The Viral Signup WordPress plugin through 2.1 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/05024ff5-4c7a-4941-8dae-c1a8d2d4e202/