The computer security vendor, which said it will release a report Monday on its findings, said the gang penetrated deeply into the banks' networks, taking time to learn about internal procedures to make their fraudulent activity less suspicious.
In some cases, the gang learned about wire transfer systems by watching administrators' computers over video.
"In this way the cybercriminals got to know every last detail of the bank clerks' work and were able to mimic staff activity in order to transfer money and cash out," Kaspersky said in a news release.
The group, called Carbanak after the malware the gang installed on computers, attempted to attack up to 100 banks and e-payment systems since 2013 in 30 countries. The gang members are suspected to be from Russia, Ukraine, other parts of Europe and China.
Some of the financial institutions affected are in Australia, Brazil, Bulgaria, Canada, China, Czech Republic, France, Germany, Hong Kong, Iceland, India, Ireland, Morocco, Nepal, Norway, Poland, Pakistan, Romania, Russia, Spain, Switzerland, Taiwan, Ukraine, the U.K., the U.S.
None of the banks or financial institutions have been named. Kaspersky said in a news release on that Interpol and Europol are involved in the investigation.
Each theft took between two and four months, Kaspersky said. Bank computers would be infected with malware through spear-phishing attacks, which involves sending targeted emails with malicious attachments or links to select employees.
Spear-phishing emails are crafted in a way to make it likely a recipient will open an attachment or click a link that appears innocuous but installs malicious software on a computer.
As much as $10 million was stolen in a raid at a time, Kaspersky said. Funds were transferred using online banking or e-payment systems to the gang's own accounts or to other banks in the U.S. and China.
In other instances, the attackers had deep control within a bank's accounting systems, inflating account balances in order to mask thefts. For example, Kaspersky said that an account with $1,000 would be raised to $10,000, with $9,000 transferred to the cybercriminals.
ATMs were also targeted, Kaspersky said. The gang commanded the machines to dispense money at a certain time, with accomplices ready to pick up the disgorged cash.
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