proxy.html

<html>
<head>
    <meta charset="UTF-8">
    <meta name="referrer" content="origin">
    <title>Proxy</title>
</head>
<body>
    <script>
        window.addEventListener('message', function(event) {
            var data = event.data;
            var source = event.source;

            if (data && data.message === 'proxyMessage' && data.message_id && data.method && data.url) {
                var xhr = new XMLHttpRequest();
                xhr.timeout = data.timeout || 60 * 1000;
                xhr.withCredentials = !!data.withCredentials;
                xhr.onreadystatechange = function () {
                    if (xhr.readyState === XMLHttpRequest.DONE) {
                        source.postMessage({
                            message: 'proxyResponse',
                            message_id: data.message_id,
                            status: xhr.status,
                            responseText: xhr.responseText
                        }, '*');
                    }
                };
                xhr.open(data.method, data.url);
                if (data.method === 'POST') {
                    xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
                }
                xhr.send(data.post_data);
            }
        });
    </script>
</body>
</html>