Introduction

Ransomware on shared storage (NFS shares, SMB/CIFS shares, cloud storage like S3) is particularly devastating because a single compromised client can encrypt files accessible to an entire organization. Unlike local ransomware, shared storage attacks can encrypt thousands of files in minutes across multiple mount points. The encrypted files typically have a new extension appended and a ransom note placed in each affected directory.

Symptoms

  • Files on shared storage have unexpected extensions (.locked, .encrypted, .crypt)
  • Ransom note files (README.txt, HOW_TO_DECRYPT.html) appear in directories
  • File sizes change after encryption (often slightly larger due to encryption overhead)
  • Users report inability to open documents on the shared drive
  • Storage I/O spikes as ransomware rapidly encrypts files

Common Causes

  • Compromised user account with write access to the shared storage
  • Malware on a workstation that has the shared drive mounted
  • Vulnerable SMB/NFS server exploited for initial access
  • Phishing email delivering ransomware payload to a user with storage access
  • Unpatched vulnerability in file sharing software (e.g., SMBv1 EternalBlue)

Step-by-Step Fix

  1. 1.Immediately isolate the affected storage:
  2. 2.```bash
  3. 3.# Unmount the shared storage from all clients
  4. 4.sudo umount -f /mnt/shared-storage
  5. 5.# For NFS
  6. 6.sudo exportfs -u *:/path/to/share
  7. 7.# For SMB, stop the share
  8. 8.sudo smbcontrol smbd close-share sharename
  9. 9.`
  10. 10.Identify the source of the encryption:
  11. 11.```bash
  12. 12.# Check NFS logs for the client that made the most write requests
  13. 13.sudo grep "WRITE" /var/log/nfsd.log | tail -100
  14. 14.# Check SMB logs
  15. 15.sudo grep "write" /var/log/samba/log.* | tail -100
  16. 16.# Check which client had the share mounted at the time
  17. 17.showmount -a # NFS
  18. 18.smbstatus # SMB
  19. 19.`
  20. 20.Assess the scope of encryption:
  21. 21.```bash
  22. 22.# Count encrypted files
  23. 23.sudo find /mnt/shared-storage -name "*.locked" -o -name "*.encrypted" | wc -l
  24. 24.# Find ransom notes
  25. 25.sudo find /mnt/shared-storage -name "README*" -o -name "HOW_TO*"
  26. 26.# Check what percentage of files are affected
  27. 27.total=$(sudo find /mnt/shared-storage -type f | wc -l)
  28. 28.encrypted=$(sudo find /mnt/shared-storage -name "*.locked" | wc -l)
  29. 29.echo "Encrypted: $encrypted / $total"
  30. 30.`
  31. 31.Restore from backups:
  32. 32.```bash
  33. 33.# Mount the backup
  34. 34.sudo mount /dev/backup-volume /mnt/backup
  35. 35.# Verify backup integrity
  36. 36.sudo rsync -avn /mnt/backup/shared-storage/ /mnt/shared-storage/ | head -20
  37. 37.# Restore
  38. 38.sudo rsync -avz --progress /mnt/backup/shared-storage/ /mnt/shared-storage/
  39. 39.`
  40. 40.Preserve evidence before restoration:
  41. 41.```bash
  42. 42.# Create a forensic image of a sample of encrypted files
  43. 43.sudo tar czf /evidence/ransomware-sample.tar.gz \
  44. 44./mnt/shared-storage/encrypted-file1.docx.locked \
  45. 45./mnt/shared-storage/README.txt
  46. 46.# Save logs
  47. 47.sudo cp /var/log/samba/* /evidence/samba-logs/
  48. 48.sudo cp /var/log/auth.log /evidence/
  49. 49.`
  50. 50.Harden the shared storage against future attacks:
  51. 51.```bash
  52. 52.# Disable SMBv1 (vulnerable to EternalBlue)
  53. 53.sudo nano /etc/samba/smb.conf
  54. 54.# Add:
  55. 55.[global]
  56. 56.server min protocol = SMB2_10
  57. 57.# Implement file permission restrictions
  58. 58.sudo chmod -R 750 /mnt/shared-storage
  59. 59.# Enable SMB/NFS access logging
  60. 60.`

Prevention

  • Implement immutable backups (WORM storage) that cannot be modified or deleted
  • Use versioned storage (S3 versioning, ZFS snapshots) for rapid file recovery
  • Restrict shared storage write permissions to the minimum necessary
  • Deploy endpoint detection on all workstations that mount shared storage
  • Implement network segmentation to limit lateral movement from compromised endpoints
  • Regularly test backup restoration procedures to ensure they work when needed