From the website above: If an attacker can establish an FTP connection passing through a Linux 2.4.x IPTables firewall with the state options allowing "related" connections (almost 100% do), he can insert entries into the firewall's RELATED ruleset table allowing the FTP Server to connect to any host and port protected by the firewalls rules, including the firewall itself. ==== You probably already have this but I can't find it as a security-level bug in the kernel. Hope this isn't a duplicate (probably is -- this appeared on http://www.slashdot.org as a featured story).
This is a known issue and an advisory will be made public any time now (if it isn't out already). It comes down to: default installs are NOT vulnerable. Only if you change from ipchains to iptables and then select FTP NAT with the 'related' feature there is a problem. That is a "don't do that then" for now.
http://www.securityfocus.com/frames/?content=/templates/advisory.html%3Fid%3D3231 has the advisory for a while now; I'm not sure why our own site doesn't show it.