An FBI-led international operation to disrupt the Gameover Zeus botnet and CryptoLocker ransomware has given people a two-week window in which to protect their computers.
Thought to comprise between 500,000 and one million computers around the world, the Gameover Zeus botnet steals bank details that crooks can use to wire money to themselves. So far it has caused known losses of around EUR75 million.
The botnet also distributes CryptoLocker, which encrypts all the files on victims' computers and demands payment for a password to unlock the files. In just two months, around $27 million has been paid out by victims.
Over the last few days the FBI - working with Europol, the UK National Crime Agency (NCA) and private firms such as Microsoft - has raided servers around the world, taking control of command and control servers used to direct the botnet.
Authorities say that this gives people a short window in which they can make sure that security software is installed and updated. In the UK, Internet Service Providers are writing to known victims and telling them to back up their data.
Andy Archibald, deputy director, national cyber crime unit, NCA, says: "By making use of this two-week window, huge numbers of people in the UK can stop that from happening to them...Our message is simple: update your operating system and make this a regular occurrence, update your security software and use it and, think twice before clicking on links or attachments in unsolicited emails."
Meanwhile, the US Justice Department has filed a complaint against a Russian citizen called Evgeniy Mikhailovich Bogachev who is accused of being the leader of the gang behind Gameover Zeus and CryptoLocker.