What is CVE-2026-5139?

Mattermost versions 11.7.x }} slash command.. Mattermost Advisory ID: MMSA-2026-00644

How severe is CVE-2026-5139?

CVE-2026-5139 has a CVSS 3.x base score of 5.4, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability low.

Is CVE-2026-5139 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (19th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-5139?

CVE-2026-5139 affects Mattermost Mattermost Server. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-5139?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-5139 have an EU (EUVD) identifier?

Yes. CVE-2026-5139 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-38246.

When was CVE-2026-5139 published?

CVE-2026-5139 was published on 2026-06-22 and last updated on 2026-06-23.

CVE-2026-5139

CVE-2026-5139 is a medium-severity vulnerability in Mattermost Mattermost Server with a CVSS 3.x base score of 5.4. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-862.

Key facts

Severity: Medium (CVSS 3.x base score 5.4)
EPSS exploit prediction: 0% (19th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-38246
Weakness: CWE-862
Affected product: Mattermost Mattermost Server
Published: 2026-06-22
Last modified: 2026-06-23

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

Frequently asked questions

What is CVE-2026-5139?: 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
How severe is CVE-2026-5139?: CVE-2026-5139 has a CVSS 3.x base score of 5.4, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability low.
Is CVE-2026-5139 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (19th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-5139?: CVE-2026-5139 affects Mattermost Mattermost Server. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-5139?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-5139 have an EU (EUVD) identifier?: Yes. CVE-2026-5139 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-38246.
When was CVE-2026-5139 published?: CVE-2026-5139 was published on 2026-06-22 and last updated on 2026-06-23.

References

https://mattermost.com/security-updates

Affected products (1)

cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*

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