Cactus (.CTS1) ransomware virus - removal and decryption options
RansomwareAlso Known As: Cactus virus
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What kind of malware is Cactus?
Cactus is ransomware that encrypts data, provides a ransom note ("cAcTuS.readme.txt"), and appends the ".CTS1" extension to filenames. For instance, it renames "1.jpg" to "1.jpg.CTS1", "2.png.CTS1", and so forth.
Cactus can use different encryption modes. Depending on the encryption mode used, Cactus may add new extensions, such as ".CTS1.CTS6", after each encryption process.
Screenshot of files encrypted by this ransomware:
Cactus ransom note overview
The ransom note left by Cactus informs the victims that the ransomware has accessed and encrypted their systems. In order to regain access to their files and avoid data exposure, victims are instructed to contact the attackers via email at cactus787835@proton.me. As a backup contact, the ransom note mentions using Tox chat.
More details about Cactus ransomware
What makes Cactus distinct from other ransomware variants is its utilization of encryption to safeguard the ransomware binary. Cybercriminals employ a batch script to acquire the encryptor binary through 7-Zip compression.
The initial ZIP archive is deleted, and the binary is executed with a specific flag to facilitate its operation. This whole process is atypical, and researchers believe it is done to evade detection of the ransomware encryptor.
Similar to typical ransomware attacks, Cactus also engages in data theft from the targeted victims. Cybercriminals employ the Rclone tool to directly transfer the stolen files to cloud storage.
Following the data exfiltration, the attackers utilize a PowerShell script named TotalExec, often linked with BlackBasta ransomware attacks, to automate the encryption process's deployment.
Ransomware in general
Ransomware is malicious software designed to encrypt files on a victim's computer, rendering them inaccessible. The perpetrators responsible for these attacks demand a ransom from the victim and promise to provide a decryption key or tool. However, it is not recommended to trust cybercriminals behind such attacks. They may not provide decryption tools even after making payments.
In most cases, victims have limited options to retrieve their files without resorting to paying a ransom. These options include restoring files from backups if they exist or looking for specialized decryption tools online.
Examples of different ransomware variants are LOCK2023, Kizu, and 2QZ3. The most common differences between ransomware attacks are the encryption algorithms employed, the ransom amounts demanded, the targeted files, and the distribution methods utilized.
How did ransomware infect my computer?
It is known that cybercriminals behind Cactus focus on gaining initial access to the networks of large commercial entities. The attackers leverage known vulnerabilities in the Fortinet VPN clients to infiltrate the victim's network.
Also, users might unknowingly download ransomware by opening malicious email attachments or clicking on malicious links within phishing emails. Visiting compromised or malicious websites can also lead to the automatic download and execution of ransomware without the user's knowledge.
In other cases, cybercriminals use malicious advertisements to redirect users to ransomware-infected sites or employ pirated software, cracking tools, third-party downloaders, P2P networks, and similar channels.
| Name | Cactus virus |
| Threat Type | Ransomware, Crypto Virus, Files locker |
| Encrypted Files Extension | .CTS1 (it may also be .CTS1.CTS6 or similar, depending on the encryption mode used) |
| Ransom Demanding Message | cAcTuS.readme.txt |
| Free Decryptor Available? | No |
| Cyber Criminal Contact | cactus787835@proton.me, Tox chat |
| Detection Names | Avast (Win64:Trojan-gen), Combo Cleaner (Generic.Ransom.Cactus.A.6A6CBCEA), ESET-NOD32 (A Variant Of Win64/Filecoder.Cactus.A), Kaspersky (Trojan-Ransom.Win32.Cactus.d), Microsoft (Ransom:Win32/Cactus.LKV!MTB), Full List Of Detections (VirusTotal) |
| Symptoms | Cannot open files stored on your computer, previously functional files now have a different extension (for example, my.docx.locked). A ransom demand message is displayed on your desktop. Cyber criminals demand payment of a ransom (usually in bitcoins) to unlock your files. |
| Additional Information | Threat actors exfiltrte files to upload them to a data leaking site |
| Distribution methods | Infected email attachments (macros), torrent websites, malicious ads. |
| Damage | All files are encrypted and cannot be opened without paying a ransom. Additional password-stealing trojans and malware infections can be installed together with a ransomware infection. |
| Malware Removal (Windows) |
To eliminate possible malware infections, scan your computer with legitimate antivirus software. Our security researchers recommend using Combo Cleaner. Download Combo CleanerTo use full-featured product, you have to purchase a license for Combo Cleaner. 7 days free trial available. Combo Cleaner is owned and operated by RCS LT, the parent company of PCRisk.com. |
How to protect yourself from ransomware infections?
Ensure timely updates for your operating system, software applications, and security tools to safeguard against potential vulnerabilities. Exercise caution when handling email attachments or clicking on links, particularly if they originate from unfamiliar or suspicious addresses or other sources. Avoid placing trust in advertisements on questionable websites.
Avoid downloading cracked software or applications from dubious websites and similar sources. Use reputable sources (such as official websites and verified stores) for software downloads. Use reliable and up-to-date antivirus software to enhance security measures.
If your computer is already infected with Cactus, we recommend running a scan with Combo Cleaner Antivirus for Windows to automatically eliminate this ransomware.
Screenshot of Cactus's text file ("cAcTuS.readme.txt"):
Text in the ransom note:
Your systems were accessed and encrypted by Cactus.
To recover your files and prevent data disclosure contact us via email: cactus787835@proton.me
Your unique ID reference: -
Backup contact TOX (hxxps://tox.chat/): 7367B422CD7498D5F2AAF33F58F67A332F8520CF0279A5FBB4611E0
Screenshot of Cactus's data leaking page:
Update December 1, 2023 – Cactus ransomware has been observed infiltrating corporate networks by exploiting vulnerabilities in the Qlik Sense data visualization and analysis tool. While back in August, Qlik released updates patching two of the known vulnerabilities on Windows; research suggests that the software is still exploitable at the time of writing. More information on these developments can be found in an article by Bill Toulas on the Bleeping Computer website.
Update March 5, 2025 – Cactus ransomware's arsenal has been expanded with the addition of the BackConnect (BC) malware. Campaigns utilizing this malicious program (and culminating in either Cactus or Black Basta ransomware infections) have been ongoing since the autumn of 2024. Multiple entities across North America and Europe have been targeted.
The known attacks relied heavily on social engineering and shared traits with tech support scams. Victims were baited with an onslaught of emails and then contacted by cyber criminals masquerading as "support"; thus, they were lured into allowing remote access to their networks (established via Microsoft Teams, Quick Assist, etc.). After the initial access was made, the attackers worked on infiltrating BackConnect malware, with the end goal being ransomware execution.
Cactus ransomware removal:
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Removal and file recovery procedure
If your files are encrypted with a new extension and a ransom note has appeared, you've been hit by ransomware. Don't pay. The procedure below works in order: contain the infection, identify the variant, remove the ransomware itself (so anything you restore doesn't get re-encrypted), and try to recover files using free decryption tools, Windows Volume Shadow Copies, or backups. Run a full system scan with Combo Cleaner to remove the infection - then move on to file recovery.
Common ransomware behaviors
VisibleEncrypts your files and renames them
The ransomware reads each file, encrypts the contents with the attacker's public key, and writes the encrypted bytes back to disk. Most variants also rename the file by appending a new extension - .net, .locked, .crypt, .lockbit, .djvu, etc. Without the matching private key, the encrypted files cannot be opened normally. Recovery depends on a free decryption tool, a recoverable Volume Shadow Copy, or a clean backup.
VisibleDrops a ransom note in every encrypted folder
A text or HTML file (usually named HOW_TO_DECRYPT.txt, README.txt, _readme.txt, or similar) is placed in every folder that contains encrypted files. The note explains the situation, lists a payment amount in cryptocurrency, gives a deadline, and provides a contact email or .onion site. Save the note - don't delete it. Its exact wording, the email address, and the bitcoin wallet are how you identify the variant in the next phase.
VisibleReplaces the desktop wallpaper
Many ransomware variants change the Windows wallpaper to a black or red image with the ransom message and instructions. The wallpaper is just a visual scare tactic - it doesn't add any actual capability and reverts to normal once the ransomware is removed. Photograph it before removal in case the contact details are useful for variant identification or for reporting to authorities.
HiddenDeletes Volume Shadow Copies
One of the first things modern ransomware does after launching is run vssadmin delete shadows /all /quiet to wipe Windows' built-in Volume Shadow Copies - the snapshots that Previous Versions and System Restore rely on. Some ransomware doesn't fully succeed (especially on systems where the user lacks full admin rights), leaving partial shadow copies recoverable. Phase 4 step 11 covers how to check what survived.
HiddenDisables Windows Defender and other security tools
Before encrypting, the ransomware tries to silence anything that could alert you or block it - Windows Defender real-time protection, third-party antivirus services, backup agents (Veeam, Acronis), and sometimes the Windows Event Log service. It often modifies Group Policy or registry keys to keep these off after reboot. Phase 3 step 9 explicitly turns Tamper Protection back on so the next ransomware attempt can't repeat the trick.
HiddenSpreads laterally to other PCs on the network
Network-aware ransomware (LockBit, Conti, BlackCat, Akira families) doesn't stop at the first machine. It scans for SMB shares, mapped drives, and other PCs on the same network, then encrypts whatever it can reach. On Windows domains, a single compromised user with admin rights can lead to all-domain encryption. This is why Phase 1 step 2 disconnects the network as the very first action - it stops the spread immediately.
HiddenSteals data before encrypting (double extortion)
Modern ransomware (LockBit, BlackCat / ALPHV, Akira, Cl0p, Royal) almost always exfiltrates data before encrypting. The attacker then runs a "double extortion" play: pay the ransom for decryption, plus pay again to prevent the stolen data from being leaked on the group's leak site. Paying the first ransom does not get the leaked data back - assume that anything the ransomware could read was uploaded. This is one more reason not to pay: the data is already out.
Don't pay the ransom
Paying the ransom does not guarantee you'll get your files back. Major ransomware response studies consistently show that a meaningful percentage of paying victims either receive a non-working decryptor, only partial decryption, or no decryptor at all. Beyond that:
- Paying confirms you as a willing target - re-attack rates within 12 months are high
- Several ransomware groups have been sanctioned (US OFAC, UK OFSI). Paying these groups may be illegal under sanctions law in your country
- The ransom funds the next attack - on you or someone else
- Modern ransomware almost always exfiltrates data first, so paying doesn't get the leaked data back. Assume anything the malware could read was uploaded
- Decryptors, even when they work, often leave files corrupted because the encryption itself was buggy or rushed
The path below tries every legal, free recovery option first. Pay only as an absolute last resort, after consulting a lawyer and your country's cyber crime authority.
Disconnect from the network and log out of cloud storage
Pull the Ethernet cable and turn off Wi-Fi on the affected PC. If multiple computers share the network, do the same on every machine you suspect could be affected, including any servers and NAS units.
Then sign out of cloud storage clients (OneDrive, Dropbox, Google Drive, iCloud) from their app settings. If you can, pause the sync. Some ransomware uses the running sync client to push encrypted versions over your clean cloud copies.
Modern ransomware spreads laterally across SMB shares, mapped drives, RDP, and reachable network neighbors. Disconnecting stops the spread the moment you do it. Logging out of cloud sync clients prevents the ransomware from encrypting the cloud copies of your files via the sync API.
Unplug external drives and USB storage
Safely eject any external drives, USB sticks, SD cards, and network-attached storage shares - then unplug them. If a drive was connected when the ransomware ran, the files on it may already be encrypted, but unplugging stops further damage.
Critically: don't reconnect any drive (including your most recent backup) to this PC until the ransomware is removed in Phase 3. Reconnecting an external backup to an actively-infected PC is the most common way that backup gets encrypted too.
Save the ransom note and a sample encrypted file
Copy the ransom note and one small encrypted file (under 1 MB) to a USB stick, or photograph the note with your phone. You'll need both for the variant identification step.
The ransom note is usually one of:
- A
.txtfile in every encrypted folder (typicallyHOW_TO_DECRYPT.txt,README.txt, or_readme.txt) - An
.htmlfile with the same purpose - A changed desktop wallpaper - photograph it
- A lock-screen interrupt that runs at startup - rarer
Don't delete the note. Variant identification depends on its exact wording, the contact email, and the bitcoin wallet address. The note is also evidence if you report to authorities.
Identify the variant with ID Ransomware
From a clean device (your phone is fine, or another PC that wasn't on the affected network), visit id-ransomware.malwarehunterteam.com. Upload the ransom note and the sample encrypted file. The site identifies the variant family and tells you whether a free decryptor exists.
ID Ransomware is run by the Malware Hunter Team. Files you upload are kept private - they're used for analysis, not published. If you're concerned about uploading data anyway, you can paste the contact email and bitcoin wallet from the note - those are often enough for identification.
The ransomware itself must be removed before any file recovery. If it's still active, anything you restore in Phase 4 gets re-encrypted within minutes.
Boot to Safe Mode with Networking and download the scanners
Safe Mode loads only the minimal Windows drivers, which prevents most ransomware from auto-running while you set up the scanners.
Windows 11: open , click Restart now next to Advanced startup. After the restart, click , then press 5 or F5.
Windows 10: click the Windows logo, hold Shift, click the Power icon and choose Restart. From the Choose-an-option screen, click , then press F5.
Once Safe Mode has loaded, turn Wi-Fi back on and download Combo Cleaner (used in step 8) and Microsoft Safety Scanner (used in step 9). Save both installers to your Desktop, then disconnect Wi-Fi again before scanning.
Run Microsoft Defender Offline
Reboot to normal Windows. Open . Select Microsoft Defender Antivirus (offline scan) and click Scan now. The PC restarts into a stripped-down environment and scans the disk before Windows fully loads.
Some ransomware variants drop bootkit components or kernel hooks that hide from any scanner running on the same Windows session that's already infected. A pre-boot offline scan sees the disk before the malware can load itself.
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Download Combo CleanerRun a full Combo Cleaner scan
Run the installer you downloaded in step 6 and start a full system scan (not the quick scan). Let it complete, review what it detected, and apply the recommended actions. Combo Cleaner quarantines the ransomware executable and the registry / scheduled-task entries that re-launch it on reboot.
Run Microsoft Safety Scanner and re-enable Defender
Run Microsoft Safety Scanner (MSERT.exe) as a second-opinion full scan. The binary expires every 10 days so the copy you downloaded has the latest signatures.
Once it finishes, open and confirm Real-time protection, Cloud-delivered protection, and Tamper Protection are all on. Many ransomware variants disable Defender during the attack - this is where you switch it back on so the next attempt can't repeat the trick.
Different scanning engines catch different threats. Running an additional Microsoft scan after Combo Cleaner is the most underused verification step in malware removal - and free.
Try the steps in order. The first decryptor that works ends the recovery process - no need to do all four.
Check the No More Ransom Project for a free decryptor
Visit nomoreransom.org and search the Decryption Tools list for the variant name returned by ID Ransomware. The project is run by Europol, the Dutch Police, and security vendors - everything listed is legitimate and free.
If a decryptor exists, download it onto a clean PC, copy it across, and follow its instructions exactly. Decryptors usually want you to point them at a sample encrypted file plus its known unencrypted version (sometimes any image you previously had a backup of works).
Free decryptors exist for parts of the STOP/Djvu, GandCrab, REvil, Avaddon, Conti and Hive families, among others. Most modern variants don't have public decryptors - if yours isn't listed, that's expected; move to the next step.
Try Windows Volume Shadow Copies
Windows automatically creates Volume Shadow Copies of files when System Restore points are made or scheduled backups run. Most ransomware tries to wipe these (vssadmin delete shadows /all /quiet), but the deletion sometimes fails partially - especially if the running user doesn't have full admin rights. When it does fail, snapshots survive that contain pre-encryption versions of your files.
First, check what's there. Open Command Prompt as Administrator and run:
vssadmin list shadows
If you see entries listed, shadow copies survived. To restore from them:
- Built-in: right-click any folder → Properties → Previous Versions tab. Pick a snapshot dated before the infection and restore.
- Better tool for browsing: install Shadow Explorer (free). It shows all snapshots as a normal file tree and lets you copy out only the files you want.
Shadow Copies recover the original files exactly, with no risk of decryption errors. If they survived, this is faster and cleaner than running a decryptor on the encrypted set.
Restore from cloud or external backup
If you have a backup that pre-dates the infection, this is the cleanest recovery available. Important precautions:
- Don't reconnect a backup drive until Phase 3 is complete. If the ransomware is still active, plugging in the backup encrypts it too.
- For cloud backups (OneDrive, Dropbox, Google Drive, iCloud), use the Version History feature to roll individual files back to pre-encryption versions.
- OneDrive specifically has a "Restore your OneDrive" feature: if it detected mass file changes, you can roll back the entire account up to 30 days. Sign in at OneDrive on the web, click the Settings gear, and choose Restore your OneDrive. Microsoft 365 subscribers also get ransomware detection alerts; non-subscribers get one free recovery.
- Verify the restored files open correctly before you delete the encrypted set. Test a few documents, photos, and any database / project files.
Try Recuva as a last-resort recovery
Recuva is a long shot, not a primary recovery tool. Most modern ransomware encrypts files in place - it reads the original, encrypts the data, and writes the encrypted bytes back to the same disk location. In that case, the original isn't deleted, just rewritten - and Recuva has nothing to find.
But for older or simpler variants that deleted originals before creating encrypted copies (rather than encrypting in place), Recuva can sometimes pull the originals from unallocated disk space. Worth trying as a last resort, especially before you use the disk much further.
If you try it:
- Stop using the disk as much as possible - every disk write reduces recovery odds
- Run Recuva from a USB stick, not from the affected disk
- Use Deep Scan mode (slower but the only mode worth running for this scenario)
Don't skip the steps above expecting Recuva to bail you out. It works in maybe 5-20% of cases, depending on the variant and how soon you try.
Report the attack to authorities
Even if your files aren't recovered, your report helps authorities track variants, attribute groups, and warn future victims. In some jurisdictions (especially the US and UK) early reporting can sometimes help recall a wired ransom payment via banking partners.
Save the report number - banks, insurers, and credit bureaus will ask for it. If you operate a business, you may also have legal disclosure obligations (GDPR in the EU, state breach laws in the US) that run on a 72-hour clock.
Plan 3-2-1 backups going forward
The single best defense against ransomware is a tested backup. Use the 3-2-1 rule:
- 3 copies of your data
- 2 different storage media (e.g., internal drive + external drive)
- 1 offline / air-gapped copy (an external drive that isn't always connected)
Recommended cadence:
- Daily: cloud sync (OneDrive, Dropbox, Google Drive) - convenient, but vulnerable if the ransomware reaches the sync folder. The Version History feature is what saves you
- Weekly: external drive that you connect briefly, then disconnect - the air-gap is what protects against ransomware
- Monthly: a second external drive stored off-site (different physical location) - protects against fire, theft, flood
Test the backup at least once. Open a few files from it, check sizes match, restore one to a different folder. An untested backup isn't a backup - it's a hope.
Patch the entry point and monitor for 30 days
Most reinfections happen via the same vector that worked the first time. Until you've patched the entry point, watch for:
- Unusual disk activity, unexpected file modifications, or unfamiliar processes in Task Manager
- Unexpected reboots or new accounts you didn't create
- Defender or your antivirus turning itself off again
Common entry vectors and what to do about each:
- Phishing email with a malicious attachment - review email filters and train yourself / staff to recognise the patterns - see pcrisk's Phishing/Scam reference
- Exposed RDP with weak passwords - disable Remote Desktop from if you don't need it. If you do, put it behind a VPN and require a long password plus MFA
- Unpatched VPN appliances or public-facing apps - check the vendor's advisories page and apply the latest firmware
- Cracked / pirated software - was the most common vector for STOP/Djvu in particular. Don't run software from torrent sites
Run a Combo Cleaner scan weekly for the first month. Update Windows, browsers, and applications aggressively.
Important: Removing a ransomware infection requires a full system scan, not a manual file delete. If you're not confident with the manual procedure, run an automated scan with Combo Cleaner and Windows Defender and stop there - that combination removes the vast majority of consumer ransomware variants. Then move to Phase 4 for file recovery, or to Phase 5 if your files are unrecoverable and you need to start fresh from a clean Windows install.
Frequently Asked Questions (FAQ)
How was my computer hacked and how did hackers encrypt my files?
Cybercriminals employ a targeted approach to infiltrate the networks of large commercial entities. They exploit known vulnerabilities in Fortinet VPN clients to gain initial access to the victim's network. Also, they may use pirated software, email, P2P networks, malicious ads, and other channels to distribute malware.
How to open ".CTS1" files, or other files encrypted by Cactus?
Your files have been encrypted due to a ransomware infection, rendering them inaccessible. To regain access and use your files, a decryption tool is necessary.
Where should I look for free decryption tools for Cactus ransomware?
In case of a ransomware attack you should check the No More Ransom project website (more information above).
I can pay you a lot of money, can you decrypt files for me?
We are unable to offer decryption services for files encrypted by ransomware. Typically, decryption without threat actors' involvement is impossible unless there are inherent flaws in the ransomware. Therefore, be cautious of third parties claiming to provide decryption for a fee, as they may act as intermediaries or attempt to scam you.
Will Combo Cleaner help me remove Cactus ransomware?
Using an antivirus program is a crucial initial step in dealing with ransomware. Combo Cleaner can scan your computer and effectively remove active ransomware infections. However, data decryption requires specialized decryption tools or assistance from cybersecurity experts.
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Tomas Meskauskas
Expert security researcher, professional malware analyst
I am passionate about computer security and technology. I have an experience of over 10 years working in various companies related to computer technical issue solving and Internet security. I have been working as an author and editor for pcrisk.com since 2010. Follow me on Twitter and LinkedIn to stay informed about the latest online security threats.
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