Kasiski ransomware: what it is and how to remove it
Kasiski ransomware is the virus that gets photos, documents, archives, video files, and other commonly used data encrypted to have the opportunity for a ransom demand. This ransomware spreads around the world and frustrates users with the money demanding message that appears on the desktop as a wallpaper and a text file.
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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Kasiski ransomware: summary
| Distribution | Spam email attachments with malicious macros get delivered to hundreds of users, so when the file is opened, and macros enabled, ransomware can drop straight on the system and start the encryption |
|---|---|
| Name | Kasiski ransomware |
| File marker | [KASISKI] prefix comes at the start of every encoded file and shows which data got locked by the threat |
| Targets | Hispanic users all over the world |
| Ransom note | INSTRUCCIONES; the text is quoted in full below |
| Ransom amount | The text file with instructions also offers to decrypt files with a decryptor for $500 |
| 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
| Distinct feature | The virus is developed for 64-bit systems, so it is not working on 32-bit devices |
|---|---|
| Detection names | No Microsoft detection name is known |
| Contact | Not recorded in our earlier report |
| Encrypted file extension | Not recorded in our earlier report |
| Decryptor | No free decryptor is known for this variant; check No More Ransom (nomoreransom.org) for updates |
| Damage | Not recorded in the old report |
| Type | File-encrypting ransomware |
| Symptoms | An unknown program in Installed apps and a ransom note left in folders |
| Evidence | 7 write-ups by security sites; no sample analysed yet |
| Free decryptor | No free decryptor is known (checked 7 October 2026) |
| First seen | 5 March 2020 |
| Facts checked | 7 October 2026 |
- Note file:
the ransom note
From our report of Mar 2020 · not reviewed since
More from our earlier report on Kasiski ransomware
- INSTRUCCIONES.txt - a file that delivers the message from criminals and displays all the needed information for the victim about encrypted files and possible recovery options.
- Kasiski ransomware removal should be performed with proper anti-malware tools, so all the malicious files get removed and virus terminated
- A full system scan with tools like can significantly improve the state of security because such utilities can repair system files and corrupted registries or functions
How Kasiski ransomware behaves
From our report of Mar 2020 · not reviewed since
Kasiski ransomware is the cryptovirus that uses unusual prefix for renaming encrypted files
Kasiski ransomware is the virus that gets photos, documents, archives, video files, and other commonly used data encrypted to have the opportunity for a ransom demand.
This ransomware spreads around the world and frustrates users with the money demanding message that appears on the desktop as a wallpaper and a text file. Criminals claim to offer the decryption tool, but that is all for faking legitimacy and trust. Cryptovirus creators only care for your money, so don't even consider paying them.
Kasiski virus is created by criminals who speak Spanish and want $500 from each victim. The threat is targetting Hispanic users, so it can be distributed around the world easily. Also, it is possible that some versions of the particular threat can aim to English-speaking users and lock files, demand ransoms.
Kasiski virus appears to be a new type of ransomware , which, surprisingly, is created for an x64 version of Windows. It seems that the threat cannot even work on 32-bit devices.
The name of this ransomware is derived from the appendix that the original names of corrupted files get. The virus renames files during the encryption process. Unlike other ransomware, it doesn't add extensions but appends prefix to the file names.
The virus can be installed by the user himself, without knowing what kind of file/program it is. As we have already mentioned, Kasiski malware encrypts almost all files with a strong cipher and then changes desktop background with a basic yellow picture with some text written on it.
Then the virus drops a file called INSTRUCCIONES.txt (on the desktop) which is known to be a ransom note - a message from malicious actors. Cybercriminals typically create such files on the system after corrupting victim's data. They aim to inform the victim of how the corrupted data can be recovered, and usually, they demand money immediately.
In this ransom note, authors of Kasiski ransomware ask the victim to pay a ransom in Bitcoins. They promise to provide a "DECRYPT TOOL" for $500. Here's how the beginning of the ransom note looks:
However, we never advise victims to pay the ransom because obviously, cybercriminals are not the people you can count on.
Unfortunately, for affected system files, you should install a program designed for system repair such as that can help with Kasiski virus damage. Then find a backup, which is needed for data recovery. If you do not have a needed backup , you can try data recovery methods provided below the article.
However, the virus is under analysis at the moment, researchers might come up with Kasiski decryption tool after a while. Until then, take care of the machine, clean virus traces, try to secure your PC from possible additional malware attacks, and try to restore all the encoded data with files stored on external devices.
Make sure to double-check before adding any data on the machine, because you may lose your files if any leftover ransomware files trigger the secondary encryption round.
From our report of Mar 2020 · not reviewed since
Kasiski ransomware virus termination procedures include AV tools
If your computer was compromised, remove Kasiski ransomware immediately using proper anti-malware tools.
Ransomware-type programs often come with various additional features such as an ability to install new malware, transmit personal information, and even more.
To make your PC safe again, it is recommended to use powerful anti-malware software such as or . Full Kasiski ransomware removal guidelines are given down below - follow them carefully, do not rush! Using those methods may help you with the proper AV scan too.
You do not want to miss infectious components because if you do so, they might weaken your PC's security, which would allow other viruses to infiltrate the system easier. When you are done with the proper elimination of all Kasiski virus components, you can go for the data recovery solutions that are also listed below. Make sure to repair system files using before though.
[GI=method-1]Reboot the system in the Safe Mode with Networking and run the AV tool then
[GI=method-2]Make the system virus free by enabling System Restore feature
Windows Previous Versions is the feature that can help with affected files
When System Restore gets enabled, you can rely on Windows Previous Versions feature and recover data individualy
ShadowExplorer restores files affected by Kasiski ransomware
The option can work when Shadow Volume Copies are left untouched
Kasiski ransomware has no decryption tool
From our report of Mar 2020 · not reviewed since
Traditional ransomware distribution techniques
How ransomware crooks distribute their malicious programs depends on their programming skills.
More advanced ones employ hacking skills to spread malicious files via ad networks, others use exploit kits , and finally, some scammers use Trojans to infect devices with ransomware.
However, the most efficient technique that is the most used by such criminals so far is also a very basic one. Malware authors only need to compose convincing email letters and send them out to hundreds or even thousands of victims. The message typically says that the user needs to view "attached contents." In most cases, attachments state about some kind of financial data like order information or invoices.
However, we advise users to think whether a) they know the sender personally or not; b) were they expecting to receive such an email or not. If the answer to these questions is no, better stay away from such letters and delete them ASAP.
Always do so with emails you were not expecting to get or notifications that contain red flags like typos, grammar mistakes, or shortened hyperlinks. Those can also send you to macro-laced sites or trigger direct downloads of malicious files, malware.
The Kasiski ransomware note
Informacion importante
Este es su numero personal (NO LO BORRE) =
Todos sus archivos fueron encryptados (bloqueados)/
Para restaurar sus archivos usted necesita un (DECRYPT TOOL)
Nosotros le ofrecemos el (DECRYPT TOOL) para restaurar sus archivos, su costo es de ($500) quinie
How to remove Kasiski 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 Kasiski 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
Kasiski 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 identify the family
Find the ransom note, usually a
.txt,.htmlor .hta file in every folder or on the desktop, and copy it with one or two encrypted files to a USB stick.From another device, upload them to ID Ransomware or No More Ransom's Crypto Sheriff, which name the family from the note and the file extension. Write down the contact address and your personal ID from the note for the report.
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 Kasiski 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 Kasiski 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.
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
Kasiski ransomware could only hurt the files that had no second copy, and the signs (kasiski ransomware in the list of installed apps) 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.
Stream videos without limitations, no matter where you are
There are multiple parties that could find out almost anything about you by checking your online activity.
While this is highly unlikely, advertisers and tech companies are constantly tracking you online. The first step to privacy should be a secure browser that focuses on tracker reduction to a minimum.
Even if you employ a secure browser, you will not be able to access websites that are restricted due to local government laws or other reasons. In other words, you may not be able to stream Disney+ or US-based Netflix in some countries. To bypass these restrictions, you can employ a powerful VPN, which provides dedicated servers for torrenting and streaming, not slowing you down in the process.
Data backups are important - recover your lost files
Ransomware is one of the biggest threats to personal data.
Once it is executed on a machine, it launches a sophisticated encryption algorithm that locks all your files, although it does not destroy them. The most common misconception is that anti-malware software can return files to their previous states. This is not true, however, and data remains locked after the malicious payload is deleted.
While regular data backups are the only secure method to recover your files after a ransomware attack, tools such as can also be effective and restore at least some of your lost data.
Questions about Kasiski ransomware
When will a decryptor for Kasiski ransomware be released?
Nobody can promise a date. When we last checked, on 7 October 2026, no free decryptor is known. Decryptors usually appear after one of three events:
- police seize the attackers' servers and keys
- the authors leak or publish the master keys
- researchers find a mistake in how the ransomware generates keys
Some families got a tool within weeks, others never. Keep the encrypted files and the note on an external drive, check No More Ransom every few months, and follow news about the Kasiski ransomware family. A decryptor may need an original file, so keep any unencrypted copies too.
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.
Did Kasiski ransomware steal my files or passwords?
We do not know yet. Nothing published so far shows data theft by Kasiski ransomware, but the only confirmed sign is kasiski ransomware in the list of installed apps, 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.
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 Kasiski ransomware get on my computer?
The way Kasiski 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 kasiski ransomware in the list of installed apps, and check the Downloads folder and Installed apps sorted by date. Keep anything suspicious for your report, but do not run it again.
Is Kasiski 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 delete the ransom note?
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. the ransom note 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.
How do I open my .docx, .jpg and other files locked by Kasiski?
You cannot open locked files normally while they stay encrypted, and renaming them will not help. Our earlier report did not record the extension that Kasiski adds, so check one locked file name and the ransom note for it.
No free decryptor is known for this variant; check No More Ransom (nomoreransom.org) for updates. Do not rename the files, change the extension or edit them, because that can make recovery harder. Copy the locked files to an external drive before you clean the system, so a decryptor released later still has them.
Will Fortect remove Kasiski 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 Kasiski 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
- Sophos Knowledge Base: Ransomware: Information and prevention (no longer online) (read October 7, 2026)
- NUI Gateway: Backup Data (read October 7, 2026)
- F-Secure: Exploit Kits (read October 7, 2026)
- Wikipedia: Macro virus (read October 7, 2026)
- CISA: StopRansomware (read October 7, 2026)