no Forms Authentication Login Page Redirect When You Don’t Want It

In an ASP.NET web application, it's very common to write some jQuery code that makes an HTTP request to some URL (a lightweight service) in order to retrieve some data. That URL might be handled by an ASP.NET MVC controller action, a Web API operation, or even an ASP.NET Web Page or Web Form. If it can return curly brackets, it can be respond to a JavaScript request for JSON.

 

http://haacked.com/archive/2011/10/04/prevent-forms-authentication-login-page-redirect-when-you-donrsquot-want.aspx