The Simple Membership WordPress plugin before 4.2.2 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 admin.
https://wpscan.com/vulnerability/b195c373-1db9-4fd7-98d0-0860dacd189e