1	GHSA-2g8v-grq3-hq2g
Go	github.com/mattermost/mattermost-server	11.6.3	GHSA-2g8v-grq3-hq2g
Go	github.com/mattermost/mattermost-server	11.5.6	GHSA-2g8v-grq3-hq2g
Go	github.com/mattermost/mattermost-server	10.11.18	GHSA-2g8v-grq3-hq2g
Go	github.com/mattermost/mattermost-server	10.11.18+incompatible, 11.5.6+incompatible, 11.6.3+incompatible, 11.7.1+incompatible	GO-2026-6480

Affected products (as reported)

Vendor	Product	Versions
Mattermost	Mattermost	11.7.0 – through 11.7.011.6.0 – through 11.6.211.5.0 – through 11.5.510.11.0 – through 10.11.17

Weakness: CWE-862 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L

References

MMSA-2026-00644 vendor-advisory