Ransomware

Ransomware is a type of malware from cryptovirology that threatens to publish the victim's data or perpetually block access to it unless a ransom is paid. While some simple ransomware may lock the system so that it is  not difficult for a knowledgeable person to reverse, more advanced  malware uses a technique called cryptoviral extortion. It encrypts the victim's files, making them inaccessible, and demands a ransom payment to decrypt them. In a properly implemented cryptoviral extortion attack, recovering the files without the decryption key is an intractable problem – and difficult to trace digital currencies such as paysafecard or Bitcoin and other cryptocurrencies are used for the ransoms, making tracing and prosecuting the perpetrators difficult. 

Ransomware attacks are typically carried out using a Trojan disguised as a legitimate file that the user is tricked into  downloading or opening when it arrives as an email attachment. However,  one high-profile example, the WannaCry Worm, traveled automatically between computers without user interaction