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.
This vulnerability has been tested on .Net frameworks 2.0 and above.
"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 article: http://msdn.microsoft.com/en-us/library/ff649487.aspx#pagguidelines0001_inputdatavalidation."
Microsoft therefore will not be releasing a fix for this issue.