File Stream Viruses: Can They Infect Your PC?

TL;DR

Yes, a virus embedded in a file stream can infect your computer if the stream is executed or processed by vulnerable software. However, it’s not as common as direct file infection and relies on specific conditions to work.

What are File Streams?

File streams (also known as alternate data streams – ADS) allow you to attach hidden data to a regular file. Think of it like having a secret compartment inside a document or program. They’re often used legitimately by operating systems and applications, but malware can exploit them.

How Viruses Use File Streams

1. Hiding Malware: A virus author might hide malicious code within a file stream to avoid detection by traditional antivirus scans that only look at the main file content.
2. Execution via Vulnerabilities: The key is getting the stream executed. This usually happens when software has a vulnerability that allows it to process ADS data as if it were part of the original file. For example, an older version of Microsoft Office might execute code in a stream without warning.
3. Auto-Execution: Some systems are configured to automatically run files based on their extension (e.g., .exe). If a malicious stream is associated with such a file and executed, the virus activates.

Steps to Protect Yourself

1. Keep Software Updated: This is the most important step! Software updates often patch vulnerabilities that malware exploits, including those related to file streams. Make sure your operating system (Windows, macOS, Linux) and all applications are current.
2. Antivirus/Anti-Malware Software: Use a reputable antivirus or anti-malware program that specifically scans for alternate data streams. Ensure it’s regularly updated with the latest virus definitions.
   • Most modern antivirus solutions will scan ADS by default, but check your settings to be sure.
3. Scan Files Regularly: Perform full system scans on a regular basis (weekly or monthly) to detect any hidden threats.
4. Be Careful with Downloads and Attachments: Avoid downloading files from untrusted sources, and never open email attachments you don’t recognize. Even if the file extension seems safe, it could contain malicious code in a stream.
5. Disable Auto-Execution (Advanced): If you are technically comfortable, consider disabling auto-execution of files on removable media or network shares. This reduces the risk of viruses activating automatically.
   • Windows: You can use Group Policy Editor to control auto-run behaviour.

Checking for Alternate Data Streams (ADS)

You can manually check files for ADS using command-line tools.

Windows

powershellGet-ChildItem C:pathtofile.txt | Select-Object Name, Length, Attributes | Format-List

This PowerShell command will show you the file’s attributes. Look for a value of ReparsePoint or Hidden which can indicate an ADS is present.

Linux

ls -laC /path/to/file.txt

The output will show file permissions and attributes. ADS are often indicated by a leading ‘@’ character in the filename (e.g., @ea_stream_name).

Removing Alternate Data Streams

Windows

powershellRemove-Item C:pathtofile.txt -Force

Warning: This command will delete the file! Use with extreme caution and only on files you are sure contain malicious streams.

A safer option is to use a dedicated ADS cleaning tool, which can remove streams without deleting the main file. Several free tools are available online.

In Summary

While file stream viruses aren’t the most common threat, they represent a potential risk. By keeping your software updated and using good security practices, you can significantly reduce your chances of infection.