The Application-Level Gateway (ALG) on the Apple Time Capsule, AirPort Extreme Base Station, and AirPort Express Base Station with firmware before 7.5.2 modifies PORT commands in incoming FTP traffic, which allows remote attackers to use the device's IP address for arbitrary intranet TCP traffic by leveraging write access to an intranet FTP server.
http://www.securitytracker.com/id?1024907
http://support.apple.com/kb/HT4298
http://lists.apple.com/archives/security-announce/2010//Dec/msg00001.html