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Seeker Adv MS-06 - .Net Cross Site Scripting - Request Validation Bypassing
From: "Seeker Research Center" <qsrc () quotium com>
Date: Wed, 29 Aug 2012 18:26:21 +0200

.Net Cross Site Scripting - Request Validation Bypassing
Seeker Research Center
By Zamir Paltiel, August 2012

A vulnerability in the .Net Request Validation mechanism allows bypassing the filter and execution of malicious scripts 
in the browsers of users via Cross Site Scripting attacks.
The exploitation technique explained here allows sending tags through the Request Validation Filter in a manner that 
will pass browser syntax and be rendered by browsers.

The .Net Request Validation mechanism prevents attackers from sending tags as the value of the parameters. It is 
however possible to bypass this mechanism and send arbitrary tags that facilitate script execution.
This is caused by the fact that although <tag> is restricted by the Request Validation filter, <%tag> is not restricted 
but parsed by Internet Explorer browsers as a valid tag.

An example of the exploitation of this vulnerability would be crafting a link to a page that reflects a parameter value 
to the user.
As the value of the parameter the attacker would provide a <%tag> with the style attribute and an expression, for 
example: http://www.vulnerablesite.com/login.aspx?param=<%tag style="xss:expression(alert(123))" >
This will bypass the filter and execute the script in the brackets.

Affected Systems
This vulnerability has been tested on .Net frameworks 2.0 and above.

Vendor Response
“The Request Validation Feature in ASP.NET is designed to perform basic input validation.  It is not designed to make 
security decisions for applications developed using ASP.NET.  Only the original developers can determine what content 
the ASP.NET application is designed to process and handle.  Microsoft recommends that all software developers perform 
input/data validation of all sources.  We do this to encourage our customers to make more robust applications that are 
less susceptible to security issues.  The Request Validation Feature was designed and released to help developers in 
this effort.  For more information about our recommendations to software developers, please see the following MSDN 
Microsoft therefore will not be releasing a fix for this issue.

This vulnerability has been identified by Zamir Paltiel, Seeker Research Center.
For more information please visit http://www.quotium.com/prod/ResearchCenter/XSS-NetrequestValidation.php 

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