Advanced Persistent Threat

Overview
An Advanced Persistent Threat (APT) involves sophisticated, determined, coordinated attackers who systematically compromise government and commercial computer networks. These attackers typically install multiple backdoors into a cyber network they are attempting to infiltrate, under the “radar” of even the most sophisticated anti-virus protections, thereby establishing a secure foothold into the network. They then install utilities to exfiltrate data to external servers. Attackers respond to attempts to eradicate infection and remediate network security by establishing additional footholds and improving sophistication. These infiltrations can persist, untraced, for months and even years.

External link
Siobhan Gorman, "Electricity Grid in U.S. Penetrated By Spies," Wall St. J. (Apr. 8, 2009).