The Easy Appointments WordPress plugin before 3.11.2 does not validate and escape some of its shortcode attributes before outputting them back in the page, which could allow users with a role as low as contributor to perform Stored Cross-Site Scripting attacks which could be used against high privilege users such as admins.
https://wpscan.com/vulnerability/3e43156a-b784-4066-be69-23b139aafbad