Read the whole article at the Security PS Blog: Session Fixation & Forms Authentication Token Termination in ASP.NET
ASP.NET applications commonly have one or more vulnerabilities associated with the use of ASP.NET_SessionId cookies and forms authentication cookies. This article briefly discusses those common vulnerabilities and explains one method of mitigating them in an ASP.NET MVC 4 application. Explanation of the exploits are not included, but I linked many of the keywords to OWASP or MSDN articles to provide more details. The security best practices for session cookies and use of sessions in general are provided in the OWASP Session Management Cheat Sheet.
ASP.NET_SessionId cookies and forms authentication cookies can be used alone or together to maintain state with a user’s browser. Each cookie works a little bit differently. The ASP.NET_SessionId cookie value is an identifier used to look up session variables stored on the server-side; the cookie itself does not contain any data. The forms authentication cookie, named .ASPXAUTH by default, contains encrypted data, stored only on the client-side. When it is submitted in a request to the server, it is decrypted and used by custom application code to make authorization decisions... Read More at the Security PS Blog.