Hackers have stolen approximately $1 billion from 100 banks. 

Kaspersky Labs reported that it has uncovered how hackers surreptitiously installed spying software on bank computers, eventually learned how to mimic bank employee workflows and used the knowledge to make transfers into bank accounts they had created for this theft, said CNN Money. 

More than 100 banks were hit, Kaspersky said, and based on the hackers' practice of stealing between $2.5 million and $10 million from each bank, it estimated "total financial losses could be as a high as $1 billion, making this by far the most successful criminal cyber campaign we have ever seen."

According to CNN Money, the banks are located in 25 countries, including the United States.

The thieves were Russian, Ukrainian, Chinese and European, CNN Money said, and the individual thefts involved no more than $10 million apiece.

"After penetrating a bank's computer systems, the hackers lurked for "two to four months" before striking in one of several ways, like changing an account balance, then transferring the excess funds into their own accounts. They also spewed cash out of ATMs when "one of the gang's henchmen was waiting beside the machine" to collect the money," said CNN Money.