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/ebe3e873-1259-43b9-a027-daa4dbd937f3