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RE: Tying a session to an IP address
From: "Tom Martin" <tom () permanentseptember co uk>
Date: Mon, 10 May 2004 20:11:57 +0100

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

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

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 Or 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...


-----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:

I'm interested in the merits of restricting a session to an IP 
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
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?



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