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Re: Our old friend Firewall-1
From: Hugo.van.der.Kooij () CAIW NL (Hugo.van.der.Kooij () CAIW NL)
Date: Wed, 15 Mar 2000 07:46:30 +0100


On Sat, 11 Mar 2000, Chris Brenton wrote:


The Dartmouth Collage security group has uncovered a problem with
Firewall-1 which could lead to the protected site handing out more IP
address info than intended.

Under certain nominal load conditions (CPU less than 40%, 200+ active
sessions) Firewall-1 will begin "leaking" packets with their private
address information in tact. The result is that the receiving site will
receive a SYN=1 that it will be unable to respond to. Once the client
attempts a resend, the target network (or anyone in the middle) can use
the source port information to enumerate the client's true IP address.

Here is a Snort trace which has been sanitized and formatted for easier
viewing:
Mar  9 14:01:19 172.30.1.10:1721 ->   192.168.1.5:80 SYN **S*****
Mar  9 14:01:48 200.200.200.5:1721 -> 192.168.1.5:80 SYN **S*****

Mar  9 14:04:35 172.30.1.10:1858 ->   192.168.1.5:80 SYN **S*****
Mar  9 14:05:05 200.200.200.5:1858 -> 192.168.1.5:80 SYN **S*****

Mar  9 14:23:25 172.16.5.20:4868 ->   192.168.1.5:80 SYN **S*****
Mar  9 14:23:51 200.200.200.5:4868 -> 192.168.1.5:80 SYN **S*****

So the first packet goes out with the private address information still
in place and SYN=1. When the client does not receive a reply, it
retransmits the SYN=1. Since FW-1 considers this to be part of the same
session, the same source port number is assigned. If the second packet
gets translated properly (as in these traces) the source port info can
potentially be used to map the legal IP address to the private address.

Of course the problem here is that a would be bad guy now knows the
client's true IP address. If enough hosts are recorded, its possible
that most of the internal network address space could be enumerated.

This problem has been noted on Firewall-1 versions 3.0b & 4.0. 4.1 has
not been checked but its expected that the same problem may exist. We
where able to reproduce the problem on a Nokia IP440 and NT. I've seen
this problem on Solaris 2.6 as well, but do not have the data to back up
the statement.

Please provide exact patchlevels. I know the problem occurs in FireWall-1
v4.0 SP4 but should be fixed in SP5 that is available as of 2000/03/14

Hugo.

--
Hugo van der Kooij; Oranje Nassaustraat 16; 3155 VJ  Maasland
hvdkooij () caiw nl     http://home.kabelfoon.nl/~hvdkooij/
--------------------------------------------------------------
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