Firefox update patches FTP port scanning flaw

Mozilla has shipped another Firefox update to patch a security flaw in the way the browser implements the FTP protocol.

Exploitation of the flaw, which is rated low-risk, could allow an attacker to perform reconnaissance on a vulnerable machine.

According to an advisory from Mozilla, the FTP protocol includes the PASV (passive) command which is used by Firefox to request an alternate data port. The specification allows the server response to include an alternate server address as well, although this is rarely used in practice.
The open-source group warned that a malicious web page hosted on a specially-coded FTP server could use the 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 said.   Patched versions of Firefox will now ignore the alternate server address.
This is the 11th Firefox security advisory released in 2007.