Microsoft Security Bulletin MS05-004

Summary
A canonicalization vulnerability exists in ASP.NET that could allow an attacker to bypass the security of an ASP.NET Web site and gain unauthorized access. An attacker who successfully exploited this vulnerability could take a variety of actions, depending on the specific contents of the website.
Solution
Microsoft has released a patch to correct this issue, you can download it from the following web site: http://www.microsoft.com/technet/security/Bulletin/MS05-004.mspx