The CryptoLocker virus can be a lethal one-two punch: It seizes control of your computer files and threatens to erase them unless you pay a ransom.
Cybercriminals email you a PDF attachment. If you open the attachment, it installs malware on your hard drive that lets hackers access your computer files. The files are then encrypted and you're unable to access them.
"What happens next is the scary part," said Alex Watson, lead researcher at Websense Security Labs. Within two days, the hackers will email saying if you don't pay up, your documents will be deleted. Watson said attackers typically demand payment via hard-to-trace Bitcoin -- a virtual currency -- and not in cash or credit card.
Small businesses are particularly vulnerable to this attack because many haven't adequately protected file-sharing between employees, said Watson. "So if one employee's computer is compromised, then every document that the company owns can be locked," he said.