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Ransomware on Hyper-V: Impact and Recovery

1️⃣ Hyper-V context and vulnerability

Microsoft Hyper-V is a widely used virtualization hypervisor designed to run multiple virtual machines (VMs) on a single physical server. Hyper-V environments mainly rely on virtual disks in VHD or VHDX format, which contain the entire operating system and data of each VM.

When ransomware targets a Hyper-V server or the storage shares accessible by the server, VHD/VHDX files can be encrypted, resulting in immediate inaccessibility of all hosted VMs.

2️⃣ Symptoms of a Hyper-V infection

When a Hyper-V hypervisor is affected by ransomware, the following symptoms are typically observed:
  1. Encrypted VHD/VHDX files
    • Extension remains unchanged but contents become unreadable or corrupted.
    • Attempts to open them in Hyper-V Manager fail.
  2. VM freeze or shutdown
    • Virtual machines no longer start.
    • Error messages indicate “inaccessible disk” or “corrupted file.”
  3. Visible ransom note
    • Some ransomware variants drop text or HTML files in the VM storage directory.
    • These files indicate that VHD/VHDX have been encrypted and provide ransom instructions.
  4. Impact on shared storage
    • If VHD/VHDX files are stored on a NAS or SAN accessible by other servers, encryption may quickly spread to additional virtual machines.
  5. Corruption of Hyper-V snapshots / checkpoints
    • Checkpoint files (.AVHDX) may also be encrypted, making rollback impossible.

3️⃣ Affected files and formats

  • VHD: legacy format used by Hyper-V up to Windows Server 2012.
  • VHDX: modern, more resilient format used since Windows Server 2012 R2.
    • Advantages: better corruption tolerance, capacity up to 64 TB, optimized for SSD storage.
  • AVHDX: checkpoint files, also commonly targeted by ransomware.
Technical note: ransomware encrypts the binary content of these files without changing the extension, preventing Hyper-V from mounting them. Metadata (size, date) often remains intact, giving the impression that the VMs are still present but inaccessible.

4️⃣ Impact on data recovery

Recovering Hyper-V VMs after a ransomware attack depends on several factors:
  1. Availability of valid backups
    • If offline or immutable backups exist (Veeam, Windows Server Backup, Azure Backup), recovery is fast.
    • Without backups, recovery requires forensic techniques and manual reconstruction of VHD/VHDX, which is complex and time-consuming.
  2. Type of encryption applied
    • Modern ransomware uses AES-256 + RSA to encrypt files, making decryption impossible without the key.
    • Partially encrypted VHD/VHDX files can sometimes be repaired, but often with partial data loss.
  3. Number of virtual machines and size of VHD/VHDX files
    • Modern VHDX files can reach several terabytes.
    • The larger the VM, the longer the recovery or reconstruction time.
  4. Snapshot corruption
    • Encrypted checkpoints prevent reverting to a previous state.
    • Data stored exclusively in checkpoints may be unrecoverable.
  5. Impact on dependent services
    • Critical production VMs, databases, file servers: prolonged downtime until full recovery.

5️⃣ Estimated time to recover the data

The recovery time varies greatly depending on the situation:
Scenario Approximate duration Comment
Valid and intact Veeam / Hyper-V Backup 1–4 hours Depends on VHD/VHDX size and network bandwidth.
Local backup on NAS / immutable tape 4–12 hours Decompression and restoration of large files.
No backup, recovery via forensic methods Several days to weeks Manual reconstruction, scanning VHD/VHDX files, partial recovery possible.
Partially corrupted VHD/VHDX 3–10 days Depends on file size and the recovery engine used.
Practical observation:
  • Specialized tools such as ScanX or R-Studio can sometimes recover damaged partitions, but for a fully encrypted VHD/VHDX, full recovery is nearly impossible without a backup.
Prevention (offline backups and immutable snapshots) remains the most effective solution.

6️⃣ Best practices to reduce the impact

  1. Isolated and immutable backups: stored offline, on separate NAS devices, or in secure cloud storage.
  2. Secure Hyper-V snapshots: use AVHDX only for testing purposes, not as the sole backup method.
  3. Network segmentation: isolate hypervisors from client machines to limit ransomware spread.
  4. Monitoring of VHD/VHDX files: alerts for suspicious modifications or access attempts.
  5. Regular restoration tests: ensure backups can be restored quickly and reliably.
A ransomware attack on Hyper-V can make all virtual machines inaccessible by encrypting VHD, VHDX, and AVHDX files. Symptoms include VMs that no longer start, unreadable files, and the appearance of ransom notes. The impact on data recovery depends mainly on:
  • the presence and condition of backups,
  • the type and extent of encryption,
  • the size of the virtual disks.
Without isolated backups, recovery may take several days to weeks, and in some cases, certain data may be irrecoverable. Implementing strong backup and monitoring practices is therefore essential to limit data loss.