Is it safe to open VHD files?

Table of Contents

What are VHD files?

VHD stands for Virtual Hard Disk. VHD files are virtual hard drive image files that are typically used with virtualization software like Microsoft Virtual PC, VMware Workstation, Oracle VM VirtualBox, etc.

VHD files contain the contents and structure of a hard drive partition. They allow you to create a virtual machine that has its own virtual hard drive that behaves like a physical hard drive. The virtual hard drive stores the operating system, applications, and data for the virtual machine.

Some key things to know about VHD files:

VHD files have a .vhd file extension.
They function like physical hard drives but are contained in a single file.
Common uses include creating virtual machines, backing up hard drive data, and migrating physical systems to virtual environments.

There are 2 main types – fixed size and dynamically expanding. Fixed VHDs take up the maximum allocated space on creation. Dynamic VHDs start small and grow as data is added.
VHDs can be attached to virtual machines as primary or additional drives.
The .vhd file contains a full partition image, including the operating system, applications, and data.

So in summary, VHD files are drive images used in virtualization to emulate physical hard drives for virtual machines. The main benefit is portability – being able to move and run complete systems by transferring single VHD files.

Are VHD files safe to open?

Whether it is safe to open a VHD file depends on the source and contents of the particular VHD file. Here are some general guidelines:

VHD files from trusted sources are generally safe to open using appropriate virtualization software like Hyper-V, VirtualBox, VMware, etc. These tools are designed to safely handle VHD files.

You should only open VHD files obtained from trusted sources like known vendors, colleagues, etc. Opening VHD files from random/unknown sources is risky.
Potential risks include malware, viruses, ransomware, and other malicious code that could be within the virtual drive and affect your physical system.
Ensure your virtualization software and physical system has up-to-date antivirus scanning enabled when working with VHDs to scan for potential threats.

Safely disconnect/detach the VHD after use so that it does not auto-run or mount on your physical system when not needed.
Take precautions such as not opening VHDs on critical systems and having backups in place.
Use a sandboxed virtual environment to isolate untrusted VHDs from physical systems as an extra safety measure.

So in summary, VHD files can be safely opened as long as proper security precautions are taken. Only open VHD files from trusted sources and use up-to-date security software to scan for and isolate any potential threats.

What are the risks of opening VHD files?

Some specific risks of improperly opening VHD files include:

Malware infection – VHD files could contain malware designed to infect host systems. This includes viruses, worms, trojans, spyware, adware and other threats that can damage files, steal data, encrypt files for ransom, etc.

Network intrusion – Malicious VHD files may be used to establish a foothold on host networks for further intrusion and attacks.
Sensitive data theft – VHD files may contain sensitive data that could be stolen if accessed by unauthorized parties.
Virtual machine escape – Flaws in virtualization software can potentially allow virtualized threats to “escape” and directly infect host systems.

Denial of service – Malicious VHD files may overload systems and networks, crash virtualization software, or cause other disruptions.
Compliance and regulatory violations – Opening unverified VHD files may go against organizational policies and industry regulations related to data security and integrity.
Destructive attacks – Wiper malware may destroy or corrupt data within improperly opened VHD files.

The likelihood and severity of these risks can vary based on the source and contents of a given VHD file, as well as the protective measures in place. But it underscores the importance of proper precautions when working with VHD files obtained from outside sources.

Steps to safely open VHD files

Here are some key steps that can be taken to open VHD files more safely:

Obtain VHD files only from trusted and verified sources – Avoid unknown third-party VHD files as much as possible.

Use a dedicated virtual machine – Open and scan any unfamiliar VHDs within an isolated VM, not your base system.
Scan before opening – Enable antivirus scanning and conduct a full scan of the VHD file before opening.
Mount read-only – Configure your virtual machine settings to mount unknown VHDs as read-only initially as an added precaution.

Disable networking – Disable networking on your VM or use a sandboxed network to prevent any potential outbound connections.
Limit resources – Restrict CPU, memory, and disk access for your VM to limit impact of any malicious activity.
Backup critical data – Ensure you have good backups of your important data in case of malware escaping and impacting files.

Monitor closely – Keep an eye on virtualization software and VM behavior when opening an unknown VHD.
Safely disconnect – Properly disconnect/detach the VHD after scanning so it doesn’t unexpectedly auto-mount.
Destroy malicious VHDs – If malware or other threats are uncovered, safely delete the problematic VHD file.

Using caution with each VHD file and following these steps can significantly improve safety. But it’s still possible that advanced threats may evade detection. Extra scrutiny should be applied to VHDs from questionable sources or that exhibit suspicious activity during scanning.

VHD malware threats

There have been various malware threats in the wild designed to spread through VHD files and target virtualization platforms. Some examples include:

VHD ransomware – Ransomware variants like Petya target MBR within VHD files to encrypt virtual drives.

Virut – An old virus from the 2000s that infected VHD files and attempted to spread via removable drives.
VHD worms – Worms that propagate through VHD files attached to shared storage like VMCI-based VHD propagation.
VHD credential theft – Malware that extracts credentials from VHD files like login names, passwords, etc.

Hypervisor platform attacks – Malware aimed at attacking weaknesses in virtualization software like VM escape vulnerabilities.
VHD wipers – Malicious VHDs designed to securely wipe or corrupt data within mounted virtual drives.

These demonstrate the diversity of malware that has utilized VHD files for propagation and attacks on virtualized environments. Continued vigilance is required as new VHD-based threats emerge.

VHD ransomware attacks

One of the most high-profile examples of VHD malware are ransomware variants that target VHD files specifically:

Petya ransomware sparked global outbreaks by encrypting the MBR within VHD files to lock access to virtual drives.
Other examples like VirLock ransomware also encrypt and lock VHD files by writing malicious bootloaders.

These threats take advantage of how VHDs are mounted automatically as drives by many systems.
The ransomware uses the mounted VHD drive letter to encrypt files on the virtual disk, disrupting operations.
Petya ransomware cost global firms like Maersk, Merck, and Fedex over $300 million combined.

Modern ransomware continues to evolve VHD attack methods, increasing risks.

Ransomware represents one of the top current threats around opening VHD files. Proper scanning and isolation of any external VHDs provides protection against these type of attacks locking down virtual environments.

Tools and best practices for safe VHD access

There are some helpful tools and best practices to enhance the safety of working with VHD files:

Antivirus scanning

Scan VHD files with up-to-date antivirus software to detect any potential malware threats before access.
Utilize tools like ClamAV that can directly scan VHD files across platforms.
Configure your virtual machines to have real-time scanning enabled as another line of defense.

Perform periodic rescans of existing VHD files to check for newly discovered threats.
Block access to VHDs that fail scanning until they can be cleaned or verified as safe.

MD5/SHA hashes

Generate hash checksums of known good VHD files to check against future alterations.

Cross-check hashes prior to mounting VHDs to ensure they match expected values.
Monitor repositories of VHDs to alert on unexpected hash mismatches indicating possible tampering.

Restricted user access

Only allow approved administrators or users to access VHD storage locations to limit risks.

Integrate VHD security with existing identify and access controls like multi-factor authentication.
Monitor access to VHD files directly as well as virtualization management systems.

Secure storage

Use encrypted storage locations like Bitlocker or dm-crypt to secure stored VHD repositories.

Backup VHDs to isolated, offline storage for protection from encryption or destruction.
Enabling versioning on VHD file shares allows rollback after malicious changes.

Applying a combo of the right tools, access controls, backups and safe storage practices makes working with VHD files much more secure.

VHD file security tools

Some specific tools that can enhance security around VHD files include:

Microsoft Virtual Machine Inspector

Free tool from Microsoft for testing and validating VHD files used with Hyper-V.
Scans VHDs for malware and provides detailed reports for any threats found.

Can integrate with System Center Endpoint Protection for automated workflows.
Provides insight into VHD file contents like OS, data, MBR, etc.

VHDTool

Utility for modifying and managing VHD files at the byte level.

Can be used to clean malware from infected VHD boot sectors or Master Boot Records.
Allows admins to directly view and edit VHD file contents for in-depth inspection.
Support for batch automation via command line usage.

VMCloak

Open source tool focused on automating VHD malware sample creation in safe environments.
Helpful for researchers and malware analysts to study virtualization-based threats.
Customizable sandboxing and isolation ensure samples don’t escape into production.

Includes inspection tools for validating newly generated malware VHDs.

These and other utilities provide enhanced visibility and control when working with VHD files to strengthen security.

Conclusion

VHD files enable convenient portability and isolation of virtual machine environments. But at the same time, their nature as virtual drive images introduces unique risks if they contain malware or are accessed improperly.

With the right security precautions, the benefits of VHDs can be safely realized:

Obtain VHD files exclusively from known, trusted sources and partners whenever possible.
Isolate and scan any external VHD files before allowing access via virtualization software.

Leverage antivirus, sandboxed virtual environments, and other isolation techniques as multiple lines of defense.
Monitor VHD files continuously even after initial verification to check for emerging threats.
Integrate VHD security into broader policies and controls around virtual infrastructure management.

VHD files enable important digital transformation initiatives like cloud migration, disaster recovery, and virtual desktops. But securing VHD files themselves against abuse must be part of an end-to-end strategy to fully realize these benefits while minimizing risk. With proper diligence around VHD file handling, organizations can safely unlock their value as a key virtualization tool.