Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. Prior to version 3.3.3, a privilege escalation vulnerability exists in the Joplin server, allowing non-admin users to exploit the API endpoint "PATCH /api/users/:id" to set the "is_admin" field to 1. The vulnerability allows malicious low-privileged users to perform administrative actions without proper authorization. This issue has been patched in version 3.3.3.