Executive brief
listmonk is a self-hosted newsletter and mailing list manager. A security flaw in its permission system allows users in a multi-user environment to access or interact with mailing lists they are not authorized to see. This could lead to unauthorized access to subscriber data or the ability to send unauthorized test emails using restricted lists.
Technical details
An authorization bypass vulnerability (CWE-639) exists in listmonk due to missing or insufficient permission checks across several API handlers. In multi-user environments, an authenticated attacker with low privileges can bypass intended restrictions to access mailing lists, export subscriber data to JSON, or trigger test emails to subscribers in lists they do not own. The vulnerability specifically impacts campaign updates, CSV imports, bulk subscriber handlers, and the test mail endpoint. The issue is resolved in version 6.1.0 by implementing granular list and subscriber permission filtering.
Affected products
- knadh listmonk >= 4.1.0, < 6.1.0
Timeline
- 2026-03-29: patched: Version 6.1.0 released
- 2026-03-31: advisory: GitHub Security Advisory published
- 2026-04-02: disclosed: CVE-2026-34584 published