function verify_secure_webhook() {

    if ( isset( $_GET['cid'] ) ) {

        $hash = md5( $_GET['cid'] . wp_fusion()->settings->get( 'access_key' ) );

        if ( $hash !== $_GET['scid'] ) {
            wp_die( 'Invalid.' );
        }

    }

}

add_action( 'init', 'verify_secure_webhook', 5 );