The plugin does not sanitise and escape some of its Widget settings, which could allow high privilege users such as admin to perform Stored Cross-Site Scripting attacks even when the unfiltered_html capability is disallowed (for example in multisite setup)
Create/edit a dashboard with an HTML widget in it and put the following payload in the Title or Description fields: <svg/onload=alert(/XSS/)>
The XSS will be triggered when viewing the Dashboard page (/wp-admin/admin.php?page=fast-flow)