A guide to how ransomware holds your data to ransom and what to do if your computer is attacked.
Computers are being locked up with user files held for ransom in the latest cyber attack to paralyse hospitals, government offices and major multinationals globally. Here's a look at how malware and ransomware work and what to do if you fall victim.
* Malware refers to software that's harmful to computers. Ransomware is a type of malware that takes over a computer and prevents users from accessing data until a ransom is paid, John Villasenor, a professor at University of California Los Angeles says.
* Software spreads ransomware infections through computers using links or attachments in malicious messages known as 'phishing' emails. The age-old advice of never clicking on a link in an email still stands as the best defence, malware researcher at California-based Malwarebytes Jerome Segura says. Once a user clinks on a bad link or opens a virus-laden document the computer is infected.
* But in a twist recent ransomware attacks - including last month's WannaCry and this current one - use leaked US National Security Agency code that permits software to spread quickly within an organisation's network.
* As the name suggests ransomware holds your files for ransom until victims pay up. University of California Los Angeles professor Peter Reiher says attackers are explicit in their demands changing the wallpaper on your computer and demanding between $US300 and $US500 in bitcoin to decrypt files. If you don't pay, the data, including photos and documents, is often lost forever.
* The price can double within 24 hours if not paid. Law enforcement officials discourage victims from paying ransoms.
* Caution is the first step to avoiding an attack. Experts say users should also be on the look out for malicious email messages that often masquerade as emails from companies or people you regularly interact with online and avoid clicking on links.
* Backing up data regularly and ensuring security updates are installed on your computer is also essential. Up-to-date backups make it possible to restore files without paying a ransom.
* WannaCry and Tuesday's attack exploited vulnerabilities in some versions of Microsoft Windows. Microsoft has released software patches for the security holes, although not everyone has installed those updates.