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Re: FireWall-1 FTP Server Vulnerability
From: mikael.olsson () ENTERNET SE (Mikael Olsson)
Date: Fri, 18 Feb 2000 05:24:26 +0100

Henrik Nordstrom wrote:

A in my opinion better approach would be to
* require that the 227 (PASV) reply is the one and only line present in
the packet

Attackers can accomplished this by setting the TCP MSS to the right size.

* that this packet properly ends with a newline

No problem in the case of the STAT command

* that this packet is not oversized for being a 227 reply

Shuoldn't be a problem

* and that the TCP byte immediately in front of the response code is a
newline, not a space or anything else. Not sure if this is possible in
the FW1 filter engine. Also, this won't help in the case of buggy

No problem in the case of the STAT command

* The firewall should also obviously only listen for a 227 reply if a
PASV command has been sent. Any 227 replies received outside this
context is an protocol violation and should be handled as such.

I think you can accomplish this by actually sending a PASV command
after the GET / STAT command. However, I don't know how the firewall
will interpret the second "227" message. Possibly by killing
the fake channel and opening the real one instead :-(

However, what if you send the fake PASV command in a packet
with a sequence number that has already been used? Such a packet
should be happily ignored by the server, but the firewall
would probably listen to it. Hmmm :)

The only solution that even begins to look "good" is to
completely reassemble the TCP stream and not make "educated"
guesses about what packet data belongs on what line and in
which order and state of the FTP protocol.

It doesn't have to be a "proxy" in order to do this, I think.
You DO need to reassemble the stream completely though.

Just my $.02


Mikael Olsson, EnterNet Sweden AB, Box 393, S-891 28 ÖRNSKÖLDSVIK
Phone: +46-(0)660-105 50           Fax: +46-(0)660-122 50
Mobile: +46-(0)70-248 00 33
WWW: http://www.enternet.se        E-mail: mikael.olsson () enternet se

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