CruelCrypt ransomware: what it is and how to remove it
CruelCrypt ransomware – the threat that creates frustration for users because it manages to encode various files on the machine, so there is a reason for money demands. Algorithms used for the encryption process allow to change the original code of the file, so the document, image, picture, or video, the audio file becomes useless and unopenable.
Facts checked October 7, 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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CruelCrypt ransomware: summary
| Distribution | The malware uses tool and injects the malicious code directly on the computer, bypasses the protection. |
|---|---|
| Name | CruelCrypt ransomware |
| Type | Cryptovirus[2] |
| Family | CXK-NMSL ransomware |
| Ransom note | readme.txt; the text of the note is not in our records |
| Extensions used to mark encoded files | .cruel and .sister appears at the end of the original file name and goes after the full name of the document or image and after the file-type extension |
| 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 12 more facts
| Danger | The blackmailing-based threat shouldn't be taken lightly because of the possible results that may lead to permanent damage to your files or loss of money. Paying is not the option, but your files, system, programs can get damaged significantly when you do not remove this threat in time |
|---|---|
| Detection names | No Microsoft detection name is known |
| Contact | Not recorded in our earlier report |
| Encrypted file extension | .sister |
| 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 | 5 write-ups by security sites; no sample analysed yet |
| Encrypted files | .sister |
| Free decryptor | No free decryptor is known (checked 7 October 2026) |
| First seen | 22 June 2020 |
| Facts checked | 7 October 2026 |
- File extension:
.sister - Note file:
readme.txt
From our report of Jun 2020 · not reviewed since
More from our earlier report on CruelCrypt ransomware
- Based on the family and tendencies of the ransomware, the primary message from virus creators should appear on the desktop and in other folders with encrypted files.
- These text files get named readme.txt or with the particular identification of the virus family/file appendix
- CruelCrypt ransomware removal is the process that shouldn't be manual only since there are various files and programs that cryptovirus can install to trigger unwanted changes and alterations
- There are various ways how this malware can alter your system.
- Adding files, programs, and other malware can significantly affect your machine.
- You need to not only terminate the virus, but also clear the virus damage, affected files, and corrupted software.
- Rely on for the job
How CruelCrypt ransomware behaves
From our report of Jun 2020 · not reviewed since
CruelCrypt cryptovirus is the simpler version of the Chinese ransomware that now uses .sister and .cruel file markers for encoded data
CruelCrypt ransomware – the threat that creates frustration for users because it manages to encode various files on the machine, so there is a reason for money demands.
Algorithms used for the encryption process allow to change the original code of the file, so the document, image, picture, or video, the audio file becomes useless and unopenable. Once the encryption is done, files receive the .cruel or .sister extensions and can be identified easily.
However, when you cannot open the file, you cannot know what value it has for you, so there are more thoughts about paying the ransom. But this is not the option because criminals behind the threat focus on getting money from you, no matter how.
CruelCrypt ransomware virus is the version of the CXK-NMSL ransomware that was a threat popular last year. This 2020 ransomware seems to be a more basic version, that is not using the Chinese language and has fewer features that are unique.
This threat may not be popular o dangerous, but there are many users who have already suffered this attack, so researchers reported the new malware and performed a full analysis.
CruelCrypt ransomware virus uses the WriteProcessMemory function, so the CertiUtil command-line tool allows to download the malicious code into the memory and bypass windows protection.
There can be some different options or variants of this threat, so the sample of the payload can be different, depending on the time it got released. The sample of the ransomware also determines the detection rate when you need to get rid of the virus.
Even though the primary version of the initial version in the same virus family focused on Chinese-speaking users, CruelCrypt ransomware is targeting people with English-language based content. The sample that got analyzed when the first wave of the infection got spotted revealed that the code was written in April 2020.
Files found on the system that got encrypted included the date stamp of July 30, 2019. this threat can easily spread around and affect the various systems, devices.
When the malicious processes running in the background, besides the encryption, CruelCrypt ransomware can control what functions get to be disabled or newly added on the machine. This is how the threat ensures persistence and makes the machine affected permanently.
CruelCrypt ransomware gets removal significantly more difficult because threat adds some files and disables security functions, blocks the AV tools that already run on the machine, and can lessen the data recovery options for you. Any executables, DLL files, registry entries can trigger many processes that sure of malicious activities.
Experts note that it is important to ensure that those affected parts of the system and files that get corrupted are fully recovered, repaired before you restore encrypted data causing your backups or third-party software. You need to remove CruelCrypt ransomware properly with AV tool or anti-malware/security app, and then clear the damage using .
Even though this CruelCrypt virus uses different .cruel and .sister file extensions, these markers get split though all the files and all the photos, documents are unreadable once the file is encoded.
It is not determined by the particular file extension. removal to repair processes is not affected by the file extension. So react tot he infection and remove the threat as soon as you can, no matter with file marker dominates on the machine.
You need to note that none of those tools that focus on. Cruel virus elimination can restore files affected by encryption. Data cannot get recovered by removing the ransomware itself. You need a proper decryption program that is designed to restore CruelCrypt ransomware affected files, encoded files.
But file recovery is a separate procedure, that can be achieved with proper tools. We have a few options for you below, but we should note that data backups are the best.
CruelCrypt ransomware is not decryptable, and there is no official decryption program that could repair any files. You may store some malware- related files and use them later on with the decryption tool. However, the initial BAT/ CXK_NMSL ransomware is still not decryptable yet. It is more likely that the CruelCrypt virus will not get decrypted any time soon.
From our report of Jun 2020 · not reviewed since
The threat can use malicious code droppers or spread the virus using malicious MS documents
This is the threat that is known for using a particular tool that can be used via hacked or malicious sites and trigger the injection of malicious code directly in the memory. This is one of the more stealthy virus distribution ways, but ransomware developers can easily use any of the available methods that various malicious actors rely on.
The WriteProcessMemory function writes data to the virtual memory of another process. This operation can get launched from a malicious macro-triggered process. When you receive emails with file attachments, get messages about supposed financial information, and open the attached file on the system, without a doubt, you can get the infection directly on the system.
These MS Office documents, OpenOffice, PDF, text files, databases, photos, music, video, image files, archives can contain scripts, direct malware payloads, and affect your machine significantly from the minute infiltration happens. Pay attention to senders, company names, file attachments, and be suspicious about everything, to keep the machine secure.
The CruelCrypt ransomware note
The note is called readme.txt.
We did not record the full text of the ransom note in our report.
How to remove CruelCrypt ransomware
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 CruelCrypt ransomware 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
CruelCrypt ransomware encrypts everything it can reach, including drives and shares you connect later. Cut the network first: cable out, or Wi-Fi off from the taskbar.
Then unplug every USB stick and backup disk and pause cloud sync, so the encrypted versions do not replace your online copies. Do not reconnect any of them until the ransomware is removed from the Windows 11 or Windows 10 PC.
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
Copy
readme.txtand one or two files ending in.sisterto a USB stick, and leave the originals where they are.From another device, upload the note and a sample file to ID Ransomware or No More Ransom's Crypto Sheriff, which name the family from the note, the extension and the file structure.
Write down the contact address and your personal ID from the note, because a decryptor or the police may ask for them.
Warning: Never contact the attackers from the infected Windows 11 or Windows 10 PC.
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
For CruelCrypt ransomware, no free decryptor is known (checked 7 October 2026). Search the family name in the No More Ransom decryption tools list and in the free decryptors of Emsisoft, Avast and Kaspersky, because new tools appear years after an attack.
Important: Keep the encrypted files even if nothing works today, and do not pay before every free option is ruled out: payment does not guarantee a working key.
Decryptors run on Windows 11 and Windows 10, but only after the ransomware itself is removed.
Full procedure with screenshots: Ransomware: first steps, finding a decryptor and recovering files
Step 4: Remove the ransomware before you restore or decrypt
Restoring or decrypting files while CruelCrypt ransomware still runs lets it encrypt them again. Run a Full scan in Windows Security > Virus & threat protection > Scan options, then the Microsoft Defender Antivirus (offline scan).
If the ransomware blocks Windows Security, start Safe Mode with Networking and scan from there. Some families install a password stealer as well, so let both scans finish on Windows 11 or 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
Open Command Prompt as administrator and run
vssadmin list shadows. If it lists copies dated before the attack, right-click a folder with encrypted files in File Explorer, choose Properties > Previous Versions, and open or restore a version from before that date.ShadowExplorer, a free tool, shows the same copies when the tab is empty or hidden. Most current families delete shadow copies, so an empty list is normal, but the check takes two minutes on Windows 11 or Windows 10.
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
Restore from an external backup, File History or OneDrive's version history, choosing a date before the attack. Connect the backup drive only after the scans are clean, or the ransomware encrypts it too.
If there is no backup, file recovery software can sometimes find the deleted originals, because some ransomware writes an encrypted copy and deletes the original file.
Stop writing to the drive and try recovery on Windows 11 or Windows 10 before new files overwrite the space.
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 Jun 2020 · not reviewed since
Remove the Cruel Crypt ransomware virus by running a proper anti-malware tool
You cannot keep this CruelCrypt ransomware virus on the system for long because this is the threat that can affect the machine significantly when it accesses system folders, files associated with some programs or parts of the software. Once crucial files get affected, you cannot repair those data recovery or security functions easily.
You should get the proper anti-malware tool and run the full system scan to remove CruelCrypt ransomware as soon as you get the ransom-demanding message on the screen. Even if you do not get the file from crypto-extortionists when the data gets encoded and marked, you need to react ASAP, so system damage is at the minimum.
CruelCrypt ransomware removal process shouldn't be the only procedure launched on the machine. Tools like or can possibly find and terminate the threat, malicious programs, or the ransomware fully. However, the procedure of system file and function remover is also crucial since the threat works in the background for a long period. and other repair utilities can find and fix files damaged by the threat.
[GI=method-1]Reboot the machine in Safe Mode with Networking and remove CruelCrypt ransomware using anti-malware tools
[GI=method-2]Use System Restore feature and recover the system in a previous state
Rely on this program and make sure to repair data affected by CruelCrypt ransomware virus
Windows Previous Versions option for file recovery
When System Restore is enabled at first, you can rely on Windows Previous Versions and use this option as the file recovery method
ShadowExplorer is the system feature that can find and repair the damage CruelCrypt ransomware made
Dceryption for the CruelCrypt ransomware virus is not possible
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.
Protect the files you restore
Restore only after the scans are clean and the PC no longer shows files that end in .sister and no longer open; otherwise the restored copies can be damaged again.
Then build a backup that survives the next infection:
- one copy in the cloud with version history
- one on an external drive that is disconnected between backups
- the working copy on the PC
Version history matters because a backup that syncs damaged files overwrites the good ones.
How to do this with the tools built into Windows: the 3-2-1 backup rule on 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 CruelCrypt ransomware
How do I open .sister files?
You cannot open them by renaming or by choosing another program. The .sister ending shows that CruelCrypt ransomware 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 CruelCrypt ransomware. Store the encrypted files on an external disk until then.
How did CruelCrypt ransomware get on my computer?
The way CruelCrypt ransomware 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 .sister 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.
Is there a free CruelCrypt ransomware decryptor?
We last checked on 7 October 2026, and for CruelCrypt ransomware 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 "CruelCrypt ransomware 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.
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.
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.
Did CruelCrypt ransomware steal my files or passwords?
We do not know yet. Nothing published so far shows data theft by CruelCrypt ransomware, but the only confirmed sign is files that end in .sister 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 CruelCrypt ransomware 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 .sister from the file names?
You can try it on a copy, but for real ransomware it will not help. The .sister 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.
Can I delete readme.txt?
Yes, the note itself is harmless text and deleting it does not affect your files. Keep at least one copy first, outside the infected PC. readme.txt contains your personal ID and the attackers' contact details, which identification services use to tell which family encrypted your files, and which a decryptor may need later.
Police reports also ask for it. Once you have saved a copy, you can remove the notes from every folder after the ransomware program has been removed and your files are restored or backed up.
Will Fortect remove CruelCrypt ransomware?
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 CruelCrypt ransomware, 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
- Twitter: New article CruelCrypt Ransomware (read October 7, 2026)
- Wikipedia: Cryptovirology (read October 7, 2026)
- Virustotal: Malware sample detection rate (read October 7, 2026)
- Dieviren: DieViren (read October 7, 2026)
- CISA: StopRansomware (read October 7, 2026)