The reason, according to GNUCITIZEN
, is because of an Adobe Flash issue in the wild. UPnP, or Universal Plug and Play, is a protocol that allows devices on a network to find each other, as well as to allow network clients (e.g. instant messengers, file sharing programs, etc.) to open ports on the router. The latter application for UPnP is the problem, in that a malicious piece of code can completely open the firewall of a router and negate any security that was in place.