IIS http request authentication issue

Hello everyone,
I am using VSTS 2008 + C# + .Net 3.5 + IIS 7.0 + ASP.Net. I have two IIS web site, site A and site B. When user request url http://sitea/abc.aspx, my ASP.Net code handler will authenticate user (using Forms authentication), and if authenticaiton passed, I will redirect user to http://siteb/bcd.aspx. current issue is, some users will access http://siteb/bcd.aspx directly to skip the authentication process of sitea. This is not what I want since I want to ensure all users who access http://siteb/bcd.aspx authenticated.
My current issue is, some users will access http://siteb/bcd.aspx directly to skip the authentication process of sitea. This is not what I want since I want to ensure all users who access http://siteb/bcd.aspx authenticated ...