Trojan:MSIL/Tarclo.INB is a malicious .NET Framework-based trojan that silently embeds itself into Windows systems to establish persistent backdoor access for remote attackers. First catalogued in Microsoft's threat intelligence database, this trojan operates by creating hidden communication channels with command-and-control servers while concealing its presence through registry modifications and process injection techniques. Unlike ransomware or adware that announce their presence, Tarclo.INB works quietly in the background, making detection difficult without specialized security tools.
This threat belongs to the Tarclo family of trojans, which are characterized by their use of managed .NET code to bypass traditional signature-based antivirus detection. The trojan typically arrives bundled with pirated software installers, fake codec packs, or through malicious email attachments disguised as invoices or shipping notifications. Once installed, it grants attackers the ability to download additional malware payloads, harvest system information, and potentially steal credentials stored in browsers or other applications.
Threat Profile
| Attribute | Details |
|---|---|
| Threat Family | Trojan:MSIL/Tarclo (backdoor/downloader variant) |
| Known Aliases | MSIL/Tarclo.INB, TrojanDownloader:MSIL/Tarclo, Backdoor.Tarclo.INB |
| Platform | Windows XP/Vista/7/8/8.1/10/11 (requires .NET Framework 2.0 or higher) |
| Detection First Reported | Mid-2010s (family); specific INB variant behavior typical for this period |
| Primary Distribution | Software bundles, email attachments, drive-by downloads, exploit kits |
| Persistence Mechanism | Registry Run keys, scheduled tasks, startup folder entries |
| Privilege Requirements | Typically runs with user-level permissions; may attempt elevation |
| Core Capabilities | Remote command execution, secondary payload delivery, system reconnaissance, process injection |
| Network Behavior | Establishes outbound connections to C2 servers (varies by campaign); may use HTTP/HTTPS on non-standard ports |
| Typical Artifacts | Random-named .exe files in %APPDATA% or %TEMP%, new registry Run entries, suspicious scheduled tasks |
| Data Exfiltration Risk | Moderate to high—capable of harvesting browser data, system info, and installed software lists |
| Removal Difficulty | Moderate—requires safe mode removal of persistence mechanisms and thorough filesystem cleaning |
How It Spreads
Trojan:MSIL/Tarclo.INB relies heavily on social engineering and deceptive bundling to gain initial access to systems. The most common infection vector involves software bundling, where the trojan is packaged alongside legitimate-looking freeware applications, video codec installers, or PDF reader updates downloaded from third-party software repositories. Users who bypass installation wizard screens by clicking "Next" repeatedly often unknowingly consent to the trojan's installation as part of a "recommended software bundle."
Email-based distribution campaigns targeting small businesses have also been documented, with attackers sending convincing invoices, shipping confirmations, or tax documents containing executable attachments or malicious macro-enabled documents. These emails often spoof legitimate companies like FedEx, UPS, or accounting services, relying on the recipient's curiosity or concern to execute the attachment. Once the attachment runs, the trojan silently installs while the victim sees either an error message or a decoy document.
Additional distribution methods include:
- Malvertising campaigns that redirect users from legitimate websites to exploit kit landing pages that attempt to deliver the trojan through browser vulnerabilities
- Pirated software cracks and keygens downloaded from torrent sites or file-sharing platforms, which frequently contain embedded trojans as "bonuses"
- Fake update notifications for Flash Player, Java, or browser plugins that appear as pop-ups while browsing compromised or malicious websites
- Infected USB drives that leverage autorun functionality or user curiosity to execute the trojan when the drive is accessed
- Remote Desktop Protocol (RDP) compromise on poorly secured systems, allowing attackers to manually install the trojan on business networks
What It Does On Your Machine
Upon successful installation, Trojan:MSIL/Tarclo.INB immediately begins establishing its foothold on the infected system. The trojan copies itself to a subdirectory within either the user's AppData folder or the Windows Temp directory, typically using a randomly generated folder name composed of alphanumeric characters or a GUID-formatted string. The executable file itself often adopts a legitimate-sounding name like "update.exe," "svchost32.exe," or a random string to avoid suspicion during casual filesystem browsing.
The trojan then modifies the Windows Registry to ensure it runs automatically every time the system boots. Common persistence techniques include adding Run or RunOnce keys under HKEY_CURRENT_USER or HKEY_LOCAL_MACHINE, creating new scheduled tasks that trigger at login or on a recurring interval, and occasionally adding entries to the Startup folder. Some variants of the Tarclo family are known to inject their code into legitimate Windows processes like explorer.exe or svchost.exe to further conceal their presence from basic task manager inspections.
Once established, the trojan initiates network communications with its command-and-control infrastructure. It sends an initial "beacon" containing system information—including the Windows version, installed antivirus software, user account name, computer name, and IP address—to the attackers' server. This information allows the threat actors to profile the infected machine and determine what additional malware payloads might be most profitable. The C2 server responds with commands that could include downloading and executing secondary malware (such as cryptocurrency miners, ransomware, or information stealers), harvesting specific files or credentials, or simply maintaining the backdoor for future exploitation.
Users rarely notice immediate symptoms when first infected with Tarclo.INB. The trojan is designed to operate with minimal system impact during its initial stages to avoid detection. However, over time, you may observe increased network activity even when the computer is idle, unexpected slowdowns (especially if cryptocurrency mining payloads are deployed), mysterious browser redirects, or new applications appearing without your installation. The trojan may also disable Windows Update or security software to protect itself from removal, and some victims report increased spam email originating from their accounts if credentials are harvested.
Manual Removal — Step by Step
Disconnect From All Networks Immediately
Before attempting any removal procedures, physically disconnect your computer from the internet by unplugging the Ethernet cable or turning off your Wi-Fi adapter. This prevents the trojan from receiving new commands, downloading additional malware, or exfiltrating any data it has collected. If you're on a business network, also notify your IT administrator that your machine is compromised to prevent lateral network spread.
Boot Into Safe Mode With Networking
Restart your computer and repeatedly press F8 (Windows 7) or use the Shift+Restart method (Windows 8/10/11) to access the Advanced Boot Options menu. Select "Safe Mode with Networking" to load Windows with minimal drivers and services, which prevents most malware from loading automatically. Safe Mode also makes it easier to delete files that might otherwise be locked by running processes.
Identify and Terminate Malicious Processes
Open Task Manager (Ctrl+Shift+Esc) and carefully examine the list of running processes. Look for unfamiliar executables, especially those with random names or located in AppData or Temp directories. Right-click suspicious processes and select "Open File Location"—if it leads to AppData\Local with a GUID-named folder or Temp directories with random-named executables, it's likely malicious. Right-click and choose "End Task" for these processes before proceeding.
Remove Registry Persistence Entries
Press Windows+R, type "regedit," and press Enter to open the Registry Editor. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run and HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run. Examine each entry carefully—look for values pointing to executables in AppData, Temp, or unusual system locations. Right-click suspicious entries and delete them. Also check RunOnce keys in the same locations.
Delete Scheduled Tasks Created by the Trojan
Open Task Scheduler by typing "taskschd.msc" in the Windows Run dialog. Expand the Task Scheduler Library and look through the Microsoft\Windows folder for tasks with suspicious names or recent creation dates. Click on each suspicious task and examine the "Actions" tab—if it points to an executable in AppData or Temp directories, right-click the task and delete it. Tarclo variants commonly create tasks under the Windows Defender or Windows Update folders to appear legitimate.
Locate and Delete the Trojan's Files and Folders
Navigate to C:\Users\[YourUsername]\AppData\Local and C:\Users\[YourUsername]\AppData\Roaming in Windows Explorer. Look for folders with GUID-formatted names (long strings of numbers and letters in curly braces) or recent creation dates that don't correspond to legitimate software you've installed. Delete these entire folders. Also check your Temp directory (Windows+R, type "%TEMP%", press Enter) and delete any suspicious executables. Empty the Recycle Bin when finished.
Run a Comprehensive Malware Scan
Download Malwarebytes Free (from malwarebytes.com using a clean device if necessary, then transfer via USB) and perform a full "Threat Scan." Allow the scan to complete—this typically takes 30-60 minutes. Malwarebytes is particularly effective at detecting trojan variants and their remnants. Quarantine and remove all detected threats. Follow up with a scan using your existing antivirus software to ensure comprehensive coverage.
Reset Browser Settings and Remove Extensions
Trojan:MSIL/Tarclo.INB sometimes installs browser hijackers or malicious extensions as secondary payloads. Open each browser you use (Chrome, Firefox, Edge) and navigate to the extensions or add-ons page. Remove any unfamiliar extensions, especially those installed recently without your knowledge. Reset your browser's homepage and search engine settings to your preferred defaults. In Chrome, consider using the built-in "Reset settings to their original defaults" option found in Settings > Advanced.
Change All Important Passwords
Because Tarclo.INB has information-stealing capabilities, assume that any passwords entered while the system was infected may have been compromised. Using a different, clean device, change passwords for your email accounts, banking sites, social media, and any other critical services. Enable two-factor authentication wherever available to add an additional security layer against unauthorized access.
Reboot Normally and Verify System Health
Restart your computer normally (not in Safe Mode) and reconnect to the internet. Monitor system behavior closely for the next several days—watch for unexpected network activity, CPU spikes, or the reappearance of suspicious processes. Run another quick scan with Malwarebytes and your antivirus software to confirm the system remains clean. Check that Windows Update is functioning properly and install any pending security updates.
Prevention
- Download software only from official sources. Avoid third-party download sites, torrent repositories, and "freeware aggregator" websites. When you need software, go directly to the developer's official website or use the Microsoft Store for Windows applications.
- Scrutinize email attachments with extreme caution. Never open attachments from unexpected emails, even if they appear to come from legitimate companies. Verify shipping notifications by logging into the carrier's website directly rather than clicking email links. Contact your accounting department or colleague directly if you receive an unexpected invoice attachment.
- Keep Windows and all software updated. Enable automatic Windows updates and regularly update third-party applications like browsers, PDF readers, and productivity software. Many trojan infections exploit known vulnerabilities that have already been patched—staying current closes these attack vectors.
- Use reputable antivirus and anti-malware protection. Install a quality security suite and keep it updated with the latest definitions. Enable real-time protection features. Supplement your primary antivirus with occasional scans using Malwarebytes or similar secondary scanning tools that catch threats through different detection methods.
- Be wary of software bundling during installations. When installing legitimate freeware, always choose "Custom" or "Advanced" installation options rather than "Express" or "Recommended." Carefully read each installation screen and uncheck any offers to install additional software, toolbars, or browser extensions.
- Implement browser security best practices. Use a reputable ad-blocking extension to prevent malvertising exposure. Disable automatic downloads and configure your browser to ask where to save files. Be immediately suspicious of any website prompting you to install updates for Flash, Java, or your browser—these are nearly always malicious.
- Create regular backups of important data. Maintain automated backups to an external drive or cloud service that isn't constantly connected to your computer. This won't prevent infection, but it ensures you can recover your files if the trojan downloads ransomware or causes system damage that requires a clean reinstall.
- Use a standard user account for daily activities. Create a separate administrator account for software installation and system changes, but use a standard (non-admin) user account for browsing, email, and routine work. This limits a trojan's ability to make system-wide changes or install deeply embedded persistence mechanisms.
When Computer Repair Roswell removes malware from your system, we include a 90-day reinfection warranty. If the same threat returns within 90 days through no fault of your own, we'll remove it again at no charge. We also provide personalized prevention advice based on how the infection occurred, helping you avoid future problems. Our virus removal service includes comprehensive scanning, manual threat hunting, and system hardening to ensure your computer returns to you cleaner and more secure than before the infection.
Bring It In
While the manual removal steps outlined above can work for technically confident users, Trojan:MSIL/Tarclo.INB often leaves behind difficult-to-detect remnants or downloads additional threats that complicate DIY removal. Computer Repair Roswell specializes in complete malware eradication using both automated tools and hands-on forensic techniques that identify every trace of infection. We'll examine your system's registry, startup locations, scheduled tasks, browser configurations, and network settings to ensure nothing remains. Most importantly, we'll determine whether any of your personal information was compromised and advise you on necessary protective measures.
Located on Canton Street in historic Roswell, we offer same-day service for virus and malware removal—just call (770) 692-4619 or stop by during business hours. Our flat-rate pricing means you'll know the cost upfront, with no surprises. We'll also show you exactly what we found, explain how the infection occurred, and provide specific recommendations to prevent future infections based on your usage patterns. Don't let a trojan infection steal your data, compromise your accounts, or serve as a gateway for more dangerous malware—bring your computer to the local experts who've been protecting Roswell residents and businesses for years.