TrojanDropper:MSIL/Perseus!rfn: what it is and how to remove it
TrojanDropper:MSIL/Perseus!rfn is the heuristic detection name that should be taken seriously because trojans are one of the more dangerous viruses. The indication of the risk related to a possible cyber infection can be related to malware that is designed to spread other threats around or directly affect your device.
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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TrojanDropper:MSIL/Perseus!rfn: summary
| Distribution | Trojans can be spread on various platforms, emails, malicious sites. Mainly it gets distributed as a malicious file[1] |
|---|---|
| Name | TrojanDropper:MSIL/Perseus!rfn |
| Type | Trojan |
| Symptoms | Trojans affect the speed and some processes of the machine, but the alert of this detection might be the only symptom you experience |
| Detection names | No Microsoft detection name is known |
| Damage | Not recorded in the old report |
| 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 4 more facts
| Evidence | 4 write-ups by security sites; details still limited |
|---|---|
| First seen | 15 January 2021 |
| Microsoft Defender name | TrojanDropper:MSIL/Perseus!rfn |
| Facts checked | 6 October 2026 |
What TrojanDropper:MSIL/Perseus!rfn does on an infected PC
From our report of Jan 2021 · not reviewed since
TrojanDropper:MSIL/Perseus!rfn is the threat indication that alerts about a trojan infection
TrojanDropper:MSIL/Perseus!rfn is the heuristic detection name that should be taken seriously because trojans are one of the more dangerous viruses.
The indication of the risk related to a possible cyber infection can be related to malware that is designed to spread other threats around or directly affect your device. The computer might become damage pretty quickly.
If you ignore the message from your AV or security tool you might end up with more issues than this warning about possible risks and issues related to TrojanDropper:MSIL/Perseus!rfn virus.
The best way to tackle such an issue would be to go for a professional anti-malware tool and scan the machine to make sure that these threats are detected and removed. You might need some help since these trojans can be extremely persistent.
You might think that it is impossible to remove TrojanDropper:MSIL/Perseus!rfn since it is sneaky, and even experts say that trojans cannot be easily found on the machine. The alert may be scary and trigger panic, but there is no need for that. You can clear the machine and eliminate the threat, but you need to take many risks into consideration.
TrojanDropper:MSIL/Perseus!rfn is the threat that triggers various background processes and can damage it, but the detection might also be a false positive. You need to run or to make sure that the program can be eliminated and the threat that bothers your device deleted.
Do not forget that TrojanDropper:MSIL/Perseus!rfn can be designed to drop other viruses on the infected machine. You cannot notice such a process also, so the ransomware or other malware can run freely until it damages the machine enough, so additional issues appear.
The issue is that TrojanDropper:MSIL/Perseus!rfn can change files, damage them, add new ones. It is possible that trojan damages the machine by altering directories or registry entries to ensure it remains persistent. This is why you need to take system issues very seriously and run or a similar tool to tackle such issues yourself.
Some issues that you might notice on the machine that is possibly related to TrojanDropper:MSIL/Perseus!rfn:
TrojanDropper:MSIL/Perseus!rfn removal process and the success of the procedure strongly depends on the time period this threat managed to run on the system and the particular intruder that is triggering the detection alerts. You can find various different names on the list when you run the AV tool, but do not panic and make sure to remove all of them to save the computer.
It is not that difficult - just follow the suggested steps. However, if you still notice that TrojanDropper:MSIL/Perseus!rfn is persistent and cannot be removed - go for the tips below this article. There are a few solutions like Safe Mode and System Restore that should help.
- The computer gets very slow.
- Applications take too long to start or even crashes.
- You see a new extension that you did not install on your browser.
- The internet is slower than usual.
- You are seeing a lot of pop-up ads, banners, other suspicious content.
- You see processes running in the background.
- You notice programs installed without your knowledge.
- You receive antivirus notifications about risks and threats.
From our report of Jan 2021 · not reviewed since
Malicious content leads to serious cyber infections
Malicious emails might look decent and legitimate when trojan developers clone email style, add logos of real companies and businesses.
However, always pay attention to file attachments, typos, or grammar mistakes, so you can be suspicious enough to avoid infiltration of the unwanted virus or a PUP that triggers issues with the machine.
Note that Trojans can get installed as a file like EXE or DLL. The downloaded DLL extracts the script to run a new executable binary, which contains the infection. It is unnoticeable, so you cannot see symptoms of the infiltration for some time. This is how the trojan becomes persistent and can run processes silently in the background.
If you notice any symptoms you can terminate the virus sooner. However, these infections more often happen when you open an insecure email or after the hacked or malicious site gets opened and visited. You can experience trojan infection when the PC already is affected by other malware. This is why you need to run an AV tool to see all of them.
From our report of Jan 2021 · not reviewed since
Terminate the TrojanDropper:MSIL/Perseus!rfn virus as soon as possible
TrojanDropper:MSIL/Perseus!rfn removal process might seem intimidating since there is no program that could be deleted from your device, but there are tools like or that can find all those pieces of malware and remove them fully form your device. It takes a few steps, but it is possible.
rely on the professional anti-malware tools and run the full system scan to determine if the threat is affecting your device already or if there is no need to remove TrojanDropper:MSIL/Perseus!rfn and this is the alert about the only possible intruder. Follow the guide below for additional help with the affected PC.
TrojanDropper:MSIL/Perseus!rfn can trigger issues with your system directly when it damages files, interferes with registry entries, deletes files or programs. So run and follow the system check to see what issues get found and need to be fixed. This PC repair solution can recover the machine and improve the performance of the computer for you.
[GI=method-1]Reboot the system in Safe Mode with Networking and then remove TrojanDropper:MSIL/Perseus!rfn properly
How to check the PC for TrojanDropper:MSIL/Perseus!rfn
Microsoft's name for TrojanDropper:MSIL/Perseus!rfn is TrojanDropper:MSIL/Perseus!rfn.
Defender sorts threats by category first, so the word before the colon tells you what kind of program was found, even if the family name means nothing to you.
Do not search for a removal tool by the detection name alone; fake "removal tools" use the same keywords. How the naming works is explained in our guide to antivirus detection names.
How to remove TrojanDropper:MSIL/Perseus!rfn
A loader's job is to install other malware, so treat the PC as infected with more than one program until the scans are clean.
Step 1: Delete scheduled tasks that bring it back
Open Task Scheduler from the Start menu and click Task Scheduler Library. Select each task you do not recognise and read the Actions tab:
- a task that starts a file in
%AppData%or%Temp% - runs
powershellwith a long encoded line - opens a web address belongs to TrojanDropper:MSIL/Perseus!rfn or a similar program
Right-click such a task and choose Delete, and check the subfolders too. Leave tasks from Microsoft, your PC's maker and programs you use. Task Scheduler is the same in Windows 11 and Windows 10.
Windows 11: Task Scheduler Library, the task selected, the Actions tab shows the file it runs. Full procedure with screenshots: Remove what malware leaves behind in Windows
- a task that starts a file in
Step 2: Remove it from startup
Whatever TrojanDropper:MSIL/Perseus!rfn installed usually starts with Windows. Open Task Manager with Ctrl + Shift + Esc, go to Startup apps in Windows 11 or the Startup tab in Windows 10, and disable what you do not know, starting with entries that have an empty Publisher column.
Open file location on the right-click menu shows the file, which helps you decide and tells you what to delete later. Disabling is safe: if something you need stops working, switch it back on.
Full procedure with screenshots: Stop apps from opening at startup On uGetFix
Step 3: Delete the folders left behind
What the uninstaller leaves behind is usually in the user folders. Open File Explorer, switch on View > Show > Hidden items (in Windows 10 View > Hidden items), and go through
%LocalAppData%,%AppData%, %ProgramData% and the two Program Files folders.Sort by Date modified and delete folders that appeared together with the problem and belong to no program you use. Folders with random names that hold
.exe,.dll, .js or .ps1 files are the strongest sign.Full procedure with screenshots: Remove what malware leaves behind in Windows
Step 4: Scan the PC, then run the offline scan
Open Windows Security > Virus & threat protection > Scan options, run a Full scan and remove everything it finds. Then select Microsoft Defender Antivirus (offline scan) and click Scan now: the PC restarts and scans before Windows loads, which finds files that hide while Windows runs.
Save your work first, because the offline scan takes about 15 minutes. Both scans are built into Windows 11 and Windows 10, and a second scanner from another vendor is a useful extra opinion.
Windows 11: Windows Security > Virus & threat protection > Scan options. Full procedure with screenshots: Run a Microsoft Defender Offline scan
Step 5: Change passwords from another device and sign out other sessions
Assume that the passwords saved in this PC's browsers, and the accounts that were open in them, are known to the attacker. From a phone or a clean computer, change the e-mail password first, then banking, shopping, social and gaming accounts.
Use each service's option to sign out of all other sessions, because stolen cookies keep a session open even after a password change. Turn on two-step verification on each account. Wait with the infected Windows 11 or Windows 10 PC until the scans are clean.
Full procedure with screenshots: Turn on two-step verification / secure a hacked account
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.
After removal: passwords, accounts and prevention
Secure your accounts after the clean-up
Assume that whatever was saved in the browsers on this PC while the PC showed the Windows Security detection TrojanDropper:MSIL/Perseus!rfn has been copied:
- passwords
- cookies
- autofill data
Work from a clean device, or from this PC once the offline scan finds nothing.
Start with your main e-mail account, because it can reset everything else, then banking and payment, then social and gaming accounts. Change each password, sign out of all sessions and turn on two-step verification: Turn on two-step verification / secure a hacked account.
The full order, including crypto wallets and card replacement, is in securing your accounts after malware.
Do not let government spy on you
The government has many issues in regards to tracking users' data and spying on citizens, so you should take this into consideration and learn more about shady information gathering practices.
Avoid any unwanted government tracking or spying by going totally anonymous on the internet.
You can choose a different location when you go online and access any material you want without particular content restrictions. You can easily enjoy internet connection without any risks of being hacked by using VPN.
Control the information that can be accessed by government any other unwanted party and surf online without being spied on. Even if you are not involved in illegal activities or trust your selection of services, platforms, be suspicious for your own security and take precautionary measures by using the VPN service.
Backup files for the later use, in case of the malware attack
Computer users can suffer from data losses due to cyber infections or their own faulty doings.
Ransomware can encrypt and hold files hostage, while unforeseen power cuts might cause a loss of important documents. If you have proper up-to-date backups, you can easily recover after such an incident and get back to work. It is also equally important to update backups on a regular basis so that the newest information remains intact - you can set this process to be performed automatically.
When you have the previous version of every important document or project you can avoid frustration and breakdowns. It comes in handy when malware strikes out of nowhere. Use for the data restoration process.
Questions about TrojanDropper:MSIL/Perseus!rfn
Defender removed TrojanDropper:MSIL/Perseus!rfn. Am I safe now?
Often yes, but check two things. First, whether the detection was caught on arrival: a file in Downloads or in an archive that Defender quarantined before it ran is the best case.
Second, whether TrojanDropper:MSIL/Perseus!rfn comes back. If it returns, or if Protection history shows several different detections around the same date, something on the PC keeps producing the file and needs to be found.
Run a full scan and, if anything remains, an offline scan. If the category names a password stealer or a backdoor, change your passwords from another device even if the file is gone.
Why does TrojanDropper:MSIL/Perseus!rfn keep coming back?
Because removing the file does not remove what creates it. The usual causes are a scheduled task, a startup entry, a service or a browser extension that downloads or rebuilds the file, or a second program that installed it in the first place.
Look in Task Scheduler and Startup apps for unknown entries, check Installed apps sorted by date, and run a Microsoft Defender Offline scan, which can remove items that are locked while Windows runs. Also check that no exclusions were added in Windows Security. If TrojanDropper:MSIL/Perseus!rfn still returns, back up documents and reset Windows.
Will Windows Security detect TrojanDropper:MSIL/Perseus!rfn?
Yes, current Microsoft Defender definitions name it TrojanDropper:MSIL/Perseus!rfn. Make sure the definitions are up to date first: open Windows Security > Virus & threat protection > Protection updates and click Check for updates. Then run a full scan and the Microsoft Defender offline scan.
If the detection comes back after you remove it, look for the program, task or extension that brings it back, using the plan above. Detection removes files and startup entries, but it does not reverse changes TrojanDropper:MSIL/Perseus!rfn already made, so follow the other steps in this guide as well.
Why didn't my antivirus stop TrojanDropper:MSIL/Perseus!rfn?
New trojan builds are packed and changed often so that signatures do not match, and some are signed with stolen or bought certificates. Many arrive inside password-protected archives, which scanners cannot open until you extract them.
Some downloads also tell the user to turn off the antivirus "because it gives false alarms", a common line in cracked software instructions. Keep real-time protection on, never disable it for an installer, and run the offline scan whenever you suspect something slipped through.
What are the signs of a trojan infection?
Most trojans try to leave no visible signs, so look for side effects. Common ones:
- Windows Security turned off or unable to update
- new entries in Startup apps or Task Scheduler
- programs in Installed apps you did not install
- browser settings that changed by themselves
- unusual network activity while the PC is idle
- password-reset or login-alert e-mails you did not trigger
None of these proves an infection on its own. Together with an antivirus alert naming TrojanDropper:MSIL/Perseus!rfn, they are a strong reason to follow the full plan.
Can TrojanDropper:MSIL/Perseus!rfn spread to other devices on my network?
Most trojans aimed at home users stay on the PC they infected, but an attacker with remote access can look at the network, open shared folders and try passwords on other devices. Loaders sometimes deliver worms or ransomware that do spread.
Disconnect the PC while cleaning, run a full scan on other Windows PCs, change the router's admin password and the Wi-Fi password if they were saved on the infected PC, and update the router's firmware. If other PCs show the same detection, treat them as infected too.
Should I reset my PC because of TrojanDropper:MSIL/Perseus!rfn?
Only if the signs point to deeper access. Reset when you see the Windows Security detection TrojanDropper:MSIL/Perseus!rfn again after removal, when Windows Security cannot start or update, when remote access tools you did not install keep appearing, or when you simply cannot trust the PC any more.
Otherwise, the plan in this guide plus an offline scan is enough. If you do reset, choose Remove everything and Cloud download for a fresh copy of Windows, restore only documents and photos, and reinstall programs from their official sites. Change important passwords from the clean system afterwards.
Is a trojan infection worth reporting to the police?
If there was harm, yes. Unauthorised payments, accounts used for fraud, blackmail or a remote session during a scam call all belong in a report, and banks often ask for its reference number before they refund anything.
If antivirus caught TrojanDropper:MSIL/Perseus!rfn before it ran and nothing was misused, there is nothing to report. Keep the evidence anyway:
- protection history
- the original download
- the dates
Businesses may also have to notify a data protection authority if personal data could have been accessed.
Is TrojanDropper:MSIL/Perseus!rfn a known trojan?
Not as a documented family, at least not yet. What is known is the visible sign, the Windows Security detection TrojanDropper:MSIL/Perseus!rfn, which matches a hidden program working for someone else.
New threats are often seen by victims weeks before researchers publish anything about them. Treat TrojanDropper:MSIL/Perseus!rfn as you would any trojan:
- remove what starts it
- run an offline scan
- change passwords from another device
If a scan reports a detection name, keep it; it usually reveals the family and whether it is known to download other malware. We update this guide when an analysis appears.
Will Fortect remove TrojanDropper:MSIL/Perseus!rfn?
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 TrojanDropper:MSIL/Perseus!rfn, 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: Malicious macros (read October 6, 2026)
- CSOonline: What is a Trojan? How this tricky malware works (read October 6, 2026)
- Dieviren: Dieviren (read October 6, 2026)
- FTC: How to recognize, remove and avoid malware (read October 6, 2026)
- Microsoft Learn: Microsoft Defender Offline (read October 6, 2026)