Home Guides Cosakos ransomware [+.cosakos virus files decryption – upd. Aug 2019]

Cosakos ransomware [+.cosakos virus files decryption – upd. Aug 2019]

7 min read
The so-called Djvu ransomware breed has been replenished with a new variant that appends a victim’s files with the .cosakos extension and uses _readme.txt note.
  1. What is the Cosakos ransomware?
  2. Cosakos ransomware automated removal and data recovery
  3. Cosakos ransomware manual removal and file recovery
  4. Ransomware Prevention Tips

What is the Cosakos ransomware?

Although extortion through the use of malicious code has plummeted in the past two years or so, it still has its very exclusive niche in the Internet-borne crime. The contemporary attacks are more refined and targeted than they used to be, and they typically don’t look like attempts to catch small fish in a big pond anymore. However, some strains follow an old-school activity pattern – the new Cosakos ransomware is a good example. Its operators cast the net wide by orchestrating ‘classic’ phishing campaigns that zero in on numerous unrelated users rather than specific organizations. This particular sample is a spinoff of the lineage catalogued as Djvu, or STOP ransomware, which has hardly evolved for many months. It bears a strong resemblance to the recent Boston version that was active throughout June 2019. Here’s a brief lowdown on the current Djvu ransomware persona: it adds the .cosakos extension to the names of encrypted files and drops a _readme.txt ransom message into affected folders.

Ransom note plus .cosakos extension files in a folder
Ransom note plus .cosakos extension files in a folder

By the way, there are quite a few other mods of this family now circulating. They use the following file extensions: .brusaf, .prandel, .masok, .zatrov, and .kovasoh. However, the Cosakos build appears to be the most widespread of them. The most likely explanation is that the pests are being distributed on a RaaS (Ransomware as a Service) basis, and some “affiliates” are more competent than the others. The bulk of the malefactors’ success relies on the distribution model. As mentioned above, the Cosakos ransomware is spreading through booby-trapped emails. These messages can be masqueraded as invoices, USPS delivery notifications, CVs, or similarly impactful subjects that the average recipient will want to explore deeper. The file attached to such a malspam email is the actual contagion. In most cases, it is a ZIP archive with a harmful JavaScript object inside. Once opened and double-clicked, this entity will instantly download the malicious program from the perpetrators’ server.

Cosakos ransomware still uses _readme.txt recovery note
Cosakos ransomware still uses _readme.txt recovery note

Once on board and running, Cosakos starts its raid with a sort of reconnaissance. It scans the hard drive and all mapped network locations for potentially important information, ignoring system-critical files and directories. Having identified the valuable data, the program encrypts it all using the AES-256 cipher. After the cryptographic job has been completed, the infection creates a recovery manual named _readme.txt in every folder that contains at least one hostage file. This note tells the victim to pay $980 worth of Bitcoin to the attackers’ cryptocurrency wallet. For ransoms submitted during the first 72 hours since the compromise, the crooks claim to provide a 50% discount, so the amount is $490.

As a reassurance that the money won’t be thrown to the wind, the malefactors can supposedly decrypt one item for free. They even include a link to a video that shows the decryption tool in action. After sending the funds, the victim is instructed to contact the villains at gorentos@bitmessage.ch or gorentos2@firemail.cc. Alternatively, there is an option to reach out to the attackers over Telegram – the account name is @datarestore.

So much for the tactic employed by the Djvu .cosakos files ransomware. What about the recovery? Thankfully, there is a breakthrough in cracking this family – however, it depends on the specific circumstances that may or may not be the case. Researchers created a tool called STOPDecrypter, which is capable of reinstating the data if the encryption was performed in offline mode.

In other words, the free recovery feature only works in case the Cosakos ransomware variant failed to obtain an encryption key from its C2 server for some reason and leveraged a key built into its code. Unfortunately, the stars don’t align that way very often, and the cryptographic workflow mostly uses an online key only the attackers possess. If so, the victims have two options: to cough up the ransom, which is definitely not recommended; or to try known white hat techniques of data recovery as described below.

Cosakos ransomware automated removal and data recovery

When dealing with the Cosakos threat, you can get a lot of mileage out of the Malwarebytes automatic security tool that will pinpoint and iron out the infection in minutes. Malwarebytes is one of the big names in the anti-malware industry with an immaculate track record of protecting systems against all prevalent forms of harmful code.

The solution uses artificial intelligence (AI) and machine learning to detect both known and zero-day threats, including potentially unwanted programs (PUPs), ransomware, exploits, and malicious sites. It boasts low system footprint, high scanning speed, and intuitive interface where every module clearly explains what it does. Follow the steps below to come through the malware predicament unscathed:

1. Download and install the latest version of Malwarebytes onto your PC.

As part of this step, you will be prompted to specify where you are installing the program (personal or work computer) and toggle the advanced setup options if you so desire. You can use all the premium features for 14 days free of charge.

Download Cosakos virus remover

2. Once the application is up and running, go ahead and click on the Scan button in the bottom middle part of the GUI. Click the Scan button

3. Wait until Malwarebytes checks for updates of its proprietary Katana detection engine and scans the memory, startup items, registry, and file system for threats. Malwarebytes scan in progress

4. When the scan is completed, go over the report listing the infections that have been identified. Make sure there are checkmarks next to all the unwanted items and click on the Quarantine button as shown below. Malwarebytes scan results

5. You will be prompted to restart your computer to finish the removal process. Click Yes to do it immediately or close the dialog and restart later on. Restart to complete the removal process

6. As soon as the threats have been moved to Quarantine, they can no longer harm your computer. If you are sure that you won’t need to restore any of those items at a later point, click on the Detection History pane at the bottom left of the Malwarebytes home screen, select the Quarantined items tab, and click Delete. Delete quarantined items

Data recovery toolkit to the rescue

Some strains of ransomware are known to delete the original files after the encryption routine has been completed. As hostile as this activity appears, it can play into your hands. There are applications designed to revive the information that was obliterated because of malfunctioning hardware or due to accidental removal. The tool called Stellar Data Recovery features this type of a capability and therefore it can be applied in ransom attack scenarios to at least get the most important files back. So use the app to get an idea of what data can be restored and let it do the recovery job. Here is a step-by-step walkthrough:

  1. Download and install Stellar Data Recovery. Download Stellar Data Recovery
  2. Open the application, select the types of recoverable files to look for, and click Next.Stellar Data Recovery main screen
  3. Choose the areas you want the tool to recover from and click the Scan button.Select which PC areas to recover from
  4. Having scanned the specified locations, the program will display a notification about the total amount of recoverable data. Close the dialog and click the Recover button. This will hopefully help you get some of your valuable files back.Recover files

Cosakos ransomware manual removal and file recovery

Some ransomware strains terminate themselves after completing the encryption job on a computer, but some don’t. Furthermore, the Cosakos virus may prevent victims from using popular antimalware tools in order to stay on board for as long as possible. Under the circumstances, it may be necessary to utilize the Safe Mode with Networking or System Restore functionality.

Remove Cosakos ransomware using Safe Mode with Networking

Remove Cosakos ransomware using Safe Mode with Networking

Get rid of Cosakos ransomware using System Restore

Get rid of Cosakos ransomware using System Restore

System Restore enables Windows users to roll back all changes made to the OS since the latest restore point creation time. This feature can help eliminate the most persistent ransomware. Before going this route, though, make sure System Restore had been enabled prior to the breach, otherwise the method will be inefficient.

  • Open Windows Advanced Options Menu as described in the previous section: hit F8 repeatedly when the PC is starting up. Use arrow keys to highlight the Safe Mode with Command Prompt entry. Hit Enter.Safe Mode with Command Prompt
  • In the Command Prompt window, type cd restore and hit Entercd restore command
  • Type rstrui.exe in the new command line and press EnterType rstrui.exe command
  • When the System Restore screen pops up, click Next, select a restore point that predates the contamination, and use the application’s controls to roll back the system to this earlier state.System Restore window

Be advised that even after the ransomware is removed, files will still be encrypted and inaccessible. The malicious code cleanup part, however, is important because it keeps a relapse of the infection from occurring further on and eliminates all opportunistic malware.

Ways of non-ransom recovery of .cosakos files

Cracking the crypto used by this ransom Trojan is more of a science fiction thing rather than an attainable prospect for the masses. This is why the troubleshooting in predicaments of this sort is a matter of two approaches: one is to pay the ransom, which isn’t an option for many victims; and the other is to apply instruments that take advantage of the ransomware’s possible weaknesses. If the latter is your pick, the advice below is a must-try.

Backups can make your day

Backups can make your day

Not only are you a lucky person in case you’ve been backing up your most important files, but you’re also a wise and prudent user. This isn’t necessarily a resource-heavy activity these days – in fact, some providers of online services are allocating a sufficient size of cloud storage space for free so that every customer can easily upload their critical data without paying a penny. Having removed the Cosakos ransomware, therefore, all you have to do is download your stuff from the remote server or transfer it all from an external piece of hardware if that’s the case.

Restore previous versions of encrypted files

Restore previous versions of encrypted files

A positive upshot of using this technique depends on whether or not the ransomware has erased the Volume Shadow Copies of the files on your PC. This is a Windows feature that automatically makes and keeps the backups of data elements on the hard drive as long as System Restore is enabled. The cryptoware in question is programmed to switch off the Volume Shadow Copy Service (VSS), but it has reportedly failed to in some cases.

Checking one’s options regarding this workaround is doable in two ways: through the Properties menu of each file or by means of the remarkable open-source tool called Shadow Explorer. We recommend the software-based way because it’s automated, hence faster and easier. Just install the app and use its intuitive controls to get previous versions of the encrypted objects reinstated.

Shadow Explorer

Alternatively, you can leverage the Previous Versions feature, which is native to Windows operating system. This method is more cumbersome that the use of ShadowExplorer, but it can help restore the most important individual files on condition that the ransomware failed to disable the Volume Snapshot Service on the computer. Right-click on a file of choice and select Properties. Then, go to the Previous Versions tab as illustrated below.

Previous Versions

Go ahead and pick the file’s latest backup version on the list. Use the Copy or Restore buttons to reinstate this object to a new path or to its original folder, respectively.

Ransomware Prevention Tips

To avoid Cosakos ransomware and other file-encrypting infections in the future, follow several simple recommendations:

  • Toggle your email provider’s anti-spam settings to filter out all the potentially harmful incoming messages. Raising the bar beyond the default protection is an important countermeasure for ransom Trojans
  • Define specific file extension restrictions in your email system. Make sure that attachments with the following extensions are blacklisted: .js, .vbs, .docm, .hta, .exe, .cmd, .scr, and .bat. Also, treat ZIP archives in received messages with extreme caution
  • Rename the vssadmin.exe process so that ransomware is unable to obliterate all Shadow Volume Copies of your files in one shot
  • Keep your Firewall active at all times. It can prevent crypto ransomware from communicating with its C&C server. This way, the threat won’t be able to obtain cryptographic keys and lock your files
  • Back up your files regularly, at least the most important ones. This recommendation is self-explanatory. A ransomware attack isn’t an issue as long as you keep unaffected copies of your data in a safe place
  • Use an effective antimalware suite. There are security tools that identify ransomware-specific behavior and block the infection before it can do any harm.

These techniques are certainly not a cure-all, but they will add an extra layer of ransomware protection to your security setup.

Revise your security status

Post-factum assessment of the accuracy component in malware removal scenarios is a great habit that prevents the comeback of harmful code or replication of its unattended fractions. Make sure you are good to go by running an additional safety checkup. Another benefit of using the antimalware tool is that it will keep ransomware threats from intruding on your computer further on.

Download Cosakos ransomware removal tool

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