Bureau-121

Bureau 121 (Unit 121 of the North Korean General Bureau of Reconnaissance) is the name of a secret cyberwarfare agency belonging to the military of North Korea. It is one of two such cyberwarfare units in the General Bureau of Reconnaissance, the other being No. 91 Office.

The activity of the agency came into public limelight in December 2014 when Sony Pictures canceled the opening of its movie The Interview after its computers had been hacked. Bureau 121 has been blamed for the cyber breach. North Korea has rejected this accusation. 

According to a report by Reuters, Bureau 121, also known as the DarkSeoul Gang is staffed by some of North Korea's most talented computer experts and is run by the Korean military.

According to Jang Se-yul, a computer expert who defected in 2007, about 1800 cyber warriors are located throughout the world. Other sources, including Prof. Kim Heung Kwang estimations vary on Unit 121's size. In 2012, South Korea asserted 3000 people belonged to Unit 121 and earlier this year predicted an increase to 5900. Many hackers of the bureau are hand-picked graduates of the University of Automation, Pyongyang. While these specialists are scattered around the world, their families benefit from special privileges at home

Much of the agency’s activity has been directed at South Korea. Prior to the attack at Sony, the agency was said to have attacked more than 30.000 PCs in South Korea affecting banks and broadcasting companies as well as a website of South Korean President Park Geun-Hye. The malicious code used in a 2012 attack on a South Korean media organization appears to be similar to the code used in the Sony hack, according to Choi Sang-myung, a senior online security researcher and adviser to Seoul's cyber warfare command. "I noticed the similarities as soon as I saw it." 

It are also has been thought to have been responsible for infecting thousands of South Korean smartphones in 2013 with a malicious gaming app. 

According to Jang Se-Yul, another North Korean defector, North Korea is very active in cyberwarfare and its capabilities have been underestimated. North Korea has sophisticated cyber warfare capabilities with cells from Bureau 121 operating around the world. One of the suspected locations of a cell is the Chilbosan Hotel in Shenyang, China.

The FBI and Justice Department's National Security Division are still investigating the "sophisticated actor" behind it. When asked how the U.S. planned to respond, White House Press Secretary, said the president's national security team was considering "a range of available responses" but did not elaborate on what that response might be.