The FTP proxy server in Apple AirPort Express, AirPort Extreme, and Time Capsule with firmware 7.5 does not restrict the IP address and port specified in a PORT command from a client, which allows remote attackers to leverage intranet FTP servers for arbitrary TCP forwarding via a crafted PORT command.
https://exchange.xforce.ibmcloud.com/vulnerabilities/56701
http://www.securityfocus.com/archive/1/509974/100/0/threaded
http://www.securityfocus.com/archive/1/509867/100/0/threaded