North Korean hackers have taken millions of dollars in virtual currencies like bitcoin in recent attacks over the internet. Virtual currencies are forms of money that are not overseen by governments but are exchanged digitally and controlled by their developers. Security experts expect more attacks as economic sanctions drive North Korea to seek new ways to get money. North Korean hackers have been blamed for a number of cyberattacks in the past, including a computer virus known as WannaCry. The cyberattack affected hospitals, banks and companies around the world this year. Experts say the fast rise in the value of bitcoin makes it and other cryptocurrencies a good target for North Korea.