Step by Step - By passing .NET Validate Request
The Microsoft .NET framework comes with a request validation feature, configurable by the ValidateRequest setting. ValidateRequest has been a feature of ASP.NET since version 1.1. This feature consists of a series of filters, designed to prevent classic web input validation attacks such as HTML injection and XSS (Cross-site Scripting). The following paper introduces script injection payloads that bypass ASP .NET web validation filters and also details the trial-and-error procedure that was followed to reverse-engineer such filters by analyzing .NET debug errors.
Paper:
http://www.procheckup.com/PDFs/bypassing-dot-NET-ValidateRequest.pdf
The original version of this paper was released in January 2006 for private CPNI distribution. This paper has now been updated in August 2008 to include additional materials such as input payloads that bypass the latest anti-XSS .NET patches (MS07-40) released in July 2007.