CryptoLocker typically propagated as an attachment to a seemingly innocuous e-mail message, which appears to have been sent by a legitimate company. A ZIP file attached to an email message contains an executable file with the filename and the icon disguised as a PDF file, taking advantage of Windows' default behaviour of hiding the extension from file names to disguise the real .EXE extension. CryptoLocker was also propagated using the Gameover ZeuS trojan and botnet.
When first run, the payload installs itself in the user profile folder, and adds a key to the registry that causes it to run on startup. It then attempts to contact one of several designated command and control servers; once connected, the server generates a 2048-bit RSA key pair, and sends the public key back to the infected computer. The server may be a local proxy and go through others, frequently relocated in different countries to make tracing them more difficult.
The payload then encrypts files across local hard drives and mapped network drives with the public key, and logs each file encrypted to a registry key. The process only encrypts data files with certain extensions, including Microsoft Office, OpenDocument, and other documents, pictures, and AutoCAD files. The payload displays a message informing the user that files have been encrypted, and demands a payment of 400 USD or Euro through an anonymous pre-paid cash voucher (i.e. MoneyPak or Ukash), or an equivalent amount in bitcoin (BTC) within 72 or 100 hours (while starting at 2 BTC, the ransom price has been adjusted down to 0.3 BTC by the operators to reflect the fluctuating value of bitcoin), or else the private key on the server would be destroyed, and "nobody and never [sic] will be able to restore files." Payment of the ransom allows the user to download the decryption program, which is pre-loaded with the user's private key. Some infected victims claim that they paid the attackers but their files were not decrypted.
In November 2013, the operators of CryptoLocker launched an online service that claimed to allow users to decrypt their files without the CryptoLocker program, and to purchase the decryption key after the deadline had expired; the process involved uploading an encrypted file to the site as a sample and waiting for the service to find a match; the site claimed that a match would be found within 24 hours. Once found, the user could pay for the key online; if the 72-hour deadline passed, the cost increased to 10 bitcoin.
Takedown and recovery of files
On 2 June 2014, the United States Department of Justice officially announced that over the previous weekend, Operation Tovar—a consortium constituting a group of law enforcement agencies (including the FBI and Interpol), security software vendors, and several universities, had disrupted the Gameover ZeuS botnet which had been used to distribute CryptoLocker and other malware. The Department of Justice also publicly issued an indictment against the Russian hacker Evgeniy Bogachev for his alleged involvement in the botnet.
As part of the operation, the Dutch security firm Fox-IT was able to procure the database of private keys used by CryptoLocker; in August 2014, Fox-IT and fellow firm FireEye introduced an online service which allows infected users to retrieve their private key by uploading a sample file, and then receive a decryption tool.