RE: How to protect against cookie stealing?

["Dawes, Rogan (ZA - Johannesburg)" <rdawes () deloitte co za>]

Well, there are only a limited number of things that one can do.

The objective is to detect a change when a request is made. What information
can we check against?

* Source IP address - can change if behind a proxy server array, doesn't
protect against other users behind the same proxy
* SSL sessionid - helps if you are using SSL, but this can also change, I
think. Particularly if the session is idle for a while?

What else? Here is a sample request:

POST http://localhost:8080/WebGoat/attack HTTP/1.0
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg,
application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword,
application/x-shockwave-flash, */*
Referer: http://localhost:8080/WebGoat/attack
Accept-Language: en-za
Content-Type: application/x-www-form-urlencoded
Proxy-Connection: Keep-Alive
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
Host: localhost:8080
Content-Length: 0
Pragma: no-cache
Cookie: JSESSIONID=5971DC264B764275ED682A353BD3D44C

* Accept header - this is unlikely to change, but is easy to guess
* Accept-Language - most likely to be en-us, but could vary. Worth adding,
anyway.
* UserAgent - one would have a reasonably good chance of guessing this. A
single incorrect guess should invalidate the session, although that would
lead to DOS, perhaps.

I would be inclined to make up a validation string comprised of a hash of
(Accept: + User-Agent: + Accept-Language:) at the time of the user's login
and check that on every request. If it ever changes, immediately invalidate
the session, and warn the real user when they make a request with the
correct string, that someone is trying to access their session.

One could also check against the source IP as a precaution, and at least
flag sessions where the source IP changes as potentially being compromised.
Quite what you would do with that, I'm not sure :-) 

One flaw with this scheme, though: If the attacker manages to execute some
script or other scheme to get the sessionid to their server, they would be
able to reap the headers as well :-( 

Bang goes that theory. :-(

That brings us back to source IP and SSL sessionid.

IIRC, proxies are supposed to add an X-Forwarded-for header to the request
headers. That could allow the server to track request that occur across
different proxies. However, an attacker that does not go through a proxy
would also be in a position to add such a spoofed header, and they would be
able to reap it from the request that delivered the sessionid. If they were
behind a proxy, they could also add that header, but it might be overwritten
by an upstream proxy?

One could, over time, build up a list of IP addresses of known proxies, and
source ranges that they serve. That could work, but would take time to build
up.

Which leaves SSL sessionid. I'm not sure how reliable that is, and it
doesn't help non secure sites.

Which I think explains why no-one has done anything about this problem! :-)

Rogan


> -----Original Message-----
> From: Phil Cox [mailto:Phil.Cox () SystemExperts com] 
> Sent: 24 July 2003 07:34 AM
> To: webappsec () securityfocus com
> Subject: How to protect against cookie stealing?
>
>
> All,
>
> I have a question on how people are handling cookie stealing 
> and session
> segregation. For example, it is possible to use session 
> cookie information
> on multiple systems for most (all?) web applications I know 
> of.  Here is a
> scenario:
>
> When a user logs in he is assigned a BLAH_SESSIONID cookie 
> which serves as
> an authorization mechanism in the application.  The 
> application  does not
> associate the cookie to any session-specific information 
> (e.g., source IP
> address), so another user can also use the BLAH_SESSIONID 
> cookie to access
> the same information (over a different TCP/IP session) that 
> the legitimate
> user can.  If an attacker obtains, or guesses a valid 
> BLAH_SESSIONID cookie
> for an active session, he can use it without the user's knowledge. For
> example, I was able to execute the following command using an 
> BLAH_SESSIONID
> that was obtained from another session: Command on Linux box: 
>
>   # curl -b "BLAH_SESSIONID=0000FDHTNLVY5WX"
> https://somesite.com/App/Function? 
>
>   Returns the page: (some portion of the returned page)
>
> Historically I have believed that having the application associate the
> BLAH_SESSIONID session cookie and its associated privileges 
> with a specific
> SSL session or source IP address would prevent this session 
> stealing. But
> recently I have been told that this solution does not work 
> because of the
> dynamic IP nature of MANY ISP's and the disassociation of 
> SSL/HTTP. I would
> like to know what others are doing to solve this problem, or 
> if they are
> just not solving it at all.
>
> Thoughts and comments are appreciated...
>
> Phil

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