CryptoLocker is a ransomware trojan which targeted computers running Microsoft Windows, believed to have first been posted to the Internet on 5 September 2013.
CryptoLocker propagated via infected email attachments, and via an existing botnet; when activated, the malware encrypts certain types of files stored on local and mounted network drives using RSA public-key cryptography, with the private key stored only on the malwares control servers. The malware then displays a message which offers to decrypt the data if a payment (through either bitcoin or a pre-paid cash voucher) is made by a stated deadline, and threatened to delete the private key if the deadline passes. If the deadline is not met, the malware offered to decrypt data via an online service provided by the malwares operators, for a significantly higher price in bitcoin.
Although CryptoLocker itself is readily removed, files remained encrypted in a way which researchers considered unfeasible to break. Many said that the ransom should not be paid, but did not offer any way to recover files; others said that paying the ransom was the only way to recover files that had not been backed up. Some victims claimed that paying the ransom did not always lead to the files being decrypted.
CryptoLocker was isolated in late-May 2014 via Operation Tovar—which took down the Gameover ZeuS botnet that had been used to distribute the malware. During the operation, a security firm involved in the process obtained the database of private keys used by CryptoLocker, which was in turn used to build an online tool for recovering the keys and files without paying the ransom. It is believed that the operators of CryptoLocker successfully extorted a total of around $3 million from victims of the trojan. Other instances of encryption-based ransomware that have followed have used the “CryptoLocker” name (or variations), but are otherwise unrelated.