The ConvertKit WordPress plugin before 2.0.5 does not validate and escapes some of its shortcode attributes before outputting them back in the page, which could allow users with a role as low as a contributor to perform Stored Cross-Site Scripting attacks, which could be used against high-privilege users such as admins.
https://wpscan.com/vulnerability/5101a979-7a53-40bf-8988-6347ef851eab