The NSA runs a massive, full-time hacking operation targeting foreign systems, the latest leaks from Edward Snowden show. But unlike conventional cybercriminals, the agency is less interested in hacking PCs and Macs. Instead, America's spooks have their eyes on the internet routers and switches that form the basic infrastructure of the net, and are largely overlooked as security vulnerabilities.

Under a $652-million program codenamed "Genie," U.S. intel agencies have hacked into foreign computers and networks to monitor communications crossing them and to establish control over them, according to a secret black budget document leaked to the Washington Post. U.S. intelligence agencies conducted 231 offensive cyber operations in 2011 to penetrate the computer networks of targets abroad.

This included not only installing covert "implants" in foreign desktop computers but also on routers and firewalls - tens of thousands of machines every year in all. According to the Post, the government planned to expand the program to cover millions of additional foreign machines in the future and preferred hacking routers to individual PCs because it gave agencies access to data from entire networks of computers instead of just individual machines.

Most of the hacks targeted the systems and communications of top adversaries like China, Russia, Iran and North Korea and included activities around nuclear proliferation.

The NSA's focus on routers highlights an often-overlooked attack vector with huge advantages for the intruder, says Marc Maiffret, chief technology officer at security firm Beyond Trust. Hacking routers is an ideal way for an intelligence or military agency to maintain a persistent hold on network traffic because the systems aren't updated with new software very often or patched in the way that Windows and Linux systems are.