Trojan:MSIL/FileCoder.GB represents a dangerous family of file-encrypting malware written in the .NET/MSIL (Microsoft Intermediate Language) framework. This ransomware variant locks users out of their documents, photos, databases, and other critical files by applying strong encryption, then demands payment—typically in cryptocurrency—for the decryption key. Like most modern ransomware, FileCoder.GB doesn't just encrypt files; it often attempts to delete Volume Shadow Copies and disable Windows recovery features to prevent victims from restoring their data through legitimate means.
What makes MSIL-based threats particularly concerning is their ability to run on any Windows system with the .NET Framework installed (which includes virtually all modern Windows installations) and their relative ease of modification by attackers. This means new variants can emerge quickly, sometimes with enhanced evasion techniques or expanded targeting. The "GB" designation indicates a specific variant within the broader FileCoder family, though behavioral patterns remain consistent across most iterations.
Threat Profile
| Family | Trojan:MSIL/FileCoder (ransomware) |
| Platform | Windows (any version with .NET Framework 2.0 or higher) |
| Aliases | MSIL/Filecoder.GB, Ransom:MSIL/FileCoder.GB, Trojan.Encoder.MSIL, FileCryptor.GB (vendor names vary) |
| Detection Rate | Moderate to high by updated antivirus engines; new variants may evade signature-based detection for 24-72 hours |
| Distribution Methods | Malicious email attachments, compromised software installers, exploit kits, Remote Desktop Protocol (RDP) brute-force attacks |
| Encryption | Typically AES-128 or AES-256 with RSA-2048 key protection; exact implementation varies by variant |
| File Extension Changes | Varies by variant; common patterns include appending .locked, .encrypted, .crypt, or randomized extensions to encrypted files |
| Ransom Note | Dropped as README.txt, HOW_TO_DECRYPT.html, or similar in affected folders and desktop; typically demands $300-$1,500 in Bitcoin |
| Persistence Mechanism | Registry Run keys, scheduled tasks; some variants execute once without establishing persistence to avoid detection |
| Targeted File Types | .doc, .docx, .xls, .xlsx, .pdf, .jpg, .png, .zip, .psd, .sql, .mdb, databases, archives—typically 200+ extensions |
| Network Behavior | Contacts command-and-control servers to register victim ID and retrieve encryption keys; may scan local network for additional targets |
| Removal Difficulty | Moderate (removing the malware itself); file recovery without backups ranges from difficult to impossible without decryption key |
How It Spreads
Trojan:MSIL/FileCoder.GB primarily relies on social engineering and software vulnerabilities to gain initial access to victim systems. The most common infection vector remains phishing emails that appear to come from legitimate sources—shipping notifications, invoice documents, tax forms, or legal notices. These emails contain attachments (often disguised as PDFs or Word documents) that are actually executable files or macro-laden documents. When opened, they download and execute the ransomware payload.
Another significant distribution method involves software bundling and fake updates. Victims may encounter this threat when downloading software from unofficial sources, torrent sites, or codec pack installers. The malware may be bundled with otherwise legitimate-looking applications, or presented as a "required update" for Flash Player, video codecs, or other common software. In corporate environments, attackers increasingly exploit weak or default credentials on Remote Desktop Protocol connections, logging in directly and manually executing the ransomware.
Common distribution vectors include:
- Phishing emails with malicious attachments masquerading as invoices, shipping notifications, or urgent business documents
- Compromised websites hosting exploit kits that target browser or plugin vulnerabilities (outdated Java, Flash, Silverlight)
- Trojanized software downloads from unofficial sites, especially cracked commercial applications or key generators
- Malvertising campaigns that redirect to automatic download pages or fraudulent tech support sites
- RDP brute-force attacks against servers and workstations with exposed remote access ports
- Secondary payload dropped by existing malware infections (banking trojans often download ransomware as a final stage)
- USB drives and removable media from untrusted sources with autorun-enabled executables
What It Does On Your Machine
Upon execution, Trojan:MSIL/FileCoder.GB immediately attempts to establish persistence and begin its encryption routine. The malware typically copies itself to a hidden location in the user's AppData folder using a randomly-generated filename to evade simple detection. It then creates registry entries or scheduled tasks to ensure it runs on system startup—though some variants execute their encryption routine immediately and delete themselves afterward to reduce forensic evidence.
Before encrypting files, the ransomware performs several preparatory actions designed to maximize damage and prevent recovery. It attempts to delete Volume Shadow Copies using the Windows vssadmin utility, disables Windows Backup services, and may terminate processes associated with databases, email clients, and document editors to ensure those files aren't locked by running applications. This preparation phase happens quickly—often in under 30 seconds—giving victims little warning before encryption begins.
The encryption process itself targets specific file extensions across all accessible drives, including mapped network shares and connected USB storage. The malware walks through the file system, encrypting documents, images, databases, and archives while typically avoiding system files to keep Windows functional (a non-functional computer can't display the ransom note or make payment). As files are encrypted, they're typically renamed with a new extension or have an identifier appended. The encryption uses strong cryptographic algorithms—usually AES for file content and RSA to protect the encryption keys—making recovery without the attacker's private key virtually impossible.
Once encryption completes, FileCoder.GB drops ransom notes in multiple locations—typically on the desktop, in the user's Documents folder, and in each directory containing encrypted files. These notes provide instructions for payment, usually demanding cryptocurrency sent to a specific wallet address within a deadline (often 72 hours). The malware may also change the desktop wallpaper to display ransom instructions and create a pop-up window that appears on every login. Some variants contact a command-and-control server to register the infection and retrieve a unique victim ID, though others operate entirely offline using pre-generated key pairs embedded in the malware binary.
Manual Removal — Step by Step
Disconnect from All Networks Immediately
The moment you suspect ransomware, physically disconnect the network cable or disable Wi-Fi. Ransomware can spread to mapped network drives, cloud storage with sync clients running, and other computers on your network. Power off the machine completely if encryption appears to be in progress (you'll see file extensions changing in real-time). This may limit damage but won't reverse encryption already completed.
Boot Into Safe Mode with Networking
Restart the computer and repeatedly press F8 (Windows 7) or Shift+F8 (Windows 8/10) during boot to access Advanced Boot Options. Select "Safe Mode with Networking." If that method doesn't work on Windows 10/11, boot from installation media and navigate to Troubleshoot > Advanced Options > Startup Settings, then select Safe Mode. Safe Mode loads only essential drivers and services, preventing most malware from running automatically.
Identify and Terminate the Malware Process
Open Task Manager (Ctrl+Shift+Esc) and look for suspicious processes—particularly anything running from %APPDATA% or %TEMP% directories with random names or posing as system services like "svchost.exe" but located in user folders (legitimate svchost.exe only runs from System32). Note the process name and file location, right-click, select "Open file location," then end the process. Do not delete the file yet; you may need it for forensic purposes or decryptor identification.
Remove Persistence Mechanisms
Press Win+R, type "regedit," and navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run and HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run. Look for entries with suspicious names pointing to executables in AppData or Temp folders—delete these entries. Also open Task Scheduler (search for "Task Scheduler" in Start menu) and review scheduled tasks for anything unfamiliar or pointing to random executable paths. Delete suspicious tasks.
Delete the Malware Executable and Associated Folders
Navigate to the file location you identified in Step 3 (typically in %APPDATA% or %LOCALAPPDATA%). Delete the entire folder containing the malware executable. Because the process is terminated and persistence is removed, the file should delete normally. Also check %TEMP% and %USERPROFILE%\Downloads for the original infection source if you can identify it (recent .exe files, suspicious email attachments).
Scan with Malwarebytes and Secondary Scanner
Download and install Malwarebytes (free version is sufficient) and run a complete Threat Scan. This will catch remnants, associated PUPs, and any secondary infections. After Malwarebytes completes, also run a scan with either HitmanPro, ESET Online Scanner, or Windows Defender Offline (if on Windows 10/11). Cross-verification with multiple scanners catches polymorphic components that single tools might miss.
Check for Available Decryptors
Visit the No More Ransom Project (nomoreransom.org) and Emsisoft's decryptor repository to see if a free decryption tool exists for your specific FileCoder variant. You'll need to upload a ransom note and sample encrypted file to identify the exact variant. For many newer ransomware families, no decryptor exists—but new ones are occasionally released as law enforcement seizes criminal infrastructure or security researchers find cryptographic flaws.
Restore from Backup or Accept Data Loss
If you have backups on external drives or cloud storage that weren't connected during infection, verify the malware is completely removed (rescan after rebooting normally), then restore your files. If no backup exists and no decryptor is available, you face a difficult choice: pay the ransom (which we strongly advise against—there's no guarantee of receiving a working decryptor, and payment funds further criminal activity) or accept the data loss. Document everything for potential insurance claims or law enforcement reports.
Change All Important Passwords
Many ransomware infections include information-stealing components that harvest credentials before encryption begins. From a known-clean device (not the infected computer), change passwords for email accounts, banking, cloud storage, and any services where you've saved passwords in browsers. Enable two-factor authentication wherever available. On the cleaned computer, clear all saved passwords from browsers after malware removal is confirmed.
Reboot Normally and Verify System Stability
Restart the computer normally (not in Safe Mode) and monitor behavior for 24-48 hours. Check that all expected programs launch correctly, no unusual network traffic occurs, and no persistence mechanisms have reactivated. Run one final full scan with your primary antivirus. If any anomalies appear—unexpected slowdowns, error messages, or suspicious network connections—the infection may not be fully resolved, and professional assistance is warranted.
Prevention
- Maintain offline backups following the 3-2-1 rule: Keep three copies of important data on two different media types with one copy stored offline (external drive disconnected when not backing up, or cloud storage with versioning that can recover from ransomware). Test restoration periodically to ensure backups actually work.
- Keep all software updated with automatic patching enabled: Windows Update should run automatically, and third-party software (browsers, PDF readers, Java, Flash) must be kept current or removed if unnecessary. Most exploit-kit infections target known vulnerabilities in outdated software.
- Use email filtering and treat attachments with extreme skepticism: Enable spam filtering on your email service, and never open attachments from unexpected senders—even if they appear to come from known companies. Call the supposed sender using a number you look up independently (not one in the email) to verify legitimacy before opening anything.
- Disable macros in Office documents by default: Navigate to File > Options > Trust Center > Trust Center Settings > Macro Settings and select "Disable all macros with notification." Only enable macros for documents from verified, trusted sources where you specifically expect macro functionality.
- Restrict RDP access and use strong authentication: If Remote Desktop Protocol must be exposed to the internet, use a VPN, require certificate-based authentication, implement account lockout policies after failed login attempts, and enable Network Level Authentication. Better yet, use zero-trust remote access solutions instead of RDP.
- Run updated antivirus with real-time protection and behavior monitoring: Windows Defender (built into Windows 10/11) provides adequate protection if kept updated and configured correctly. Commercial alternatives like Kaspersky, Bitdefender, or ESET offer additional layers. Ensure real-time protection remains enabled—not just on-demand scanning.
- Apply least-privilege principles to user accounts: Don't use an administrator account for daily activities. Create a standard user account for regular work; this limits malware's ability to modify system files, install persistence mechanisms, or disable security features. Use the administrator account only when installing legitimate software.
- Disable AutoRun for removable media: Press Win+R, type "gpedit.msc," navigate to Computer Configuration > Administrative Templates > Windows Components > AutoPlay Policies, and enable "Turn off AutoPlay" for all drives. This prevents malware from automatically executing when USB drives are inserted.
Bring It In
Ransomware infections represent one of the most stressful computer emergencies you can face—your files are held hostage, your work is disrupted, and decisions must be made quickly under pressure. While the manual removal steps above can eliminate the malware itself, file recovery often requires specialized tools, forensic analysis to identify the exact variant, and sometimes negotiation with attackers (which we handle so you don't have to communicate with criminals directly). Our technicians have successfully recovered data from dozens of ransomware infections using a combination of decryption tools, shadow copy recovery, and professional data recovery techniques.
Don't wait and don't pay a ransom before consulting with professionals. Bring your infected computer to our Roswell location at 1394 Canton Road or call (770) 674-6820 to discuss emergency service options. We offer same-day diagnostics for active infections, flat-rate pricing (no surprises), and honest assessments of recovery possibilities. If your data can be recovered, we'll recover it. If it can't, we'll tell you that up front rather than taking your money for false hope. We're here Monday through Saturday, and we've seen just about every ransomware variant that's made the rounds in Georgia over the past decade.