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Addendum to Firewall-1 FTP Server Vulnerability
From: paul () MOQUIJO COM (Paul Cardon)
Date: Tue, 29 Feb 2000 22:18:41 -0500


What follows is a clarification of a statement in the advisory by John
McDonald and Thomas Lopatic which can be retrieved in its entirety from
the BUGTRAQ archive:

38A1B3D9.9D16EBAE () dataprotect 
com">http://www.securityfocus.com/templates/archive.pike?list=1&date=2000-02-8&thread=38A1B3D9.9D16EBAE () dataprotect 
com</A>

Here is the relevant portion:

"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."

It then goes on to describe some restrictions on this TCP connection one
of which is that "it cannot be to a port that is listed in FireWall-1's
list of well-known TCP
services."  This is true of the default inspect code in the base.defs
file.

It really depends on the definition of the NOTSERVER_TCP_PORT macro.  If
the macro is modified the behavior changes.  This is the reason for the
difference between v3.0 and v4.x that is mentioned in the original
advisory.  See http://www.phoneboy.com/fw1/faq/0106.html for discussion
of a change to NOTSERVER_TCP_PORT that removes the restriction on ports
that are in FW-1's list of TCP services and as a result only restricts
ports <1024.

A Nokia machine running FW-1 4.0 with a base.defs modified according to
the phoneboy FAQ item listed above has been tested and a connection can
be made to any port *not matched* by the NOTSERVER_TCP_PORT macro.

-paul


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