Spoof File Extensions in Windows

TL;DR

Yes, file extensions can be easily spoofed in Windows. The operating system relies on the extension to determine which program opens a file, but this association is not secure. Renaming a file doesn’t change its underlying data format; it just changes how Windows thinks about the file. This means you can disguise a malicious file as something harmless.

How File Extensions Are Spoofed

1. Renaming Files: The simplest method is to simply rename a file in Windows Explorer. For example, change document.exe to document.txt.
2. Hidden Attributes: You can hide the true extension by setting the ‘hidden’ attribute on the file. This won’t prevent someone from seeing it if they choose to show hidden files, but it can mislead casual users.
3. Command Prompt/PowerShell: Use command-line tools for more control.
   • Renaming:

     ren document.exe document.txt

   • Setting Hidden Attribute:

     attrib +h document.txt

Steps to Spoof a File Extension

1. Identify the Target File: Choose the file you want to disguise.
2. Rename the File: Right-click on the file, select ‘Rename’, and change the extension to something innocuous (e.g., .txt, .jpg, .pdf).
3. (Optional) Hide the True Extension:
   • Open Command Prompt or PowerShell as an administrator.
   • Navigate to the directory containing the file using the cd command (e.g.,

     cd C:UsersYourNameDocuments

     ).

   • Use the attrib +h filename.newextension command to hide the file attribute.
4. Test the Spoof: Double-click the renamed file. Windows will attempt to open it based on the new extension. If successful, you’ve spoofed the extension.

How to Detect a Spoofed Extension

1. File Size and Type: Be suspicious of files with unexpected sizes or types for their claimed extensions. A small text file shouldn’t be several megabytes in size.
2. Show File Extensions: In Windows Explorer, go to ‘View’ > ‘Options’ > ‘Change folder and search options’. On the ‘View’ tab, uncheck ‘Hide extensions for known file types’. This will reveal the true extension.
3. File Header Analysis: Use a hex editor or file analysis tool (like TrIDNet) to examine the file’s header. The header contains information about the actual file type, regardless of the extension.
   • Download and install a hex editor (e.g., HxD).
   • Open the file in the hex editor.
   • Look for magic numbers or patterns that identify the true file type. For example, ‘MZ’ often indicates an executable file.
4. cyber security Software: A good antivirus program can detect malicious files even if their extensions are spoofed.

Important Considerations

• Renaming a file does not change its internal structure or code. It only changes how Windows interprets the filename.
• Spoofing file extensions is often used in malware distribution to trick users into opening malicious files.
• Always be cautious when opening files from untrusted sources, even if they have seemingly harmless extensions.