How severe is CVE-2026-6673?

CVE-2026-6673 has a CVSS 3.x base score of 6.4, rated medium severity. It is exploitable over network with high attack complexity, requires low privileges and user interaction. Impact on confidentiality is none, integrity high, and availability high.

Is CVE-2026-6673 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (21st percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-6673?

CVE-2026-6673 affects Mattermost Mattermost Server. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-6673?

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

Does CVE-2026-6673 have an EU (EUVD) identifier?

Yes. CVE-2026-6673 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-38249.

When was CVE-2026-6673 published?

CVE-2026-6673 was published on 2026-06-22 and last updated on 2026-06-23.

CVE-2026-6673

CVE-2026-6673 is a medium-severity vulnerability in Mattermost Mattermost Server with a CVSS 3.x base score of 6.4. 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-306.

Key facts

Severity: Medium (CVSS 3.x base score 6.4)
EPSS exploit prediction: 0% (21st percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-38249
Weakness: CWE-306
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 authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret and disrupt the Jira integration via POST to /ac/installed during the pending-install window.. Mattermost Advisory ID: MMSA-2026-00654

Frequently asked questions

What is CVE-2026-6673?: 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 authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret and disrupt the Jira integration via POST to /ac/installed during the pending-install window.. Mattermost Advisory ID: MMSA-2026-00654
How severe is CVE-2026-6673?: CVE-2026-6673 has a CVSS 3.x base score of 6.4, rated medium severity. It is exploitable over network with high attack complexity, requires low privileges and user interaction. Impact on confidentiality is none, integrity high, and availability high.
Is CVE-2026-6673 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (21st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-6673?: CVE-2026-6673 affects Mattermost Mattermost Server. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-6673?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-6673 have an EU (EUVD) identifier?: Yes. CVE-2026-6673 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-38249.
When was CVE-2026-6673 published?: CVE-2026-6673 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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