RE: Tying a session to an IP address

["Tom Martin" <tom () permanentseptember co uk>]

Its fairly trivial to spoof ones IP address and I would say MOST ISPs
routers won't particularly care what IP address you are pretending to
use.

The reason I would say IP address spoofing doesn't totally invalidate
this idea is that IP address spoofing is fine as long as you don't need
to see the replies to your packets - not a problem if you DDOSing
someones web server but slightly more challenging if you're trying to
read their webmail. But actually I can think of ways round that as
well...

Maybe address spoofing does kind of make this one redundant.

-----Original Message-----
From: Scovetta, Michael V [mailto:Michael.Scovetta () ca com] 
Sent: 10 May 2004 15:35
To: exon; webappsec () securityfocus com
Subject: RE: Tying a session to an IP address


..."I'd say it doesn't do diddly squat to add to security, since it's 
trivial to spoof ones address."

Is that really true? Is it trivial to spoof an arbitrary, specific
address? Can you make my traffic log think that you came from 
158.4.24.21? Or 127.0.0.1? I agree that within a subnet or behind a
hacked router, sure, but at some point a router in the downline is going
to say, "WTF! I don't know about the 158.4 subnet, screw that!"

Unless I totally misunderstand the issues at hand in spoofing IPs...

Mike

-----Original Message-----
From: exon [mailto:exon () home se]
Sent: Monday, May 10, 2004 10:01 AM
To: webappsec () securityfocus com
Subject: Re: Tying a session to an IP address


Paul Johnston wrote:
> Hi,
>
> I'm interested in the merits of restricting a session to an IP 
> address.
> I realise this isn't great security as often many users will appear to

> come from the same IP address (NAT, proxies, etc.) However, if you 
> consider the case where an attacker uses an XSS vulnerability to steal

> the session ID, then the IP address restriction raises the bar 
> considerably for an arbitrary remote attacker to exploit this. I'm 
> worried that the IP address restriction wouldn't work for all users - 
> e.g. if their ISP uses load-balanced web caches. Does anyone know how 
> common such arrangements are in practice? Perhaps something to be done

> then is just check the top 16 bits of the IP address. This is likely
to 
> work for all such network arrangements and still raises the bar a lot 
> for remote attacks.
I'd say it doesn't do diddly squat to add to security, since it's 
trivial to spoof ones address.

> Does anyone here already restrict sessions by IP address?
>
> Regards,
>
> Paul