Kook virus: what it is and how to remove it
Kook ransomware - a version of the notorious cryptovirus that is known as Djvu. Since this is one of many versions in the prominent malware family, there are many features and functions that haven't changed much for the past year.
Facts checked October 6, 2026. Removal steps tested on Windows 11 (26H2) and checked against Microsoft's and the browser makers' current documentation. Sections marked as our earlier report are the original text: they describe the threat as it was then and have not been reviewed since. The 2026 status, the removal steps and the questions are current.
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Kook virus: summary
| Distribution | The threat uses methods involving malicious files. The virus can be spread via email attachments with malicious macros or from torrent platforms, pirating sites when malicious scripts get injected on software package files |
|---|---|
| Name | Kook virus |
| Type | Ransomware[1] |
| Family | STOP virus/ Djvu ransomware |
| File extension | .kook – the file appendix that comes after a filetype extension and indicates encrypted files |
| Amount demanded from victims | $980 or $490, when the discount is offered |
| Removal | Scan the PC with security software to find and remove the malware and anything installed with it. Fortect scans Windows for malware and repairs the system files and settings it damaged. Remove it nowTo use the full-featured product, you have to purchase a license for Fortect. The scan is free. |
Show 13 more facts
| Ransom note | _readme.txt – a file that contains a direct message from criminals |
|---|---|
| Contact emails | helpmanager@mail.ch and restoremanager@airmail.cc |
| Detection names | No Microsoft detection name is known |
| Encrypted file extension | .kook |
| Decryptor | No free decryptor is known for this variant; check No More Ransom (nomoreransom.org) for updates |
| Damage | Not recorded in the old report |
| Symptoms | Files renamed with a new extension and a ransom note left in folders |
| Evidence | 4 write-ups by security sites; no sample analysed yet |
| Encrypted files | .kook |
| Attacker contacts | helpmanager@mail.ch |
| Free decryptor | No free decryptor is known (checked 6 October 2026) |
| First seen | 4 August 2020 |
| Facts checked | 6 October 2026 |
- File extension:
.kook - Note file:
_readme.txt - Contact:
helpmanager@mail.ch
From our report of Aug 2020 · not reviewed since
Kook ransomware is a malicious program designed to lock your files and keep them hostage until a ransom is paid
Kook ransomware - a version of the notorious cryptovirus that is known as Djvu.
Since this is one of many versions in the prominent malware family, there are many features and functions that haven't changed much for the past year. Since August 2019, cybercrooks changed the encryption algorithm from AES to RSA, which made established decryption tools obsolete.
This particular version of ransomware is appending files with .kook extension, so it can be distinguished from other variants, which also use the same ransom note - the text file named _readme.txt.
Unfortunately, the Kook ransomware virus is no different from other variants that came out in 2020, so there is little to no possibility to get your files recovered when the encryption algorithm is used to change the original code.
The ransom note states about payment options and encourages people to contact criminals via helpmanager@mail.ch and restoremanager@airmail.cc emails. However, when you try to get more information about the payment, you may get tricked instead, and the sum of $490 or $980 might be lost forever.
In some cases, there is a tool that helps - Emsisoft Djvu decrypter. There is an issue of online vs offline IDs, so only some of the encoded files get decrypted this way. You can check your encrypted files with this tool and see if you deal with the older or newer variant of the STOP/Djvu virus.
This fact determines the offline and online ID issue. You might have the option to decrypt files marked by .kook ransomware. However, the best option is to remove the threat completely and recover from your separate data backups.
Kook ransomware can trigger changes in the system, so your device is not working as it supposed to. In most cases, cryptovirus affect data recovery options, file restoring features, security software, and other programs that could help with virus removal or file restoring functionalities.
Since the threat focuses on keeping malicious activities and files on the system, Kook ransomware triggers these changes immediately after the encryption. The behavior of the stealthy threat is not easily noticed because these changes happen in the background.
The victim of the .Kook files virus can notice the infection when files get marked using the .kook extension, and the ransom note is delivered on the screen, placed on the desktop, in other folders. The message in _readme.txt states:
This message should be ignored because there is no need to contact the criminals behind the Kook ransomware. Those people are not concerned about victims' files. The only purpose of this file virus is to get cryptocurrency from people directly by scaring them.
Victims cannot know what to do when this text is displayed, and all the files get locked, so there are the ones who decide to pay. Unfortunately, it is not recommended by experts.
Kook ransomware displays only a part of the malicious activities on the screen, so many changes can happen in the background. This is why you need to react as soon as possible. Also, when the time that ransomware creators give ends your files may get damaged even further, so removing the virus as soon as you can, may save your data.
Make sure to remove Kook ransomware properly from the system before you attempt any file restoring methods. Especially, when you rely on data backups from external devices that need to be plugged into the computer. You may lose all your data when the secondary encryption is launched.
From our report of Aug 2020 · not reviewed since
More from our earlier report on Kook virus
- To properly remove Kook ransomware from the system, you need a trustworthy anti-malware tool that
- The system gets affected while alterations in system functions get made.
- Make sure to repair them or at least find affected parts with
How Kook virus behaves
From our report of Aug 2020 · not reviewed since
Kook files virus is equipped with multiple features
The primary goal of ransomware, such as the Kook file virus, is to access your device, scan for susceptible files, lock them, and then demand ransom for their redemption.
However, Djvu variants are a bit more sophisticated than that, as malicious actors seek to gain maximum benefits from each of the infections.
If you had no backups, Kook file recovery might not be possible - this fact is devastating by itself. However, it is important to note that malware also equipped with additional modules that could cause even more damage than permanent data loss. Here are a few examples and reasons why you should hurry to remove Kook ransomware from your system:
- Djvu ransomware variants are known to modify Windows "hosts" file in order to prevent victims from seeking help on security-focused websites, including 2-spyware.com. As a result, you might not be able to access these sites when seeking help. To revert this process, you should visit the following location and delete the "hosts" file: C:\Windows\System32\drivers\etc\
- If you keep Kook ransomware running in the background, it might begin stealing information via your web browser. As a result, your banking details, various account information and other data can be stolen and sold for profits on the dark web; This malware family is known to be collaborating with other strains, and people infected with Djvu were also found banking Trojan AZORult on their systems. In other words, it is possible that malicious actors might install other malware on your machine.
The Kook virus ransom note
ATTENTION!
Don't worry, you can return all your files!
All your files like photos, 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.
You can get and look video overview decrypt tool:
https://we.tl/t-gSEEREZ5tS
Price of private key and decrypt software is $980.
Discount 50% available if you contact us first 72 hours, that's price for you is $490.
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:
helpmanager@mail.ch
Reserve e-mail address to contact us:
restoremanager@firemail.cc
Your personal ID:
How to remove Kook virus
Tools you'll need
All of these are free except where noted. Download them on a clean device if the infected PC is offline.
- A USB stick: to keep the ransom note, two or three encrypted files and screenshots off the infected PC.
- Microsoft Defender Offline: built into Windows 11 and Windows 10; scans before Windows starts, so running malware cannot hide.
- Microsoft Safety Scanner: a second, portable scanner with current signatures; each download works for 10 days.
- ID Ransomware: identifies the family from the note and one encrypted file and says whether a decryptor exists.
- No More Ransom: the free decryptors from police and security companies; check it again every few months.
- Fortect (optional): scans Windows for malware and repairs the system files and settings it damaged. The free scan is in the box above.
How to remove Kook virus and get your files back
Work in this order.
Disconnecting comes first, removal comes before any restore, and nothing here asks you to contact the attackers.
Step 1: Disconnect the PC and unplug backup drives
Unplug the network cable or turn off Wi-Fi, and disconnect USB drives, external disks and network shares, so Kook virus cannot reach more files. Pause OneDrive, Google Drive or Dropbox sync, because synced folders upload the encrypted copies over the good ones.
Leave the PC on but offline while you read the next steps, since a restart can let the ransomware run again. This applies to Windows 11 and Windows 10 alike.
Windows 11: turn off Wi-Fi to take the PC offline. Full procedure with screenshots: Ransomware: first steps, finding a decryptor and recovering files
Step 2: Save the ransom note and confirm the family
Your files now end in
.kookand the instructions are in_readme.txt. Save both to a USB stick, a copy of the note and two small encrypted files, before anything else.On another device, check them with ID Ransomware or Crypto Sheriff: the family name decides which decryptor, if any, can help. Keep the note's ID and contact line for your report.
Windows 11: the ransom note and encrypted files to copy for identification. Full procedure with screenshots: Ransomware: first steps, finding a decryptor and recovering files
Step 3: Check for a free decryptor
Our last check found that for Kook virus, no free decryptor is known (checked 6 October 2026). Look again yourself in the No More Ransom list and the free decryptor pages of Emsisoft, Avast and Kaspersky, which add new families every year.
A decryptor needs the ransomware gone first, or it encrypts the files again. Keep at least one copy of the encrypted files on an external drive, even if no tool works yet.
Full procedure with screenshots: Ransomware: first steps, finding a decryptor and recovering files
Step 4: Remove the ransomware before you restore or decrypt
Removing Kook virus does not bring the files back, but it has to come first. Start with Defender's Full scan, then the offline scan from the same Scan options page, which checks the disk before Windows loads.
If the scan cannot start, use Safe Mode with Networking. Delete the ransom notes only after you have saved a copy, because removal tools sometimes leave them behind on Windows 11 and Windows 10.
Windows 11: Windows Security > Virus & threat protection > Scan options. Full procedure with screenshots: Run a Microsoft Defender Offline scan
Step 5: Look for shadow copies of the files
Windows keeps shadow copies for restore points and backups, and some ransomware fails to delete them.
vssadmin list shadowsin an administrator Command Prompt tells you at once whether any exist.If they do, right-click the folder that held your files, open Properties > Previous Versions, select a version from before the attack, and click Open to check it before you Restore or copy the files out. Windows 11 and Windows 10 both have the tab.
Windows 11: vssadmin list shadows shows whether shadow copies exist. Full procedure with screenshots: Ransomware: first steps, finding a decryptor and recovering files
Step 6: Restore the files from a backup or recover deleted originals
A backup made before the attack is the surest way back. Connect it only once the PC is clean, then restore from File History, Windows Backup, OneDrive's Restore your OneDrive or your own external copies.
Without a backup, try file recovery: the originals that Kook virus deleted may still be on the disk until something overwrites them. Install nothing new on the drive you want to recover from on the Windows 11 or Windows 10 PC.
Full procedure with screenshots: Recover deleted files (Recycle Bin, backups, OneDrive) On uGetFix
Instructions for each browser and system
The detailed steps for every browser and system this guide covers. Open the one you use.
Manual removal using Safe Mode
Important! →
Manual removal guide might be too complicated for regular computer users. It requires advanced IT knowledge to be performed correctly (if vital system files are removed or damaged, it might result in full Windows compromise), and it also might take hours to complete. Therefore, we highly advise using the automatic method provided above instead.
Step 1. Access Safe Mode with Networking
Manual malware removal should be best performed in the Safe Mode environment.
Windows 7 / Vista / XP
- Click Start > Shutdown > Restart > OK.
- When your computer becomes active, start pressing F8 button (if that does not work, try F2, F12, Del, etc. - it all depends on your motherboard model) multiple times until you see the Advanced Boot Options window.
- Select Safe Mode with Networking from the list.
Windows 10 / Windows 8
- Right-click on Start button and select Settings.
- Scroll down to pick Update & Security.
- On the left side of the window, pick Recovery.
- Now scroll down to find Advanced Startup section.
- Click Restart now.
- Select Troubleshoot.
- Go to Advanced options.
- Select Startup Settings.
- Press Restart.
- Now press 5 or click 5) Enable Safe Mode with Networking.
Step 2. Shut down suspicious processes
Windows Task Manager is a useful tool that shows all the processes running in the background. If malware is running a process, you need to shut it down:
- Press Ctrl + Shift + Esc on your keyboard to open Windows Task Manager.
- Click on More details.
- Scroll down to Background processes section, and look for anything suspicious.
- Right-click and select Open file location.
- Go back to the process, right-click and pick End Task.
- Delete the contents of the malicious folder.
Step 3. Check program Startup
- Press Ctrl + Shift + Esc on your keyboard to open Windows Task Manager.
- Go to Startup tab.
- Right-click on the suspicious program and pick Disable.
Step 4. Delete virus files
Malware-related files can be found in various places within your computer. Here are instructions that could help you find them:
- Type in Disk Cleanup in Windows search and press Enter.
- Select the drive you want to clean (C: is your main drive by default and is likely to be the one that has malicious files in).
- Scroll through the Files to delete list and select the following:
Temporary Internet Files
Downloads
Recycle Bin
Temporary files - Pick Clean up system files.
- You can also look for other malicious files hidden in the following folders (type these entries in Windows Search and press Enter):
%AppData%
%LocalAppData%
%ProgramData%
%WinDir%
After you are finished, reboot the PC in normal mode.
From our report of Aug 2020 · not reviewed since
The different ways for Kook file recovery
Since Kook ransomware is the variant from a known virus family, it is known that previously developers used offline IDs, and the method allowed many victims to get their files back. Unfortunately, the technique is no longer used by these 2020 variants. Each victim gets a unique ID that is needed for the decryption process. It means that decryption tool development is even harder.
Even though the decryption is not possible, there are some options for the file recovery. Some variants that use offline IDs still can be decrypted, some types of data also have solutions. Nevertheless, to get back to the system that works properly, you need a thorough system cleaning and Kook ransomware removal process that can eliminate the virus. For that purpose, you need anti-malware or security tools.
As for the data that is affected by .Kook file-encrypting virus, you need to rely on trustworthy data recovery options. A few listed below the guide. You can try to restore files from the cloud database or archive stored on the external device.
Remember to repair system files also, so the machine can run as it supposed to. You can rely on for this purpose since the program can show affected files and corrupted functions for you.
From our report of Aug 2020 · not reviewed since
Removing Kook virus cannot unlock your files
Your files will remain locked and encrypted even when you remove the virus using security tools or renewing the operating system entirely.
Kook ransomware virus can alter various system settings, folders, and functions of the computer to remain persistent. The reversed encryption process is the best option, but there is no such decryption tool that could work at the moment.
You need to remove the Kook virus from the system, stop it from running, so it can no longer encrypt your files and then clear all the traces. This is how you completely terminate the threat. If you risk replacing encrypted files with safe copies while the virus is active, you can permanently lose files and money if you decide to pay up. Do not consider these options at all.
From our report of Aug 2020 · not reviewed since
You need to pay attention to avoid the difficult Kook ransomware virus removal
As we mentioned Kook ransomware virus is distributed using various malicious files included on email attachments or packages with licensed software, game cheats, cracked program versions.
These files get installed automatically and trigger the payload drop of the ransomware.
Kook virus removal gets affected by the processes and files planted in the background. Some security functions can get disabled, so you have fewer options for the elimination. However, tools like or are the best ones for such instances.
Unfortunately, these anti-malware or security tools cannot recover files encrypted by the virus or help to repair or remove Kook ransomware damage. Try for the virus damage repair. Then fully restore your device and all the affected files yourself.
[GI=method-1]Rebooting the machine in Safe Mode with Networking can help with the Kook ransomware removal
[GI=method-2]System Restore is the feature that can be used as a virus removal method because it recovers the machine in a previous state
Windows Previous Versions feature is helpful with files encrypted by Kook ransomware
If you used System Restore before, you can try Windows Previous Versions for data recovery
ShadowExplorer - a method for file restoring after Kook ransomware attack
This method works when Shadow Volume Copies are left untouched
Kook ransomware decryption options
You can try the Djvu decryption tool for some of the versions of Kook ransomware virus
Report it and recover your files
Report it
Report the attack even if you do not expect the files back: insurers and banks ask for the report number, and police use the contacts in the note to link cases.
- United States
- FBI IC3 · FTC ReportFraud
- United Kingdom
- Report Fraud (formerly Action Fraud) · NCSC
- Australia
- ReportCyber (ASD)
- EU countries
- Europol: national reporting sites
Give the victim ID, the note and the date the files were encrypted. A business that holds personal data may also have to notify its data protection authority, in the EU within 72 hours.
Make the next attack harmless
Kook virus could only hurt the files that had no second copy, and the signs (files that end in .kook and no longer open) showed exactly which ones those were.
The 3-2-1 rule fixes that:
- three copies
- two media
- one disconnected
A practical version for a home PC: OneDrive or another cloud backup with version history, plus an external disk that you plug in once a week for File History and then unplug. Test a restore now and then by opening a few files from the backup on another device.
Details, schedules and what not to back up are in our 3-2-1 backup guide for Windows.
Choose a proper web browser and improve your safety with a VPN tool
Online spying has got momentum in recent years and people are getting more and more interested in how to protect their privacy online.
One of the basic means to add a layer of security - choose the most private and secure web browser. Although web browsers can't grant full privacy protection and security, some of them are much better at sandboxing, HTTPS upgrading, active content blocking, tracking blocking, phishing protection, and similar privacy-oriented features.
However, if you want true anonymity, we suggest you employ a powerful VPN - it can encrypt all the traffic that comes and goes out of your computer, preventing tracking completely.
Lost your files? Use data recovery software
While some files located on any computer are replaceable or useless, others can be extremely valuable.
Family photos, work documents, school projects - these are types of files that we don't want to lose. Unfortunately, there are many ways how unexpected data loss can occur:
- power cuts
- Blue Screen of Death errors
- hardware failures
- crypto-malware attack
- even accidental deletion
To ensure that all the files remain intact, you should prepare regular data backups. You can choose cloud-based or physical copies you could restore from later in case of a disaster. If your backups were lost as well or you never bothered to prepare any, can be your only hope to retrieve your invaluable files.
Questions about Kook virus
How do I open .kook files?
You cannot open them by renaming or by choosing another program. The .kook ending shows that Kook virus encrypted the content, and only the matching key can reverse it. Renaming a file back to .docx or .jpg changes nothing except the icon, and it can confuse a future decryptor, so leave the names as they are.
To get the content back, use a backup, an unencrypted copy elsewhere, or a decryptor listed on No More Ransom if one exists for Kook virus. Store the encrypted files on an external disk until then.
Should I pay the ransom?
We advise against it, and so do the FBI, Europol and national cyber agencies. Payment does not guarantee a working tool: some attackers never reply, some tools damage files, and some variants have no decryptor at all. Paying also funds further attacks and can make you a target again.
Before considering payment, try every recovery option in this guide and report the attack. Companies must involve their legal adviser and insurer, because payments to sanctioned groups can be illegal. If files are truly irreplaceable, store the encrypted copies and wait; decryptors sometimes appear later.
Is there a free Kook virus decryptor?
We last checked on 6 October 2026, and for Kook virus no free decryptor is known. We check No More Ransom, which collects free tools from police and security companies, and the decryptor pages of the major antivirus vendors. A working decryptor exists only when the encryption has a flaw or the keys were leaked or seized.
Ignore sites and videos that offer a "Kook virus decryptor" for download or for a fee: these are usually scams or malware. Real decryptors are free and come from known security companies or law enforcement. Keep the encrypted files in case a tool appears later.
Can a data-recovery company decrypt my files for a fee?
Only if a decryptor already exists or the company pays the attackers for you. Some "ransomware recovery" services advertise that they can decrypt families with no known flaw; in practice they negotiate with the attackers and add their own fee. Others use the same free tools listed on No More Ransom.
Before you hire anyone, ask in writing how they will recover the files, whether they will contact the attackers and what happens if they fail. A legitimate service answers clearly. If a free decryptor exists for your family, you can run it yourself.
How did Kook virus get on my computer?
The way Kook virus spreads has not been documented yet, so look at your own recent activity. On home PCs, ransomware most often comes with cracked programs, game cheats, key generators and fake updates, or with an e-mail attachment that was opened. On business networks, attackers usually log in through Remote Desktop with a stolen or guessed password.
Think back to what was downloaded or installed in the days before files that end in .kook and no longer open, and check the Downloads folder and Installed apps sorted by date. Keep anything suspicious for your report, but do not run it again.
Did Kook virus steal my files or passwords?
We do not know yet. Nothing published so far shows data theft by Kook virus, but the only confirmed sign is files that end in .kook and no longer open, which says nothing about what happened before. Many current ransomware attacks copy files or run a password stealer first, so it is wise to act as if they did.
From a clean device, change the passwords that were saved in the browsers on this PC, starting with e-mail and banking, sign out of all sessions and turn on two-step verification. Watch bank statements and account activity for the next few weeks.
Is Kook virus the same as other ransomware with a similar name?
Not necessarily. Ransomware names come from the file extension, the note or a word in the code, so unrelated families often end up with similar names, and one family can appear under several names. The difference matters: a decryptor or advice for one family does not fit another and can damage files.
Compare the ending added to your files and the exact name of the note with the summary table at the top of this guide, then upload the note and one encrypted file to ID Ransomware from a clean device. If the result names another family, follow the guide for that family instead.
Can I just remove .kook from the file names?
You can try it on a copy, but for real ransomware it will not help. The .kook extension is only a label; the file content has been scrambled with a key the attackers hold.
If a renamed copy opens normally, the malware only renamed files, which happens with some fake ransomware, and you can rename the rest back. If it does not open, leave the files as they are. Renaming encrypted files can confuse a decryptor later, so always work on copies.
What does _readme.txt tell me about the ransomware?
Quite a lot. The file name _readme.txt, the wording, the contact addresses and the format of the personal ID are typical for each family. Uploading the note together with one encrypted file to ID Ransomware or No More Ransom's Crypto Sheriff usually names the family within seconds.
That name decides your options: some families have free decryptors, some can be partially recovered, and some cannot be decrypted at all. The note's claims about stolen data should be taken seriously but not at face value.
Will Fortect remove Kook virus?
Fortect scans Windows for malware and unwanted programs and repairs the system files and settings they change, and its free scan shows what it finds on your PC before you decide anything.
For Kook virus, follow the plan above as well: the browser steps take back permissions and settings that no scanner treats as a threat, and uninstalling the program that brought it removes the source.
Run Microsoft Defender's full scan and, if anything was found, its offline scan as a second opinion. If the symptoms are gone after the plan and both scans are clean, there is nothing more to do.
Sources
- Wikipedia: Cryptovirology (read October 6, 2026)
- Dieviren: Dieviren (read October 6, 2026)
- Dikstuna: Media-Repair. Small repair tool for MP3 and WAV files affected by STOP/DJVU (read October 6, 2026)
- CISA: StopRansomware (read October 6, 2026)
- No More Ransom (read October 6, 2026)