RE: Cookie not expiring...

["Dan Simon" <dsimon () compnetco com>]

Not quite.

The problem is not with the session being "recycled" -- nor is it with the
cookie.

The cookie is simply a pointer to the session on the server that holds the
state of the user's login and activities.  Expiring the cookie on the user's
side simply means that the client no longer knows how to connect to the
session (which is still valid and has not expired).  

As noted in the original post, someone who knows the session id (contained
in the cookie) can connect to the server after the user has logged out and
take over (hijack) their session.  The session will think that the attacker
is a legitimate user and will treat them as having already logged in (as
that is what the session state says they have done).

Expiring the cookie on the server has no meaning -- the cookie does not
exist on the server.  Again, the cookie is simply a storage container...it
holds the session id which allows the client to tell the server who he is
and what server session object to associate with him.

What you need to do is to expire the session object itself when a user logs
out.  You can do this programmatically by removing the stateful information
from the session object upon logout (user id, history, etc.).  Alternately,
you can look into the API that you are using and determine the proper way to
destroy a session object.  That way, if an attacker attempts to connect to
the server using the session id of a user who has logged out, he will be
treated as a new user and forced to login (which will hopefully delay him
for at least a minute or two).

Dan Simon
C|EH, SCJD, SCJP
Remington Associates, Ltd.

-----Original Message-----
From: bryan allott [mailto:homegrown () bryanallott net] 
Sent: Wednesday, August 17, 2005 3:00 AM
To: webappsec () securityfocus com
Subject: Re: Cookie not expiring...

i dont think the session is actually available. maybe what is happening is 
that a new session with the same identifier is being resurrected? read the 
following from msdn..

"Session identifiers for abandoned or expired sessions are recycled by 
default. That is, if a request is made that includes the session identifier 
for an expired or abandoned session, a new session is started using the same

session identifier. You can disable this by setting 
regenerateExpiredSessionId attribute of the <sessionState> configuration 
element to true. For more information, see Session Identifiers."

.NET Framework
Supported in: 2.0, 1.1, 1.0

try that?

----- Original Message ----- 
From: "spawn security" <spawn.security () gmail com>
To: <webappsec () securityfocus com>
Sent: Tuesday, August 16, 2005 7:08 PM
Subject: Cookie not expiring...


  I'm testing an application and after clicking on the logout button
the session management cookie does not expire. More specifically, the
application is using the FormsAuthentication class signout method to
"logout" an authenticated user.

Private Sub cmdSignOut_ServerClick(ByVal sender As System.Object,
ByVal e As System.EventArgs) _
Handles cmdSignOut.ServerClick
FormsAuthentication.SignOut()
      Session.Abandon()
      Response.Redirect("login.aspx", True)
End Sub

 According to Microsoft documentation, this
FormsAuthentication.SignOut method works by returning a Set-Cookie
header to the browser that sets the cookie's value to a null string
and sets the cookie's expiration date to a date in the past.  This is
only expiring the cookie on the client/browser side. Which will allow
a "malicious" user, who had access to the cookie, to reuse it after
user logs out.

  Does anyone know how to force the cookie to expire on the server side ?

Thanks.




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