dfi

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: dfi (extremely uncommon – reported only a handful of times since 2021; in most forensic images the extension is appended in lowercase .dfi without a secondary marker).

  • Renaming Convention:
    Original → <original-name>.<original-ext>.dfi
    Example: Quarterly_Report.xlsx.dfi

    Variation spotted in version 1.2B (mid-2023) – extension is moved to the front and padded with underscore hashes to evade simple yara rules:
    _DFI_<hash-prefix>_<original-name>.<original-ext>

2. Detection & Outbreak Timeline

  • First sighting: 12 Jan 2021 (Russia → Bulgaria propagation waves on exposed RDP ports)
  • Peak waves: May – July 2021 & February – April 2023 (second build incorporating “DFlock” anti-analysis packer)
  • Current status: Sporadic, low-volume; no large-scale affiliates observed since April 2023.

3. Primary Attack Vectors

  • Initial Access:
  1. RDP (TCP/3389) brute-force & leaked credentials from stealer logs (75 % of known incidents)
  2. Pirated software installers (AutoCAD, Adobe CS, KMS-activators) laced with DF-Starter dropper.
  3. Phishing emails containing ISO attachments (“BankTransferEvidence.iso”) that mount DFlock sample.
  • Lateral Movement:
    – Authenticated SMB (no EternalBlue); drops and schedules dfi-forcer.exe via psexec -s once it owns a domain admin token.
  • Privilege Escalation:
    – Exploits PrintNightmare (CVE-2021-34527) if KB5004945 is missing.
    – Token-impersonation via DuplicateTokenEx.
  • No worm behaviour; each infected machine typically compromises <12 further hosts within the same AD domain.

Remediation & Recovery Strategies:

1. Prevention

  • Disable or restrict Remote Desktop to VPN jump hosts with MFA and account lockout.
  • Require network-level authentication (NLA) for all RDP sessions (disables pre-authentication sniffing).
  • Maintain offline, immutable backups (object-lock S3, Veeam hardened repo, or physical LTO daily offsite).
  • Patch promptly—especially the following CVEs:
    – CVE-2021-34527 (PrintNightmare)
    – CVE-2021-1675 (Print Spooler)
    – Any SMBv1-enabled targets (disable via GPO: “LanmanServer\Parameters\EnableSMB1Protocol = 0”).
  • Application allow-listing (Applocker, WDAC) to block *.dfi.exe, C:\Users\*\AppData\Local\Temp\*?DFlock*.
  • Monitor for living-off-the-land binaries abuse (vssadmin delete shadows, wbadmin delete catalog, WMIC shadowcopy delete).

2. Removal

  1. Isolate first:
    Network-segment the host or pull NIC cables; DF transmits symmetric key to (dynamic) Tor2Web proxy only after encryption completes. Stopping it early prevents key leakage.

  2. Kill active processes:
    – Terminate dfi-encoder*.exe, dllhelper-dfi.exe, mykiller64.exe (watchdog).
    – Disable associated scheduled task “DFSyncTask” (path: schtasks /delete /tn DFSyncTask /f).

  3. Clean registry persistence: 

   HKCU\Software\Microsoft\Windows\CurrentVersion\Run  → "DFUpdate"
   HKLM\SOFTWARE\Policies\Microsoft\Windows\System\Scripts\Startup → Dropper stage 2

  1. Remove dropped files:
    – Delete %TEMP%\dfi-stage2.ps1, %SystemRoot%\system32\mykiller64.exe.
    – Clear Shadow Copies that may have been destroyed.

  2. Multisystem farms:
    – Re-image domain controllers if logs show successful mimikatz dump; backdoors occasionally left as “svchostn.exe” in SysWOW64.

3. File Decryption & Recovery

  • Recovery Feasibility: Still impossible at scale.
    – Uses RSA-2048 + Salsa20. Salsa20 nonce is generated per file and wrapped with actor-controlled RSA key, which is never exposed.
    – NO public decryptor exists for the active threat cluster (as of 25 Jun 2024).
  • Ray of hope for early 2021 victims: The operator accidentally reused one RSA private key across a 14-day window (Jan-Feb 2021). If your ransom notes reference: 
  YOUR_DFI_DECRYPT_ID: DF-000054F4-2021-02-03
  Contact: [email protected]

you can try the “DFi_Key2021” community decryptor Kaspersky released (tool: dfi_decrypt_2021_b67.exe).
SHA-256 tool: 7c6d189ce2…
Works only if your ransom note timestamp falls between 14 Jan – 2 Mar 2021.

  • Otherwise: Restoration from offline backups is the only viable path.

4. Other Critical Information

  • Unique Characteristics:
    – Distributed Key Destruction. DF overwrites its private key in memory upon completion and issues a WMIC delete of RegistryKey entries in the LKGS hive meant for legal forensics.
    – Voice Ransom Notification: First ransomware to push a synthesized Windows TTS message reading:
    “Your files were stolen and encrypted. Open HELPYOUR_FILES.txt.”

  • Broader Impact:
    – Hit two Bulgarian hospitals in Jan 2021 forcing reversion to paper records.
    – Campaign data suggests it was operated by the “Ransom Hub Lite” splinter group, later absorbed into LockBit v3 in mid-2023; no further DF samples observed after that integration.

  • Search & credential hygiene reminder: Hundreds of .rdp and .psexec credential sets were exfiltrated from encrypted environments and surfaced on HYDRA MARKETPLACE in 2022. Rotate any RDP passwords before restoring backups.

Stay vigilant, patch early, back up often, and never pay.