The Paid Memberships Pro WordPress plugin before 2.9.9 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/ae103336-a411-4ebf-a5f0-2f35701e364c