Paid Memberships Pro < 2.9.9 - Contributor+ Stored XSS via Shortcode

The plugin 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.

1. Insert the following shortcode into a page/post:

[pmpro_checkout_button class='" onmouseover="alert(1)"']

2. Browse the page and hover over the button.