ProfilePress < 3.1.11 - Unauthenticated Cross-Site Scripting (XSS) in tabbed login/register widget

Description

The plugin's widget for tabbed login/register was not properly escaped and could be used in an XSS attack which could lead to wp-admin access.  Further, the plugin in several places assigned $_POST as $_GET which meant that in some cases this could be replicated with just $_GET parameters and no need for $_POST values.

Proof of Concept

1. Add the registration shortcode to a page.
2. Add the tabbed login/register widget on the same page.
3. Visit the page logged out and add this to the end of the URL /?tabbed-login-name="><script>alert(1)</script>

This could also be replicated by posting to any page with the widget present, which would not need the registration shortcode to be present on the page.