Auth bypass in Mattermost

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

Vulnerability class: Broken Authentication

Published · last modified .

CVSS v3 metric

CVSS v3 base score 6.4 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:N/I:H/A:H.

EPSS exploit prediction

EPSS: 0.003 (20.3th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 20th percentile — 20.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-6673: held from 0.003 to 0.003.

EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: 0.000

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-6673?
CVE-2026-6673 is a medium-severity vulnerability in Mattermost, classified under Missing Authentication for Critical Function. CVSS score: 6.4/10. Published 2026-06-22.
How severe is CVE-2026-6673?
Medium severity. CVSS v3 base score is 6.4 out of 10.