|Isolation||June 2, 2014|
|Operating system(s) affected||Windows|
CryptoLocker was a ransomware trojan which targeted computers running Microsoft Windows and was first observed by Dell SecureWorks in 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 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 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 malware's operators, for a significantly higher price in Bitcoin.
Although CryptoLocker itself is readily removed, files remained encrypted in a way which researchers considered infeasible 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 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.
CryptoLocker typically propagated as an attachment to a seemingly innocuous e-mail message, which appears to have been sent by 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 which claims to allow users to decrypt their files without the CryptoLocker program, and to purchase the decryption key after the deadline expires; the process involves uploading an encrypted file to the site as a sample, and waiting for the service to find a match, which the site claims would occur within 24 hours. Once a match is found, the user can pay for the key online; if the 72 hour deadline has passed, the cost increases to 10 Bitcoin.
The success of CryptoLocker spawned a number of unrelated and similarly named worms working in essentially the same way, such as CryptoLocker 2.0—which was originally thought to be a variant of CryptoLocker, but was ultimately considered by security researchers to be a copycat due to notable differences in its internal architecture (such as being written in a completely different programming language, and using a different type of encryption).
Takedown and recovery of files
On June 2, 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.
While security software is designed to detect such threats, it might not detect CryptoLocker at all, or only after encryption is underway or complete, particularly if a new version unknown to the protective software is distributed. If an attack is suspected or detected in its early stages, it takes some time for encryption to take place; immediate removal of the malware (a relatively simple process) before it has completed would limit its damage to data. Experts suggested precautionary measures, such as using software or other security policies to block the CryptoLocker payload from launching. Symantec estimated that 3% of users infected by CryptoLocker chose to pay.
Due to the nature of CryptoLocker's operation, some experts reluctantly suggested that paying the ransom was the only way to recover files from CryptoLocker in the absence of backups (in particular, offline backups made before the infection that are inaccessible from the network, and thus cannot be infected by CryptoLocker). Due to the length of the key employed by CryptoLocker, experts considered it practically impossible to use a brute-force attack to obtain the key needed to decrypt files without paying; the similar 2008 worm Gpcode.AK used a 1024-bit key that was believed to be large enough to be computationally infeasible to break without a concerted distributed effort, or the discovery of a flaw that could be used to break the encryption. Sophos security analyst Paul Ducklin even speculated that CryptoLocker's online decryption service involved the brute forcing of its own encryption.
In August 2014, FireEye and Fox-IT have released a free service decryptolocker.com that allows users to upload an encrypted file and then receive the private key via email. Using this private key and a separately downloaded executable, users are able to decrypt the files. It's not entirely clear how the companies accessed the private keys and they state "Through various partnerships and reverse engineering engagements, Fox-IT and FireEye have ascertained many of the private keys associated with CryptoLocker." 
In December 2013 ZDNet traced four Bitcoin addresses posted by users who had been infected by CryptoLocker, in an attempt to gauge the operators' takings. The four addresses showed movement of 41,928 BTC between 15 October and 18 December, about US$27 million at that time.
A survey by researchers at the University of Kent found that 41% of British victims decided to pay the ransom, a figure much larger than expected; 3% had been conjectured by Symantec, and 0.4% by Dell SecureWorks. Following the shutdown of the botnet that had been used to distribute CryptoLocker, it was calculated that about 1.3% of those infected had paid the ransom; many had been able to recover files which had been backed up, and others are believed to have lost huge amounts of data. Nonetheless, the operators were believed to have extorted a total of around $3 million.
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- DecryptCryptolocker, online recovery service