Ransomware, Cryptolocker, hits systems

Ransomware, Cryptolocker, hits systems

Cryptolocker, a ransomware Trojan virus, encrypts a victim’s files and then demands payment for the key, and is indicative of the lengths nefarious types will go to for a few dollars of ill-gotten gains.

Ransomware is on the rise and thanks to more than a few nefarious types and their victims, is proving to be an all too common way for electronic extortion to move into an enterprise. In many cases, it proves to be cheaper to pay for the privilege to unlock your data than it would be to remediate the impacted system, which only makes matters worse.

Take for example Cryptolocker, a ransomware Trojan that encrypts files and can spread in many ways, including in phishing emails that contain malicious attachments or links, or via drive-by download sites. Often, Cryptolocker arrives as a file with a double extension, such as *.pdf.exe and can be hard to recognize, simply because Windows hides file extensions by default – that file may look like a PDF file rather than an executable.

Double clicking on the Cryptolocker infected file launches an executable, which infects computers just like any other malware by placing its files in Windows directories and creating registry entries that allow it to restart after a reboot. Cryptolocker also attempts to contact its command and control (C&C) server using a random domain name generation algorithm to try and find a current C&C server. Some sample Crytpolocker domains might look like this:



Once Cryptolocker contacts its C&C, it generates a public/private cryptographic key for the specific computer, using very strong and standard RSA and AES 2048-bit encryption. The private key is only stored on the attacker’s C&C servers, but the public key is saved in a registry entry on the computer. Cryptolocker then uses that key pair to encrypt many different types of files on the computer, including

*.odt, *.ods, *.odp, *.odm, *.odc, *.odb, *.doc, *.docx, *.docm, *.wps, *.xls, *.xlsx, *.xlsm, *.xlsb, *.xlk, *.ppt, *.pptx, *.pptm, *.mdb, *.accdb, *.pst, *.dwg, *.dxf, *.dxg, *.wpd, *.rtf, *.wb2, *.mdf, *.dbf, *.psd, *.pdd, *.pdf, *.eps, *.ai, *.indd, *.cdr, *.jpg, *.jpe, *.jpg, *.dng, *.3fr, *.arw, *.srf, *.sr2, *.bay, *.crw, *.cr2, *.dcr, *.kdc, *.erf, *.mef, *.mrw, *.nef, *.nrw, *.orf, *.raf, *.raw, *.rwl, *.rw2, *.r3d, *.ptx, *.pef, *.srw, *.x3f, *.der, *.cer, *.crt, *.pem, *.pfx, *.p12, *.p7b, *.p7c.

After the encryption process completes, Cryptolocker displays screen with a warning that requires a payment of either $300 or £200 within 72 hours to regain access to the files.

What should I do if I get infected?

If you are infected with Cryptolocker, the first thing you should do is disconnect the infected PC from the internet. If Cryptolocker can’t access its C&C, it can’t encrypt files. Disconnecting the machine may prevent further files from being encrypted.

There are many tools that will totally clean a Cryptolocker infection, but most victims are more concerned with recovering encrypted files. Unfortunately, you will not be able to crack Cryptolocker’s encryption. It uses a very strong and reliable public/private key implementation that is similar to what commercial encryption products use. It would take decades to centuries to crack today.

If Cryptolocker encrypts some of your files, you should check if you have a backup, which would be the best chance for recovering the lost data. Adding insult to injury is that there are reports claiming Cryptolocker’s decryption does work, and paying the ransom may only result in the loss of your money.

How can I avoid Cryptolocker?

Most commercial antivirus (AV) products can detect many variants of Cryptolocker, which means protection starts with using both host-based and network-based AV products that are kept up to date. However, Cryptolocker’s authors are very aggressive at re-packing their malware to make the same executable file look different on a binary level, which helps it evade some AV solutions. In short, though AV helps, some variants may get past some AV solutions. Other defenses are becoming a must as well, such as reputation based defense systems that keep track millions of malicious URLS and web sites. That means access to sites that distribute or support malware can be blocked, effectively preventing infected hosts from reaching C&C servers.

Awareness proves to be one of the best defenses, Cryptolocker typically spreads via some obvious phishing emails. The emails may pretend to be FedEx or UPS related messages, which contain zip files that hide a double-extension executable. Training users to recognize some of the common phishing and malware signs, such as unsolicited emails from shipping providers, double-extension files, links that point to the wrong sites, and so on should prove to be an effective first line of defense.

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