Re: Session security with cookies

["Aaron Katz" <atkatz () gmail com>]

Ron,

It's unclear exactly what problem you're trying to solve.  Encrypting
& signing the cookie may stop tampering, but it won't prevent stealing
the cookie and replaying it.  So, you've basically got two types of
data that might be in the cookie: the session identifier, or the
state.

In either case, you have to decide whether or not to tie the cookie to
the client.  If you don't tie the cookie to the client, (or possibly
the current time, with a very short timeout, and automated
replacement), then nothing prevents replay of the cookie.

If you tie the cookie to the client, you've got the problem of
uniquely identifying the client.  You've got to use stuff that's
available to the server, so that the server can confirm the cookie.
The obvious ideas would be IP address, user agent, and such, but these
don't seem to work very well.  The IP address can be "duplicated" for
multiple clients (NAT), and can theoretically change at inopportune
times (DHCP).  User agent agent isn't unique, and provides such little
entropy (most people choose from the same small set of browsers (IE,
Fx, Mozilla, Opera, Lynx)) that the dictionary attack to discover it
is so easily overcome that it's probably not worth the effort.

The problem of uniquely tying the cookie to the client seems to rely
upon the problem of uniquely identifying the client.  Uniquely
identifying the client is what the session cookie is trying to solve.
To improve this, you might consider client certificates, but that
comes with its own problems.

If you're tying the cookie to the current time (directly, by time
stamp, or indirectly, if you choose to include the session identifier
in a cookie that contains state), then you have to be sure that the
cookie doesn't contain anything that absolutely must live longer than
the timeout.  For example, some shopping carts retain their content,
between multiple sessions.

Also, in some cases, it may be entirely appropriate to allow tampering
(I can't think of one off the top of my head, but it seems like a
reasonable assumption that this would be good, in some cases).


As implied, above, it would be easier to comment on your thoughts if
you identify the problem that you're trying to solve :)

--
Aaron

On Dec 4, 2007 1:47 PM, Ron <ronlists () skullsecurity com> wrote:
> Something I've recently been toying with is the idea of
> encrypting/signing cookies with a private key on the server. The the
> cookies can't be read or tampered with by the user, nor can they be
> stolen by cross-site attacks and the like.
>
> This isn't something I've done a lot of work with, however, so I may be
> missing something obvious, and am open to comments on the idea.
>
> Ron
>
> Till Elsner wrote:
> > Hi, i'm investigating in web application security this time and i'm
> > trying to find some information about session management with cookies
> > and related security issues. Can anyone point me to tips on how to make
> > cookie based sessions more secure and how to prevent session hijacking?
> > How secure is session handling using cookies and what are the main
> > risks? Is anyone aware of good literature on that topic?
> > Thanks and have a nice day
> > Till
> >
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Sponsored by: Watchfire 
Methodologies & Tools for Web Application Security Assessment 
With the rapid rise in the number and types of security threats, web application security assessments should be 
considered a crucial phase in the development of any web application. What methodology should be followed? What tools 
can accelerate the assessment process? Download this Whitepaper today! 

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