WASHINGTON — The trail that led American officials to blame North Korea for the destructive cyberattack on Sony Pictures Entertainment in November winds back to 2010, when the National Security Agency scrambled to break into the computer systems of a country considered one of the most impenetrable targets on earth.
Spurred by growing concern about North Korea’s maturing capabilities, the American spy agency drilled into the Chinese networks that connect North Korea to the outside world, picked through connections in Malaysia favored by North Korean hackers and penetrated directly into the North with the help of South Korea and other American allies, according to former United States and foreign officials, computer experts later briefed on the operations and a newly disclosed N.S.A. document.
A classified security agency program expanded into an ambitious effort, officials said, to place malware that could track the internal workings of many of the computers and networks used by the North’s hackers, a force that South Korea’s military recently said numbers roughly 6,000 people. Most are commanded by the country’s main intelligence service, called the Reconnaissance General Bureau, and Bureau 121, its secretive hacking unit, with a large outpost in China.