The FTP protocol includes the PASV (passive) command which is
used by Firefox to request an alternate data port. The specification
of the FTP protocol allows the server response to include an alternate
server address as well, although this is rarely used in practice. reported that a malicious web page hosted on
a specially-coded FTP server could use this feature to perform a
rudimentary port-scan of machines inside the firewall of the victim.
By itself this causes no harm, but information about an internal
network may be useful to an attacker should there be other vulnerabilities
present on the network.Mozilla clients will now ignore the alternate server address.