Question: What is UrlScan?
Answer: UrlScan is a security tool that screens all incoming requests to the server by filtering the requests based on rules that are set by the administrator. Filtering requests helps secure the server by ensuring that only valid requests are processed. UrlScan helps protect Web servers because most malicious attacks share a common ? characteristic they involve the use of a request that is unusual in some way. For instance, the request might be extremely long, request an unusual action, be encoded using an alternate character set, or include character sequences that are rarely seen in legitimate requests. By filtering unusual requests, UrlScan helps prevent such requests from reaching the server and potentially causing damage.
Details:
http://technet.microsoft.com/en-us/security/cc242650.aspx
