• User session sensitive information such as cookies can be stolen.
• XSS can enable client-side worms which could modify, delete or steal other users' data within the application.
• The website can be redirected to a new location, defaced or used as a phishing site.

The issue occurs because the browser interprets the input as active HTML, Javascript or VbScript. To avoid this, all input and output from the application should be filtered. Output should be filtered according to the output format and location. Typically the output location is HTML. Where the output is HTML ensure that all active content is removed prior to its presentation to the server.

Prior to sanitizing user input, ensure you have a pre-defined list of both expected and acceptable characters with which you populate a white-list. This list needs only be defined once and should be used to sanitize and validate all subsequent input.

There are a number of pre-defined, well structured white-list libraries available for many different environments, good examples of these include, OWASP Reform and Microsoft Anti Cross-site Scripting libraries are good examples.

• XSS Cheat Sheet
• OWASP - Cross-site Scripting
• XSS Shell
• XSS Tunnelling Paper

<System.Web>
     < httpRuntime enableVersionHeader="false" /> 
     <customErrors mode="On" defaultRedirect="~/error/GeneralError.aspx">
          <error statusCode="403" redirect="~/error/Forbidden.aspx" />
          <error statusCode="404" redirect="~/error/PageNotFound.aspx" />
          <error statusCode="500" redirect="~/error/InternalError.aspx" />
     </customErrors>
</System.Web>