Cosakos Ransomware - How to remove

Cosakos ransomware has recently been reported to be infecting computers as a new version of STOP ransomware, usually spread disguised as useful programs online and downloaded by the victims accidentally. According to those researching this virus, Cosakos uses symmetric encryption to lock most of the data on the infected computer. To mark the locked files, a “.cosakos” extension is applied to the files, and that’s what Cosakos ransomware is named after. Variants of STOP ransomware are also known to install a password stealer and block certain websites from being accessed on the infected machine.

Cosakos can be removed without necessarily wiping your PC, but the free decryption solutions are limited and depend a lot on circumstances unique to each victim. However, some victims do manage to restore all of their data without ever dealing with the criminals who released the Cosakos virus — even some of those who did not have backups of their data.

Cosakos encrypts your files

The ransom note that the criminals programmed their virus to place in your folders, _readme.txt, include a message from the extortionists. The message is supposed to encourage the victims to pay the ransom which costs hundreds of dollars. Luckily, a few solutions exist which don’t require anyone to contact the criminals.

Cosakos changes the type of the locked data into “.cosakos”, but removing the extension will not fix the files. The internal contents of the files are actually edited in a way that makes it impossible to recognize what the data is supposed to be. While cryptography allows a file that’s been encrypted to be restored to the previous state, a decryption key is required for this, as well as the correct implementation of the algorithm. Only the developers behind Cosakos know the decryption keys, so the victims are in a complicated position. So far, many STOP ransomware variants have been released (like Djvu, Todarius, Litar, and others), and they don’t have complete decrypters.

Can you avoid ransomware?

There are a few things that everyone should do if they want to avoid Cosakos and other ransomware, adware, other malware, and scams. It’s worth taking these measures and being careful because some malware really does cost people real money, time, and grief.

• Back up your data. That’s the best defense against Cosakos and any other ransomware. While Cosakos is theoretically decryptable, some ransomware viruses are not. Some of them simply corrupt the files with no way to reverse that. Not to mention that physical storage breaks all the time and backups are useful there, too.
• Update your software. While there are legitimate reasons for why some people avoid updates, it’s important to realize that each public and unpatched vulnerability in their browser, media player, or operating system increases the risk of a malware infection. Some malware is very successfully distributed by exploiting security vulnerabilities which have had patches released.
• Don’t pirate software. Cosakos mostly spreads by being downloaded be people who are looking for ways to activate their trial software for free. There are tools online that promise to do this, but it’s important to remember that not only is this illegal, but malware developers use cracks and other pirated programs to hide viruses in.
• Scan each file before you open it. Be especially careful with files that came in email from unfamiliar senders. Malicious email spam is still one of the main ways that ransomware is distributed.

Decrypt and remove Cosakos virus

Cosakos, the trojan which it installs, and most other malware can be removed with almost any competent antivirus tool, for example, Spyhunter. It might also require you to boot the infected computer in safe mode. Use a trusted antivirus program to find and remove the viruses and avoid using the computer for anything important and sensitive until you’re sure that the PC is clean.

As for decrypting the files, a researcher @Demonslay335 has been dedicated to solving the STOP ransomware problem and has released a free decrypter. The problem is, it only works in limited circumstances, so only some of the victims can take advantage of this solution.

Automatic Malware removal tools

How to recover Cosakos Ransomware encrypted files and remove the virus

Step 1. Restore system into last known good state using system restore

1. Reboot your computer to Safe Mode with Command Prompt:

for Windows 7 / Vista/ XP

• Start → Shutdown → Restart → OK.
• Press F8 key repeatedly until Advanced Boot Options window appears.
• Choose Safe Mode with Command Prompt.

for Windows 8 / 10

• Press Power at Windows login screen. Then press and hold Shift key and click Restart.
• Choose Troubleshoot → Advanced Options → Startup Settings and click Restart.
• When it loads, select Enable Safe Mode with Command Prompt from the list of Startup Settings.

 

2.Restore System files and settings.

• When Command Prompt mode loads, enter cd restore and press Enter.
• Then enter rstrui.exe and press Enter again.
• Click “Next” in the windows that appeared.
• Select one of the Restore Points that are available before Cosakos has infiltrated to your system and then click “Next”.
• To start System restore click “Yes”.

 

Step 2. Complete removal of Cosakos Ransomware

After restoring your system, it is recommended to scan your computer with an anti-malware program, like Spyhunter and remove all malicious files related to Cosakos. You can check other tools here.  

Step 3. Restore Cosakos Ransomware affected files using Shadow Volume Copies

If you do not use System Restore option on your operating system, there is a chance to use shadow copy snapshots. They store copies of your files that point of time when the system restore snapshot was created. Usually Cosakos tries to delete all possible Shadow Volume Copies, so this methods may not work on all computers. However, it may fail to do so. Shadow Volume Copies are only available with Windows XP Service Pack 2, Windows Vista, Windows 7, and Windows 8. There are two ways to retrieve your files via Shadow Volume Copy. You can do it using native Windows Previous Versions or via Shadow Explorer. a) Native Windows Previous Versions Right-click on an encrypted file and select Properties → Previous versions tab. Now you will see all available copies of that particular file and the time when it was stored in a Shadow Volume Copy. Choose the version of the file you want to retrieve and click Copy if you want to save it to some directory of your own, or Restore if you want to replace existing, encrypted file. If you want to see the content of file first, just click Open.

b) Shadow Explorer It is a program that can be found online for free. You can download either a full or a portable version of Shadow Explorer. Open the program. On the left top corner select the drive where the file you are looking for is a stored. You will see all folders on that drive. To retrieve a whole folder, right-click on it and select “Export”. Then choose where you want it to be stored.

Step 4. Use Data Recovery programs to recover Cosakos Ransomware encrypted files

There are several data recovery programs that might recover encrypted files as well. This does not work in all cases but you can try this:

• We suggest using another PC and connect the infected hard drive as slave. It is still possible to do this on infected PC though.
• Download a data recovery program.
• Install and scan for recently deleted files.

Note: In many cases it is impossible to restore data files affected by modern ransomware. Thus I recommend using decent cloud backup software as precaution. We recommend checking out Carbonite, BackBlaze, CrashPlan or Mozy Home.