WannaCry ransomware is the most aggressive malware attack launched so far, affecting users in more than 150 countries. The ransomware uses vulnerabilities in older Windows versions (starting with Windows 8 and down) which allows spreading like a worm in Windows-based networks. Once a computer is infected, all files are encrypted and the personal data is held hostage. WannaCry displays a screen that supposedly will allow you to decrypt the files if you pay a ransom using Bitcoins, an untraceable virtual currency - although this does not guarantee that the files will be decrypted.
This security update resolves vulnerabilities in older Windows versions. Depending on your operating system, these are the direct download links to the Windows patches that protects your computer from WannaCry ransomware (also called WannaCrypt):