A type of malicious software that encrypts a victim’s files, making them inaccessible until a ransom is paid. It is commonly used to target individuals and businesses, often through email phishing scams or infected website downloads. Examples include the WannaCry attack that affected the NHS in 2017, the CryptoLocker attack that targeted Microsoft Windows users in 2013, and the recent Ryuk ransomware attack on the printing company RR Donnelley in 2019.