The Click to Chat WordPress plugin before 3.18.1 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/1666f91d-3aa2-487d-a31b-44d051ab0124