Webkit’s XSS Auditor explained and current exploits

Webkit is a open source browser engine used by Safari and Chrome. To prevent cross site scripting attacks (number 3 in this years in the security vulnerabilities list of OWASP), Webkit filters all the web traffic with a auditor.

This auditor, called the XSS auditor, can be looked up online: https://github.com/WebKit/webkit/blob/master/Source/WebCore/html/parser/XSSAuditor.cpp

What does it do?
It prevents cross site scripting (XSS) by replacing malicious scripts with an empty script, so ” <script> </script> “.

As an example, we have our insecure web application.
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Which has a simple input value:
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When inserted a malicious XSS value into a input field, like ” /><script>pay /* test */ &;lt/script></br ”
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Then we see that after submitting the page, the malicious script has been removed.
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But wait! There are exploits.
It is good to know that the auditor doesn’t reflect all possible output contexts, like in JSP:

<script type="text/javascript">
    var a = "<%= request.getParameter("a") %>";
    document.write("<text>Welcome "+ a + "</text>");

When this code is called as follows in our insecure web application


Then we get to see our session cookie!
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