The hackers have been active since at least the end of 2013 and infiltrated more than 100 banks in 30 countries, according to Russian security company Kaspersky Lab.
After gaining access to banks' computers through phishing schemes and other methods, they lurk for months to learn the banks' systems. Then the hackers have programmed ATMs to dispense money at specific times or set up fake accounts and transferred money into them.
Most of the targets have been in Russia and Eastern Europe. But Kaspersky says banks in the U.S., Asia and elsewhere in Europe were also targeted.