CryptoLocker is a Trojan that runs on Microsoft Windows platform, which was first detected in 2013. It is a ransom Trojan, which enters the system through various sources. One of them is thorough an email ZIP attachment. The ZIP file has an EXE file with icon of a PDF file. Now, as we all know, Windows does not show the file extension. People who are professionals will definitely check the file's extension, but a person who is not so familiar with the computers will become an easy target. more info: http://nabzsoftware.com/types-of-threats/cryptolocker/