'Petya' ransomware attack: what is it and how can it be stopped?

Companies have been crippled by global cyberattack, the second major ransomware crime in two months. We answer the key questions

Many 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 past 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.