Update the WordPress Ninja Forms plugin to the latest available version (at least 188.8.131.52).
Muhammad Talha Khan discovered and reported this Open Redirection vulnerability in WordPress Ninja Forms Plugin. This could allow a malicious actor to redirect users from one site to the other due to the redirect URL not being validated. Users could be tricked to visiting a legitimate site to then be redirected to a malicious site and cause a phishing incident. This vulnerability has been fixed in version 184.108.40.206.