Everest Forms Pro <= 1.9.12 - Unauthenticated Remote Code Execution via Calculation Field

The Everest Forms Pro plugin for WordPress is vulnerable to Remote Code Execution via PHP Code Injection in all versions up to, and including, 1.9.12. This is due to the Calculation Addon's process_filter() function concatenating user-submitted form field values into a PHP code string without proper escaping before passing it to eval(). The sanitize_text_field() function applied to input does not escape single quotes or other PHP code context characters. This makes it possible for unauthenticated attackers to inject and execute arbitrary PHP code on the server by submitting a crafted value in any string-type form field (text, email, URL, select, radio) when a form uses the "Complex Calculation" feature.