Authorities are seeking a suspected Russian cybercriminal Evgeniy Bogachev, after a worldwide coordinated operation to shut down botnet command and control servers.
Businesses should take immediate action to defend against the GameOver Zeus Trojan, because cyber criminals could restore the botnet within a fortnight.
The botnet has also been distributing crypto locker ransomware, which locks victims' computers, and offers them a way out, if ransom money is paid.
In a press conference on Monday, the US Department of Justice believed Mr Bogachev was last known to be living in Anapa, Russia.
His charges, filed in a court in Pittsburgh, included conspiracy, wire, bank and computer fraud, and money laundering.
The UK government, US government, Microsoft, Symantec and Trend Micro have posted advice on how to protect both personal computers and businesses. The advice included links to malware removal tools.
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