Organizations in Europe and the US have been crippled by a ransomware attack known as ‘Petya’. The malicious software has spread through large firms including the advertiser WPP, food company Mondelez, legal firm DLA Piper and Danish shipping and transport firm Maersk, leading to PCs and data being locked up and held for ransom.

It’s the second major global ransomware attack in the last two months. In early May, Britain’s National Health Service (NHS) was among the organizations infected by WannaCry, which used a vulnerability first revealed to the public as part of a leaked stash of NSA-related documents released online in April by a hacker group calling itself the Shadow Brokers. Like WannaCry, ‘Petya’ spreads rapidly through networks that use Microsoft Windows.

The ransomware takes over computers and demands $300, paid in Bitcoin. The malicious software spreads rapidly across an organization once a computer is infected using the EternalBlue vulnerability in Microsoft Windows.

The ransomware infects computers and then waits for about an hour before rebooting the machine. While the machine is rebooting, you can switch the computer off to prevent the files from being encrypted and try and rescue the files from the machine, though if you are infected and the system reboots with the ransom note, don’t pay the ransom – the “customer service” email address has been shut down so there’s no way to get the decryption key to unlock your files anyway. Disconnect your PC from the internet, reformat the hard drive and reinstall your files from a backup. Back up your files regularly and keep your anti-virus software up to date.