FormsAuthentication.SignOut() does not log the user out

Using two of the above postings by x64igor and Phil Haselden solved this:

1. x64igor gave the example to do the Logout:

• You first need to Clear the Authentication Cookie and Session Cookie by passing back empty cookies in the Response to the Logout.

  public ActionResult LogOff()
  {
      FormsAuthentication.SignOut();
      Session.Clear();  // This may not be needed -- but can't hurt
      Session.Abandon();
  
      // Clear authentication cookie
      HttpCookie rFormsCookie = new HttpCookie( FormsAuthentication.FormsCookieName, "" );
      rFormsCookie.Expires = DateTime.Now.AddYears( -1 );
      Response.Cookies.Add( rFormsCookie );
  
      // Clear session cookie 
      HttpCookie rSessionCookie = new HttpCookie( "ASP.NET_SessionId", "" );
      rSessionCookie.Expires = DateTime.Now.AddYears( -1 );
      Response.Cookies.Add( rSessionCookie );
  

2. Phil Haselden gave the example above of how to prevent caching after logout:

• You need to Invalidate the Cache on the Client Side via the Response.

      // Invalidate the Cache on the Client Side
      Response.Cache.SetCacheability( HttpCacheability.NoCache );
      Response.Cache.SetNoStore();
  
      // Redirect to the Home Page (that should be intercepted and redirected to the Login Page first)
      return RedirectToAction( "Index", "Home" ); 
  }
  

Users can still browse your website because cookies are not cleared when you call FormsAuthentication.SignOut() and they are authenticated on every new request. In MS documentation is says that cookie will be cleared but they don't, bug? Its exactly the same with Session.Abandon(), cookie is still there.

You should change your code to this:

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

// clear authentication cookie
HttpCookie cookie1 = new HttpCookie(FormsAuthentication.FormsCookieName, "");
cookie1.Expires = DateTime.Now.AddYears(-1);
Response.Cookies.Add(cookie1);

// clear session cookie (not necessary for your current problem but i would recommend you do it anyway)
SessionStateSection sessionStateSection = (SessionStateSection)WebConfigurationManager.GetSection("system.web/sessionState");
HttpCookie cookie2 = new HttpCookie(sessionStateSection.CookieName, "");
cookie2.Expires = DateTime.Now.AddYears(-1);
Response.Cookies.Add(cookie2);

FormsAuthentication.RedirectToLoginPage();

HttpCookie is in the System.Web namespace. MSDN Reference.