Security Weakness in Asp.Net

on clicking of logout button remove all the sessions (Session.Abandon()). and on every page check a particular session (create a common functiion) if it is null than redirect to your default error page.

and i give u link k hope it will more help

ASP.NET web application security review: Do's & Don'ts[^]

You will need to make sure the pages behind the login (those that can only be accessed after login) are not cached in the browser:

Page.Response.Cache.SetCacheability(HttpCacheability.NoCache);

If using forms authentication, you will also need to abandon the session and logout:

Session.Abandon();
FormsAuthentication.SignOut();

Browser back button[^]
Disable back button[^]

Please refer a good answer on same question:
How to disable browser back bottun after logout.[^]

Another solved answer:
After Logout Their is Back Button Problem[^]

Some more useful links:
Back Button issue after Logout in ASP.NET[^]
How to disable back button of the browser after logout[^]

refer the following url, where Sandeep Mewara has posted Tip for such issue.:
Browser back button issue after logout[^]

hope it helps.