Cryptolocker – What is cryptolocker malware and how to avoid been infected by malware!


CyptoLocker is part of the family of ransomware whose business model (yes, malware is big business to some hitec criminlas!) is based on extorting money from users. This continues the trend started by another infamous piece of malware which also extorts its victims, the so-called ‘Police Virus’, which asks users to pay a ‘fine’ to unlock their computers. However, unlike the Police Virus, CryptoLocker hijacks users’ documents and asks them to pay a ransom (with a time limit to send the payment) normally in some form of crypto currency as this makes tracking where your payment goes is pretty impossible.


Malware installation

CryptoLocker uses a number social engineering techniques to trick the user into running it. More specifically, the victim receives an email with a password-protected ZIP, excel document or PDF file purporting to be from a logistics company, or now the criminals are using hijacked email accounts to send their infected payloads which will get through many spam filters as they come from legit email addresses with genuine file attachments, the malware script is in a link with in the attached document.

The Trojan gets run when the user opens the attached ZIP file or other attached file such as excel document or pdf, by entering the password included in the message, and attempts to open the PDF it contains. CryptoLocker takes advantage of Windows’ default behavior of hiding the extension from file names to disguise the real .EXE extension of the malicious file.

As soon as the victim runs it, the Trojan goes memory resident on the computer and takes the following actions:

  • Saves itself to a folder in the user’s profile (AppData, LocalAppData).
  • Adds a key to the registry to make sure it runs every time the computer starts up.
  • Spawns two processes of itself: One is the main process, whereas the other aims to protect the main process against termination.

File encryption

The Trojan generates a random symmetric key for each file it encrypts, and encrypts the file’s content with the AES algorithm, using that key. Then, it encrypts the random key using an asymmetric public-private key encryption algorithm (RSA) and keys of over 1024 bits (we’ve seen samples that used 2048-bit keys), and adds it to the encrypted file. This way, the Trojan makes sure that only the owner of the private RSA key can obtain the random key used to encrypt the file. Also, as the computer files are overwritten, it is impossible to retrieve them using forensic methods.



Once run, the first thing the Trojan does is obtain the public key (PK) from its C&C server. To find an active C&C server, The Trojan incorporates a domain generation algorithm (DGA) known as ‘Mersenne twister’ to generate random domain names.  This algorithm uses the current date as seed and can generate up to 1,000 different fixed-size domains every day.

After the Trojan has downloaded the PK, it saves it inside the following Windows registry key: HKCUSoftwareCryptoLockerPublic Key. Then, it starts encrypting files on the computer’s hard disk and every network drive the infected user has access to.

CryptoLocker doesn’t encrypt every file it finds, but only non-executable files with the extensions included in the malware’s code these can include the list below:

Additionally,CryptoLocker logs each file encrypted to a registry key:

When the Trojan finishes encrypting every file that meets the conditions, it displays a message asking the user to make a ransom payment, with a time limit to send the payment before the private key kept by the malware writer is destroyed.


Oddly enough, the malware doesn’t ask users for the same amount of money, but incorporates its own currency conversion table.

How to avoid CryptoLocker and other malware infections

This malware spreads via email by using social engineering techniques. Therefore, our recommendation are:

  • Being particularly wary of emails from senders you don’t know, especially those with attached files.
  • Disabling hidden file extensions in Windows will also help recognize this type of attack.
  • We’d like to remind you of the importance of having a backup system in place for your critical files. This will help mitigate the damage caused not only by malware infections, but hardware problems or any other incidents as well.
  • If you become infected and don’t have a backup copy of your files, our recommendation is not to pay the ransom. That’s NEVER a good idea, as it turns the malware into a highly profitable business model and will contribute to the flourishing of this type of attack.

If you find yourself infected with the cryptolocker family of malware contact Goldfield to see how we can help get your systems backup and running. We also offer a off site backup  service which backups up your data offsite at a secure location.