Mautic allows you to update the application via an upgrade script.
The upgrade logic isn't shielded off correctly, which may lead to vulnerable situation.
This vulnerability is mitigated by the fact that Mautic needs to be installed in a certain way to be vulnerable.
With access to edit a Mautic form, the attacker can add Cross-Site Scripting stored in the html filed. This could be used to steal sensitive information from the user's current session.
The logic in place to facilitate the update process via the user interface lacks access control to verify if permission exists to perform the tasks. Prior to this patch being applied it might be possible for an attacker to access the Mautic version number or to execute parts of the upgrade process without permission. As upgrading in the user interface is deprecated, this functionality is no longer required.