FormsAuthentication.SignOut() does not log the user out

Question

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.

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" ); 
  }
  

Sounds to me like you don't have your web.config authorization section set up properly within . See below for an example.

<authentication mode="Forms">
  <forms name="MyCookie" loginUrl="Login.aspx" protection="All" timeout="90" slidingExpiration="true"></forms>
</authentication>
<authorization>
  <deny users="?" />
</authorization>

The key here is that you say "If I type in a URL directly...".

By default under forms authentication the browser caches pages for the user. So, selecting a URL directly from the browsers address box dropdown, or typing it in, MAY get the page from the browser's cache, and never go back to the server to check authentication/authorization. The solution to this is to prevent client-side caching in the Page_Load event of each page, or in the OnLoad() of your base page:

Response.Cache.SetCacheability(HttpCacheability.NoCache);

You might also like to call:

Response.Cache.SetNoStore();