For those who are not yet familiar with XSS attacks and using or planning to use AJAX technologies in you applications I recommended to download
From Microsoft article :
Cross-site scripting (XSS) attacks exploit vulnerabilities in Web-based applications that fail to properly validate and/or encode input that is embedded in response data. Malicious users can then inject client-side script into response data causing the unsuspecting user’s browser to execute the script code. The script code will appear to have originated from a trusted-site and may be able to bypass browser protection mechanisms such as security zones.
These attacks are platform and browser independent, and can allow malicious users to perform malicious actions such as gaining unauthorized access to client data like cookies or hijacking sessions entirely.
Simple steps that developers can take to prevent XSS attacks in their ASP.NET applications include (see How To: Prevent Cross-Site Scripting in ASP.NET in the patterns & practices series for more detail):
For defence in depth, developers may wish to use the Microsoft Anti-Cross Site Scripting Library to encode output. This library differs from most encoding libraries in that it uses the “principle of inclusions” technique to provide protection against XSS attacks. This approach works by first defining a valid or allowable set of characters, and encodes anything outside this set (invalid characters or potential attacks). The principle of inclusions approach provides a high degree of protection against XSS attacks and is suitable for Web applications with high security requirements.”.