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VMware ESXi encrypted by a ransomware

1️⃣ Symptoms of an ESXi infection by ransomware

When a VMware ESXi hypervisor is infected by ransomware, several distinct signs appear:
  1. Encryption of VMDK files
    • Hosted virtual machines (VMs) are impacted by the encryption of VMDK (Virtual Machine Disk) files, which contain the virtual hard disks.
    • Files may keep their original names, but the ransomware may add or change the extension, e.g., .vmdk.locked, .vmdk.anubis, .vmdk.enc.
    • File size often remains unchanged, but the content becomes unreadable.
  2. Ransom notes on the datastore
    • Presence of text or HTML files in ESXi datastores indicating the attack and the ransom demand.
  3. Inaccessibility of VMs
    • Attempts to start VMs 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 may delete or encrypt snapshots (.vmsn, .vmsd) to prevent quick rollback.

2️⃣ Impact on VMDK files

The VMDK is the core of a VM. The impact depends on the type of ransomware:
Type of impact Description
Full encryption The VM data is completely encrypted, making the virtual system unusable.
Partial encryption Only certain parts of the VMDK are encrypted, sometimes detectable by changes in block size.
Metadata destruction The VMDK header or descriptor (.vmdk file) is modified or deleted, preventing the VM from opening even if data blocks are intact.
Snapshot deletion vSphere restore points are lost, removing a quick recovery option.
Direct consequences:
  • VMs cannot be started without specialized intervention.
  • Standard ESXi recovery tools fail if the VMDK header is corrupted or if strong encryption is used.
  • Non-isolated backups (NAS, network-connected datastore) may also be encrypted if accessible from the compromised ESXi host.

3️⃣ Impact on data recovery

Recovery depends on several factors:
  1. Integrity of the VMDK files
    • 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 the descriptor is affected, it is possible to rebuild the VMDK and recover the VM.
  2. Type of datastore
    • VMFS on local disk or SAN: faster recovery since data blocks are localized and accessible.
    • NFS / NAS: encryption of the datastore affects all hosted VM files, making recovery more complex.
  3. Recovery tools
    • Standard tools such as Veeam / ESXi Recovery may fail if the header is corrupted.

4️⃣Immediate post-attack recommendations

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

5️⃣Summary 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 the entire data set of the virtual machine, and their corruption or encryption makes recovery highly complex. Rapid intervention, the availability of isolated backups, and the use of specialized tools are the key factors that determine the success and the time required for recovery.