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Cryptolocker: How to avoid getting infected and what to do if you are

Jonathan Hassell | Oct. 28, 2013
There's a new piece of ransomware in town and here's how to protect your company's assets

There's a big threat wiling around on the Internet right now: a particularly nasty piece of ransomware called Cryptolocker. Many, many organisations are being infected with this malware, but fortunately, there are surefire ways to avoid it and also ways to mitigate the damage without letting the lowlifes win.

What is Cryptolocker?
Cryptolocker comes in the door through social engineering. Usually the virus payload hides in an attachment to a phishing message, one purporting to be from a business copier like a Xerox that is delivering a PDF of a scanned image, from a major delivery service like UPS orFedEx offering tracking information or from a bank letter confirming a wire or ACH transfer.

Cryptolocker's ransom note to infected users.

The virus is, of course, an executable attachment, but interestingly the icon representing the executable is a PDF file With Windows' hidden extensions feature, the sender simply adds ".pdf" to the end of the file (Windows hides the .exe) and the unwitting user is fooled into thinking the attachment is a harmless PDF file from a trusted sender. It is, of course, anything but harmless.

Once Cryptolocker is in the door, it targets files with the following extensions:

*.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, img_*.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

When it finds a file matching that extension, it encrypts the file using a public key and then makes a record of the file in the Windows registry under HKEY_CURRENT_USER\Software\CryptoLocker\Files. It then prompts the user that his or her files have been encrypted and that he or she must use prepaid cards or Bitcoin to send hundreds of dollars to the author of the malware.

Once the payment has been made, the decryption usually begins. There is typically a four-day time limit on the payment option; the malware's author claims the private key required to decrypt files will be deleted if the ransom is not received in time. If the private key is deleted, your files will essentially never be able to be decrypted - you could attempt to brute force the key, but as a practical matter, that would take on the order or thousands of years. Effectively, your files are gone.

Currently, the only versions of Cryptolocker in existence target files and folders on local drives and mapped drives. The malware does not currently attempt to perform its malfeasance over network -based universal naming convention paths, although one would surmise this would be a relatively simple change for the author of the ransomware to make.


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