ClassificationTrojan horse
Isolation2 June 2014
Operating system(s) affectedWindows

The CryptoLocker ransomware attack was a cyberattack using the CryptoLocker ransomware that occurred from 5 September 2013 to late-May 2014. The attack utilized a trojan that targeted computers running Microsoft Windows,[1] and was believed to have first been posted to the Internet on 5 September 2013.[2] It propagated via infected email attachments, and via an existing Gameover ZeuS botnet;[3] 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 malware's 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 it will threaten 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 malware's operators, for a significantly higher price in bitcoin. There is no guarantee that payment will release the encrypted content.

Although CryptoLocker itself was easily removed, the affected 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.


CryptoLocker typically propagated as an attachment to a seemingly innocuous e-mail message, which appears to have been sent by a legitimate company.[4] 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.[5][6][7]

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.[1][6] The server may be a local proxy and go through others, frequently relocated in different countries to make tracing them more difficult.[8][9]

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.[7] 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),[10] or else the private key on the server would be destroyed, and "nobody and never [sic] will be able to restore files."[1][6] Payment of the ransom allows the user to download the decryption program, which is pre-loaded with the user's private key.[6] Some infected victims claim that they paid the attackers but their files were not decrypted.[4]

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.[11][12]

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.[5][13][14]

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.[15][16]

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