April 22nd, 2002, 02:00 PM
Vulnerability: Raptor Firewall FTP Bounce
The Raptor Firewall can make an FTP server behind it vulnerable to the well-known
FTP bounce vulnerability even if the FTP server used is not susceptible to this issue.
While performing a penetration test for a customer, we discovered that their FTP server
was vulnerable to the well-known FTP Bounce attack from the Internet. However, subsequent
conversation with the customer showed that the FTP server itself (a recent version of
wu-ftp) was not vulnerable to the FTP bounce attack.
It appears that the Raptor Firewall's FTP proxy was somehow making the FTP server vulnerable
to the FTP bounce vulnerability even though the FTP server itself was immune to this problem.
The Firewall vendor (Symantec) have been informed of this issue.
Firewall: Raptor 6.5.3i on Sun Solaris 7
FTP Server: wu-ftpd on internal network with anonymous access
Config: Using built-in Raptor FTP proxy for inbound FTP access from Internet
We verified and analysed the vulnerability using the following setup:
1. "attacker" - A Linux system on the Internet that connects to the FTP server and
exploits the vulnerability
2. "victim" - A second Linux system on the Internet that is the target of the bounce attack
3. "server" - The FTP server. External address 220.127.116.11, internal 10.1.13.5
4. "Firewall" - The Raptor Firewall
We verified the FTP bounce vulnerability from the Internet and used the "tcpdump" packet
sniffer on the Internet "attacker", the Internet "victim" (target of the ftpbounce test) and the
FTP server to determine what was going on.
It turns out that the Raptor Firewall re-writes the inbound FTP "PORT" command and
changes the IP address to be the Hacker's IP rather than the Victim's, and the port number
to be another ephemeral port. This means that the FTP server cannot detect the FTP
bounce attack because it sees the correct IP address (the one of the hacker rather than
the victim) and an ephemeral port. However, when the FTP Server makes the outbound
connection to this IP address and port, the Firewall re-writes the IP address and port in
the packet to be the IP address and port of the victim which was originally specified by
Thus, the Raptor Firewall prevents the FTP Server from detecting the FTP bounce attack, and
permits the attack to take place. Because the FTP Server will always see the "correct" IP
address and port in the PORT command, it cannot determine that an FTP bounce attack is
being carried out and will accept the command.
Further information, including annotated "tcpdump" packet traces are available at:
April 22nd, 2002, 02:07 PM
I have to say: this is a very cool method to hide your IP if you plan to attack a site.