KOOL Virus (.kool File) Ransomware

The Kool virus, known as a ransomware infection, is part of the STOP/DJVU family. This malware family is famous for malignant file encryption operations. Once the Kool virus gets into a computer, it scans the system for files such as documents, videos, photos, and more. It changes the file structure and appends the “.kool” extension to each file, making them inaccessible and unusable without the decryptor.

Kool Virus Overview

Kool

🤔 Kool virus is ransomware that belongs to the DJVU/STOP family. Its primary purpose is to lock files that are important to you. Then, ransomware virus asks its victims for a ransom payment – ($499 – $999) in Bitcoin.

The Kool virus is a type of malware that encrypts your files and makes you pay for their recovery. This ransomware encrypts a wide selection of file types. ciphered files become identifiable by a distinct “.kool” extension. As a result, the affected files become impossible to access and use.

Then, the ransomware asks for a ransom payment in Bitcoin from its victims, ranging from $499 to $999. Typically, a ransom text file is named as “_readme.txt“.

Kool Ransomware uses Salsa20 encryption algorithms to cipher the files. Since Kool virus uses such a strong encryption method, it becomes incredibly hard, if even possible, to find the decryption key without cooperating with the attackers.

Once the files are encrypted, the Kool virus displays a ransom note to the user, asking for a ransom payment in exchange for the decryption key. The ransom note contains instructions on how to make the payment and also often contains threats of data loss or ransom amounts surge if the demands are not met within a specified timeframe.

I have made a comprehensive list of the solutions, tips, and approaches to stop the Kool ransomware and decrypt your files. In some cases, it may be feasible to restore your files, while sometimes, it may be impossible.

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📌 Important Reminder!

It is essential to voice that paying off the bill does not guarantee the successful files recovery. The scoundrels who stand behind the Kool virus are known for their untrustworthiness. There were the cases where people have paid the ransom, only to be denied by the cybercriminals with the decryption key provision.

Kool employs a unique key for each victim, with one exception:

If Kool fails to establish a connection with its command and control server (C&C Server) before starting the encryption process, it resorts to using offline keys. This key is not unique and is the same for all users, allowing for the decryption of files encrypted by the ransomware.

The Kool virus is similar to other DJVU ransomware variants such as Nood, Wiaw, Wisz, and Lkfr. This virus encrypts a wide range of common file types and appends its distinct “.kool” extension to all files. For instance, a file named “1.jpg” would be altered to “1.jpg.kool” and “2.png” to “2.png.kool“.

Upon successful encryption, the virus creates a special text file named “_readme.txt” and places it in every folder containing the encrypted files. It also adds the readme file to the desktop, so the victim will not miss its appearance even without opening folders.

The image below provides a visual representation of files with the “kool” extension:

Encrypted Files by STOP/DJVU Ransomware

Name	Kool Virus
Ransomware family¹	DJVU/STOP² ransomware
Extension	.kool
Ransomware note	_readme.txt
Ransom	From $499 to $999 (in Bitcoins)
Contact	[email protected], [email protected]
Detection	MSIL/BadJoke.AMU, TrojanDownloader:Win32/Ainslot.DAA!MTB, Trojan:Win32/Trickler!pz
Symptoms	Installs password-stealing Trojan on the system, like Vidar Stealer or RedLine Stealer; Adds a list of domains to HOSTS file to block access to certain security-related sites; Encrypts most of your files (photos, videos, documents) and adds a particular “.kool” extension; Manages to install a SmokeLoader backdoor; Can delete Volume Shadow copies to make victim’s attempts to restore data impossible;
Fix Tool	To remove possible malware infections, scan your PC: 6-day free trial available.

This message asking payment is for restore files via decryption key:

_readme.txt (STOP/DJVU Ransomware) – The terrifying alert insisting users to pay off the ransom to unlock the encoded data contains these frustrating admonitions.

Kool ransomware arrives as a set of actions that are used to perform different tasks on a victim’s computer. One of the initial processes being launched is winupdate.exe, a sneaky process that shows a fraudulent Windows update alert during the attack. Such a trick helps to convince the victim that their PC has become slow because of a Windows update.

Simultaneously, the ransomware runs another process (which name commonly consists of 4 random characters) which starts scanning the computer for target files and encrypting them. Then the ransomware removes Volume Shadow Copies from the disk through the following CMD command:

vssadmin.exe Delete Shadows /All /Quiet

Once erased, it becomes impossible to retrieve the previous computer state using System Restore Points. The problem is, ransomware operators are eliminating any built-in Windows techniques that could help the victim to restore files for free. In addition, the crooks modify the Windows HOSTS file by including a list of domains to it and linking them to the localhost IP. As a result, the victim will witness a DNS_PROBE_FINISHED_NXDOMAIN error when trying to access one of the listed websites.

We noticed that ransomware attempts to block websites that publish various how-to guides for computer users. It is evident that by hindering specific domains, the perpetrators are attempting to prevent the victim from reaching relevant and helpful ransomware-attack-related information online. This malware also keeps two .txt files on the victim’s computer that present information related to the attack – the victim’s public encryption key and personal ID. These two files are named bowsakkdestx.txt and PersonalID.txt.

After all these modifications, the malware doesn’t cease. Variants of STOP/DJVU tend to install Vidar password-stealing Trojan on compromised systems. This threat has an extensive list of capabilities, including:

Stealing Steam, Telegram, Skype login / password;
Stealing cryptocurrency wallets;
Downloading other malware to the computer and running it;
Stealing browser cookies, saved passwords, browsing history, and more;
Viewing and manipulating files on victim’s computer;
Allowing the hackers to perform other tasks on the victim’s computer remotely.

The encryption algorithm in STOP/Djvu ransomware is AES-256. So, if your data got encrypted with an online decryption key, the possibility to get your files back are quite low. Thing is, online key is unique for each victim, and finding a suitable one will take too much time.

Retrieving the online decryption key in another way is also merely impossible. It is stored on a remote server owned by the crooks who promote the Kool infection. For receiving unlocking key the payment should be $999. To see the payment details, the victims should contact the fraudsters by email ([email protected]).

The message by the ransomware states the following information:

ATTENTION!

Don't worry, you can return all your files!
All your files like pictures, databases, documents and other important are encrypted with strongest encryption and unique key.
The only method of recovering files is to purchase decrypt tool and unique key for you.
This software will decrypt all your encrypted files.
What guarantees you have?
You can send one of your encrypted file from your PC and we decrypt it for free.
But we can decrypt only 1 file for free. File must not contain valuable information.
Do not ask assistants from youtube and recovery data sites for help in recovering your data.
They can use your free decryption quota and scam you.
Our contact is emails in this text document only.
You can get and look video overview decrypt tool:
https://we.tl/t-hPAqznkJKD
Price of private key and decrypt software is $999.
Discount 50% available if you contact us first 72 hours, that's price for you is $499.
Please note that you'll never restore your data without payment.
Check your e-mail "Spam" or "Junk" folder if you don't get answer more than 6 hours.


To get this software you need write on our e-mail:
[email protected]

Reserve e-mail address to contact us:
[email protected]

Your personal ID:
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

How To Remove STOP/Djvu Ransomware?

Video Guide

How To Decrypt .kool Files?

How to restore big files?

Try removing .kool extension on a few big files and opening them. This malware has issues with large files encryption. Either the virus accessed and did not encode the file, or it bugged and did not add the filemarker. If you have files that are very large (2GB+), the latter is most likely. Please, text me in the comments whether this helped you or not.

The newest extensions released around the end of August 2019 after the criminals made changes. This includes Nood, Wiaw, Wisz etc.

As a consequence of the alterations made by the criminals, STOPDecrypter is no longer backed. It has been replaced with the Emsisoft Decryptor for STOP Djvu Ransomware developed by Emsisoft.

You can download free decryption tool here: Decryptor for STOP Djvu.

Select folders for decryption.

Based on the default configurations, the decryptor will by default choose the available directories in order to unlock the currently available drives (the connected ones), including the network drives. Additional (optional) locations can be selected with the help of the “Add” button.

Decryptors generally offer several options considering the specific malware family. The existing possible options are shown in the Options tab and can be turned on or disabled there. You may discover a detailed list of the currently active Options below.

Click on the “Decrypt” button.

As soon as you add all the desired locations for decryption into the list, click on the “Decrypt” button in order to initiate the decryption procedure.

Note that the main screen may turn you to a status view, letting you know of the active process and the decryption statistics of your data:

The decryptor will notify you as soon as the decryption procedure is completed. If you need the report for your personal papers, you can save it by choosing the “Save log” button. Note that it is also possible to copy it directly to your clipboard and to paste it into emails or messages here if you need to do so.

The Emsisoft Decryptor might display different messages after a failed attempt to restore your files:

✓ Error: Unable to decrypt file with ID: [your ID]

Emsisoft does not have a corresponding key in the database.

✓ No key for New Variant online ID: [your ID]

Notice: this ID appears to be an online ID, decryption is impossible

Malware used an online key to cipher your files. Your key is unique and is kept on the command server, controlled by criminals. File recovery without paying the ransom is impossible. 🙁

✓ Result: No key for new variant offline ID: [example ID]

This ID appears be an offline ID. Decryption may be possible in the future.

An offline key was used, but files could not be restored (the offline decryption key isn’t available yet). But, receiving this message is good news for you, since it might be possible to restore your files in the future. 🙂

It can take a few weeks or months until the decryption key gets found and uploaded to the decryptor. Please follow updates regarding the decryptable DJVU versions here.

✓ Remote name could not be resolved

It’s an indication of a DNS issue on your PC. Try resetting your HOSTS file to default.

How to Restore .kool Files?

In some case ransomware is not doom for your files…

The Kool ransomware encryption mechanism is next: it encodes every file byte-by-byte, then saves a duplicate, erasing (not overriding!) the initial file. Hence, the data of the file location on the disk drive is lost, but the original file is not deleted from the drive. The cell, or the sector where this file was stored, can still contain this file, but it is not listed by the file system and can be overwritten by data that has been loaded to this drive after the deletion. Hence, it is possible to recover your files using special software.

I recently had my pc infected by the this virus. It managed to bypass 2 Antivirus software and 2 malware fighters.

Anyway, after realizing it was an online algorithm, it is impossible to retrieve my encrypted files. I also had my backup drive plugged in at the time of the virus, and this was also infected, or so I thought. Every folder within my backup drive had been infected and was encrypted. However, despite losing some important files, I retrieved almost 80% of my 2TB storage.

When I started going through the folders, I noticed the readme.txt ransom note in every folder. I opened some of the folders and found that all files that were not in a subfolder within that folder had been encrypted. However, I found a flaw and glimmer of hope when I went into the subfolders in other folders and found that these files had not been encrypted. Every folder within my c and d drives, including subfolders, had been encrypted, but this was not the case with the backup drive. Having subfolders created within a folder has saved 80% of my data.

As I said, I believe this to be only a small loophole on a backup drive. I’ve since found a further 10 % of my data on another hard drive on a different pc. So my advice is if you use a backup drive, create subfolders. I was lucky, I guess. But I was also unlucky that the virus hit as I was transferring some files from my backup.

Hopefully, this can help some other people in my situation.

Jamie Newland

Some pointers for recovery repair of Kool files (true for all STOP/DJVU variants):

I have seen Stop/Djvu variants fail to encrypt deeper nested folders, so that you can check that. You may find those are not encrypted.

This ransomware saves encrypted data to a new file, deletes the original. So there’s a slight chance part of that deleted file can be recovered using file recovery software. It’s unlikely the folder structure can be restored, so a free tool like PhotoRec may be as good as any.

This ransomware only partially encrypts (about the first 150 KB), so depending on file size and type of data, the not encrypted part may be recoverable.

Joep

Recovering your files with PhotoRec

PhotoRec is an open-source program, which is originally created for file retrieval from damaged disks, or for files recovery in case if they are unintentionally deleted. However, with time flow, this program adopted the ability to restore the files of 400 different file types. Hence, it can be used for file restoration after the Kool attack.

First, you need to download this app. It is 100% free, but the developer notifies that there is no warranty that your files will be recovered. PhotoRec is distributed in a pack with other program of the same developer – TestDisk. The downloaded archive will bear TestDisk name, but don’t worry. PhotoRec files are right inside.

To run PhotoRec, find and open “qphotorec_win.exe” file. No installation is required – the program has all the files it need inside of the archive, hence, you can mount it on your USB drive, and try to help your friends/parents/anyone who has been attacked by Kool ransomware.

Upon the start, you will witness the screen showing you the entire list of your disks. However, this details are presumably unhelpful, because the required menu is placed a bit higher. Click this menu, then choose the disk which was attacked by ransomware.

After choosing the disk, you need to select the destination folder for the retrieved files. This menu is located at the lower part of the PhotoRec window. The best choice is to export them on USB drive or any other type of removable disk.

Then, you need to specify the file formats. This setting is located at the bottom, too. As it was mentioned, PhotoRec can restore the files of about 400 different formats.

Finally, you can start files restoration by pressing the “Search” button. You will see the screen where the outcomes of the scan and recovery are shown.

Kool files recovery Video Guide.

Frequently Asked Questions

🤔 How can I open “.kool” files?

No way. These files are encrypted by ransomware. The contents of encrypted files are not available until they are decrypted.

🤔 This virus has blocked infected PC: I can’t get the activation code.

In this situation, you need to prepare the memory stick with a pre-installed

🤔 Decryptor did not decrypt all my files, or not all of them were decrypted. What should I do?

Have patience. You are infected with the new version of STOP/DJVU ransomware, and decryption keys have not yet been released. Follow the news on our website. We will keep you posted on when new keys or new decryption programs appear.

🤔 What can I do right now?

The Kool ransomware encrypts only the first 150KB of files. So MP3 files are rather large, some media players (Winamp for example) may be able to play the files, but – the first 3-5 seconds (the encrypted portion) will be missing. You can try to find a copy of an original file that was encrypted: Files you downloaded from the Internet that were encrypted and you can download again to get the original.

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Brendan Smith