Input validation vulnerability in Simplr Registration Form Plus+ 2.3.4

The Simplr Registration Form Plus+ plugin for WordPress is vulnerable to a type of malicious attack called Reflected Cross-Site Scripting. This vulnerability exists in versions of the plugin up to and including 2.3.4. This means any users of the plugin with a version that old are at risk. It happens because the plugin failed to properly sanitize input and escape output when using a certain type of command (add_query_arg()). This allows an attacker to inject malicious code into a page that then executes if they can get a user to perform an action like clicking a link.