An XSS vulnerability in the popup-builder plugin before 3.64.1 for WordPress allows remote attackers to inject arbitrary JavaScript into existing popups via an unsecured ajax action in com/classes/Ajax.php. It is possible for an unauthenticated attacker to insert malicious JavaScript in several of the popup’s fields by sending a request to wp-admin/admin-ajax.php with the POST action parameter of sgpb_autosave and including additional data in an allPopupData parameter, including the popup’s ID (which is visible in the source of the page in which the popup is inserted) and arbitrary JavaScript which will then be executed in the browsers of visitors to that page. Because the plugin functionality automatically adds script tags to data entered into these fields, this injection will typically bypass most WAF applications.
