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Exorcising CTB Locker from your Computer: What you Need to Know

Ransomware, a type of malware which holds your files to ransom by encrypting them and then demanding a ransom for their “release”, i.e. by decryption, is nothing new. Cyber criminals make a lot of money by extorting funds from victims all over the world.

The latest family of widely distributed ransomware is called CTB Locker. In this blog we have decided to provide information about CTB Locker in the form of an FAQ so that our customers and the general public globally may be well-informed about the dangers of this malware family, learn how to avoid it, and be reassured about our robust response to it.

FAQ

  • How do you prevent your computer from becoming infected by CTB Locker?

Let’s begin with this question as it is the most important one to keep your computer safe. Prevention is always better than cure.

The initial spreading vector for CTB Locker is a spam email with enticing content which uses social engineering techniques to convince the potential victim to unzip a ZIP archive attachment (extension ‘.zip’) and execute its embedded file.

This embedded file, which is currently around 40KB in size, may have misleading extensions such as ‘.scr’ in order to masquerade as a screensaver application. This file is the downloader component for CTB Locker’s main payload, which then does the actual file encryption and makes ransom demands. We urge you to be vigilant against such spam emails as it is very first line of defence against CTB Locker as well as a host of other malware families which also use the same old time-tested technique to spread.

If an email comes from an unknown or unexpected source containing an attachment or a website link requesting you to open the attachment or click on the link, please exercise extreme caution. We would suggest simply deleting such emails if they are not already quarantined by your spam filter.

The spam emails tend to be targeted at English-speaking countries and at least 3 European countries given that the malware payload provides its ransom messages in German, Dutch and Italian.

This ransomware is not targeted at Indian users per se but given the ubiquitous nature of spam there will be “collateral damage” resulting in not just Indian victims but also many other hapless victims in other non-target countries.

  • What should you do when you discover your computer is infected with CTB Locker?

If you have seen messages demanding a ransom as shown above, it is likely that many, if not all, of your personal files such as Microsoft Office documents, PDF, TXT, ZIP and even ‘C’ source code files will be in an encrypted state, i.e. appear to contain random binary junk. Files encrypted by CTB Locker will have filenames such as yourfile.ext.<7 random lowercase letters>, e.g. 253667.PDF.iryrzpi

Executable files, e.g. EXE, DLL, OCX, etc, and files with extensions unknown to the malware will not be touched.

First and foremost, we would request that you do not attempt to pay the ransom to get your files back. Even if the cyber criminals do actually decrypt your files, the money they get from you will only serve to encourage them to continue their nefarious practices, investing R&D in enhancing their capabilities and global reach. Cyber criminals must be stripped of their Return on Investment incentive to create malware.

Once you have decided not to pay the ransom we would recommend removing the malware immediately. This can be done most easily by:

  1. updating your product
  2. rebooting into Safemode
  3. performing an on-demand scan on your computer
  4. removing the detected components. Note, the main CTB Locker payload is detected as ‘Trojan ( 0049d83b1 )’ and its downloader component is detected as ‘Trojan-Downloader ( 00499db21 )’

  • Is it possible to decrypt files encrypted by CTB Locker?

The malware itself demonstrates that files can be decrypted by randomly choosing 5 samples to decrypt.

However, the malware uses a high-grade encryption algorithm with a key which is unique to your computer, rendering it effectively impossible to force a decryption en masse.

  • How to restore files encrypted by CTB Locker?

It may not be possible to restore all files encrypted by CTB Locker. However, if your Windows operating system supports System Restore it may be possible to recover some of the contents of some of your folders to a recent restore point before the infection took place. However, typically System Restore will not restore files from user areas such as images and documents.

The most reliable solution, though, is to restore your critical files from regular backups. If you don’t backup your important files regularly then we urge you to start doing so ASAP. Apart from a CTB Locker infection, there are numerous other factors which could render your files irrecoverable in the future, including a hard disk failure. Note, it may also be possible to use deep forensics tools to recover some critical files if they still exist on sectors on the hard disk, but this is not an alternative to regular backups.

  • Will paying the ransom actually decrypt your files?

We refuse to pay any ransom so we are unable to confirm whether payment will actually result in your files being released. Once again, we would request you to not attempt to pay the ransom for the reasons mentioned earlier.

  • Why did K7 not detect and remove CTB Locker?

At K7 Threat Control Lab we are constantly monitoring and acting against CTB Locker infections, including coding robust generic detection for all components of CTB Locker. However, the cyber criminals behind the CTB Locker family have been investing considerable resources in morphing, i.e. changing the appearance of, all their components and spam emails such that they may sometimes be able to get past security scanners, not just K7’s, albeit for a very short period of time. We at K7, and our colleagues at other security companies, are working hard to stay ahead of CTB Locker in order to protect all our customers across the planet.

Samir Mody
Senior Manager, K7TCL

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