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VMware ESXi encrypted by ransomware

1️⃣ Symptoms of an ESXi ransomware infection

When a VMware ESXi hypervisor is infected by ransomware, several distinct signs can be observed:
  1. Encryption of VMDK files
    • Hosted virtual machines (VMs) are affected through the encryption of VMDK (Virtual Machine Disk) files, which contain the virtual hard disk.
    • Files may retain their original names, but the extension can be modified by the malware, e.g. .vmdk.locked, .vmdk.anubis, .vmdk.enc.
    • File size often remains unchanged, but the content becomes unreadable.
  2. Ransom notes on datastore
    • Presence of text or HTML files in ESXi datastores indicating the attack and payment instructions.
  3. Inaccessible VMs
    • VM startup attempts fail.
    • Common vSphere errors include: “Cannot open the disk”, “VMware cannot access the virtual disk”, or “file is encrypted”.
  4. Modification or deletion of snapshots
    • Advanced ransomware variants delete or encrypt snapshots (.vmsn, .vmsd) to prevent quick recovery.
  5. Suspicious network events
    • Massive file transfers or data exfiltration before encryption (double-extortion models).

2️⃣ Impact on VMDK files

The VMDK is the core of a VM. The impact depends on the ransomware type:
Type of impact Description
Full encryption The VM’s data is completely encrypted, making the virtual system unusable.
Partial encryption Only certain parts of the VMDK are encrypted, sometimes detectable through modified block sizes.
Metadata destruction The VMDK header or descriptor file (.vmdk) is altered or erased, preventing access even if data blocks remain intact.
Snapshot deletion vSphere restore points are lost, eliminating a quick recovery method.
Direct consequences:
  • VMs cannot be started without expert intervention.
  • Standard ESXi recovery tools fail if the VMDK header is corrupted or if encryption is strong.
Non-isolated backups (NAS, network-connected datastores) may also be encrypted if they are accessible from the compromised ESXi host.

3️⃣ Impact on data recovery

Recovery depends on several factors:
  1. Availability of offline backups
    • Offline Veeam, Veritas, or Altaro backups are critical.
    • If a backup is encrypted by the ransomware, recovery chances are low without advanced intervention.
  2. Integrity of the VMDK
    • If the VMDK is fully encrypted with a strong algorithm (AES-256 or ECIES), direct recovery is nearly impossible without the key.
    • If only the header or descriptor is affected, reconstructing the VMDK and recovering the VM may be possible.
  3. Datastore type
    • VMFS on local disk or SAN: faster recovery, since blocks are localized and accessible.
    • NFS / NAS: datastore encryption affects all hosted VM files, making recovery more complex.
  4. Recovery tools
    • Specialized tools (ScanX, R-Studio, UFS Explorer, or internal provider tools) can analyze VMDK blocks, rebuild the descriptor, and extract VM files.
    • Standard Veeam / ESXi recovery tools may fail if the header is corrupted.

4️⃣ Estimated time to recover the data

The recovery time depends on:
Factor Impact on time
Number and size of VMs The more VMs and the larger they are (VMDK > 1 TB), the longer the analysis and reconstruction process takes.
VMDK condition Intact header: 1–2 days to extract files. Damaged header: 1–3 weeks depending on complexity.
Backup availability With offline backup: fast recovery (<24h per VM). Without backup: slow and partial reconstruction.
Hardware resources Powerful recovery servers + SSD accelerate analysis. Recovery on HDD or a saturated SAN increases time.
Concrete examples:
  • 500 GB VM with intact header: ~24 to 48 hours for full restoration.
  • Partially corrupted VMDK or encrypted header: manual reconstruction + file recovery: 7 to 21 days.
ESXi with 10 VMs and a fully encrypted NFS datastore: 2 to 4 weeks depending on expertise and specialized tools.

5️⃣ Immediate post-attack recommendations

  1. Isolate the ESXi host from the network to prevent spread to other hypervisors or NAS systems.
  2. Do not restart the encrypted VMs to avoid further disk writes.
  3. Identify the ransomware if possible (file extensions, ransom note).
  4. Check the state of backups and immediately secure an offline copy.
Contact a specialized provider for post-ransomware recovery on ESXi.

6️⃣ Summary of key impacts

Element Impact
VMDK files Full or partial encryption, modified header
Snapshots Often deleted, loss of restore points
Datastore Encrypted NFS or VMFS, possible spread to all VMs
Backups Risk of encryption if accessible, major impact on recovery
Recovery time 1–2 days (intact VMDK) to 3–4 weeks (damaged header + multiple VMs)
In conclusion, a ransomware attack on a VMware ESXi hypervisor is extremely critical: VMDK files contain all of a VM’s data, and their corruption or encryption makes recovery highly complex. Rapid intervention, availability of isolated backups, and specialized tools determine the success and timeframe of the recovery process. Even with expert assistance, some data may remain unrecoverable if the attack includes a destructive module or if snapshots were deleted.