The Federal Bureau of Investigation (FBI) and Microsoft Corp have launched a major assault on one of the world’s biggest Global cyber crime rings, which they believe was involved in stealing over $500m from bank accounts during past eighteen months.

The authorities of about 80 countries, including Australia, Germany, Brazil, Ecuador, Hong Kong, Holland, Spain, Iceland, Indonesia, India, and UK are aiding FBI and Microsoft in this operation.

The Digital Crimes Unit of Microsoft has reportedly taken-down about 1,000 of an assumed 1,400 malicious computer networks – known as Citadel Botnets -.

Citadel is responsible of infecting as many as 5 million computers across the globe. Microsoft believes that the network was used to steal whopping amounts from scores of financial institutions, including Bank of America, American Express, JPMorgan Chase, PayPal of eBay, Citigroup, Credit Suisse, Wells Fargo and Royal Bank of Canada.

No arrests have been made following the crackdown as the authorities are still investigating about the identities of ringleaders with the help of overseas authorities.

A botnet, also known as a zombie army, is a series of computers connected in a synchronized fashion for malicious purposes without the knowledge of their owners. Each computer in a botnet is called a bot. The botnet is controlled by a third-party and is used in committing financial crimes, distributing spam computer viruses and transmitting spam.

Citadel is one of the biggest botnets in operation today. According to Microsoft, the creator of software packed it with the pirated versions of the Windows OS to gain the control of personal computers in US, Western Europe, Australia, Hong Kong and India.

Microsoft said the Citadel was used to steal over $500 million from banks in the United States and other countries. However, the company didn’t reveal the details of losses at individual accounts or organizations.

Microsoft further said that the Citadel software is designed in such a way that it not attacks the PCs and financial institutions in Russia or Ukraine, apparently because the hackers operate from those countries and want to prevent themselves from law enforcement agencies there.