The Ultimate Member WordPress plugin before 2.12.1 does not filter administrator-level...

The Ultimate Member WordPress plugin before 2.12.1 does not filter administrator-level capabilities from the roles it makes selectable on its registration forms, and its post-registration safeguard against elevated accounts is disabled by default, allowing unauthenticated users to register with a site-defined role that carries administrator capabilities and gain administrative access, when such a role exists and a role-selection field is present on a published registration form.