NotificationX <= 3.3.3 - Missing Authorization to Unauthenticated Arbitrary Content Injection via 'nx_id' Parameter on /popup-submit REST Route

The NotificationX plugin for WordPress is vulnerable to Missing Authorization leading to arbitrary content injection in versions up to, and including, 3.3.3 The /notificationx/v1/popup-submit REST route is registered with permission_callback => '__return_true', and the handler at PopupNotification::handle_popup_submission() accepts an attacker-controlled nx_id, loads whichever NotificationX campaign that id points to, and copies that campaign's own source into the new entry without verifying that the campaign is a Popup Notification or Exit Intent Popup (the two sources this form is intended to serve). This makes it possible for unauthenticated attackers to inject forged social-proof entries (name, title, message, email, theme) into any enabled non-popup NotificationX campaign — WP Comments, WooCommerce/EDD/Give sales, reviews, etc.