The Happyforms WordPress plugin before 1.26.3 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/5a9fd64b-3207-4acb-92ff-1cca08c41ac9/