The Forminator Forms WordPress plugin before 1.57.2.1 does not restrict which classes may be instantiated when it deserialises a value taken from an XML-RPC request, allowing users who hold its forms-management permission to write a file of their choosing and execute arbitrary code. That permission belongs to an administrator by default, and to any role the site has granted it through the Forminator Forms WordPress plugin before 1.57.2.1's own settings, so the issue is reachable well below administrator on sites that use that feature.