One evening in April, Dave Winston stood in a convenience store in suburban Charlotte, N.C., uneasily shoving $20 bills into a slim automated-teller machine unlike any he had ever seen. He was buying bitcoin, a digital currency unknown to him a few hours earlier, before hackers took over his computer.
Mr. Winston, crew chief with the Circle Sport-Leavine Family Nascar race team, is among a growing number of victims of a pernicious type of malicious software called ransomware, which has earned millions of dollars for cybercriminals by encrypting computer files and holding them hostage.
Ransomware dates to the late 1980s, but attacks spiked this year amid the growing use of bitcoin and improved encryption software. Malicious code turned Mr. Winston’s Excel spreadsheets and Word documents into unreadable gobbledygook, and hackers instructed him to pay $500 in bitcoin to unscramble them.
Mr. Winston doesn’t know how the software infected his computer, but security experts say attacks often start with an email message containing an attachment or a link to a website that then quietly