CVE-2026-6673: Mattermost Jira plugin had unauthenticated {{/ac/installed}} lifecycle callback during pending Jira Cloud install

Severity: medium (CVSS 6.4 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-306

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

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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