Tying a session to an IP address

[Paul Johnston <paul () westpoint ltd uk>]

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.

Does anyone here already restrict sessions by IP address?

Regards,

Paul

--
Paul Johnston
Internet Security Specialist
Westpoint Limited
Albion Wharf, 19 Albion Street,
Manchester, M1 5LN
England
Tel: +44 (0)161 237 1028
Fax: +44 (0)161 237 1031
email: paul () westpoint ltd uk
web: www.westpoint.ltd.uk