Add Session Management to ASPX pages with Minimum Code Changes

I have an ASP .NET application which has a number of aspx pages. These pages were not having any login or session management features and any user could directly type the url and visit any page. Now I want to add state management feature to all the pages. I created a new signup and a basic login page which accepts username and password and has necessary database for it. It is fine to use any server or client state management features, but how can I implement it with very minimum code changes.

One straight forward solution here is to implement session variable in server side to store logged in user details and check for it in the Page_Load event handler of each and every page. But here, code change has to be made in all the pages and I don't want that as there are so many pages. Any easier way will help. Any ideas please??