The Popup box WordPress plugin before 3.8.6 does not sanitise and escape some of its 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)
https://wpscan.com/vulnerability/f1eb05e8-1b7c-45b1-912d-f668bd68e265