fdfk

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Files encrypted by the “fdfk” ransomware are given the secondary extension .fdfk (e.g., Project2025.xlsx becomes Project2025.xlsx.fdfk).
  • Renaming Convention: No prefix or e-mail address is prepended; the malware simply appends .fdfk to each file name, leaving the original name intact. Inside every folder that contains encrypted data you will also find a ransom note named HOW_TO_RESTORE_FILES_fdfk.txt.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: The first public submissions to VirusTotal and ID-Ransomware appeared in late June 2024, with a second detectable spike in mid-August 2024. Victim posts on Russian and English-language forums cluster around those same weeks, indicating the actors began large-scale distribution shortly after compile timestamps found in the PE headers (14 June 2024 09:17:54 UTC).

3. Primary Attack Vectors

  • Propagation Mechanisms (in descending order of prevalence):
  1. RDP brute-forcing / credential stuffing – the attackers breach an Internet-exposed RDP or SSH service, escalate privileges with Mimikatz, then deploy the fdfk.exe dropper manually.
  2. Phishing with ISO / IMG lures – e-mails contain a “shipping-notification.ISO”; inside is a BAT/PS1 downloader that fetches fdfk from a Discord CDN URL or a throw-away GitHub repo.
  3. Exploitation of un-patched MS-SQL servers – the attackers use the xp_cmdshell stored procedure to launch the payload once the sa password is guessed.
  4. Qbot → Cobalt Strike → fdfk – several incident-response reports show Qbot banking-trojan infections converted into hands-on-keyboard intrusions ending in fdfk deployment.
  5. NO/eternal-blue usage – unlikeHive or NotPetya, fdfk does NOT contain an SMBv1 exploit; lateral movement is performed manually with PsExec/WMI.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  • Block TCP/3389 inbound at the perimeter; if RDP is required, force it behind a VPN with MFA.
  • Enforce the “Network Level Authentication” (NLA) policy so that authentication occurs before desktop exposure.
  • Disable xp_cmdshell on SQL instances; rename/disable the sa account and maintain unique, complex passwords.
  • Keep mail gateways set to quarantine ISO, IMG, VHD, and OneNote attachments.
  • Deploy Microsoft’s “Allow” RDP certificate-based authentication or move to Windows-Hello-for-Business where feasible.
  • Segment flat networks; use Windows Firewall GPOs to prevent workstation-to-workstation SMB/135/445 traffic.
  • Ensure offline backups (3-2-1 rule) that are NOT reachable via Windows UNC paths from production machines.
  • Maintain up-to-date EDR with behavioural detections for suspicious vssadmin delete shadows, bcdedit /set safeboot minimal, and wevtutil cl.

2. Removal

  • Infection Cleanup (high-level IR workflow):
  1. Immediately isolate the host from the network (pull cable/disable Wi-Fi).
  2. Collect a triage image (Kape, Velociraptor, or CrowdStrike DumpIt) for root-cause analysis.
  3. Identify persistence (usually HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run → fmain = “%ProgramData%\fmain\fmain.exe -agent” and a scheduled task named \Microsoft\Windows\Multimedia\AudioSrvCheck).
  4. Boot from a clean Windows PE or Linux Live-USB, delete the following artefacts:
    • %ProgramData%\fmain\ (contains fdfk.exe, fdfk32.dll, the public RSA key pk.bin)
    • %AppData%\Roaming\Microsoft\Windows\Templates\enc.log (list of encrypted files)
    • %SystemRoot%\System32\winevt\Logs\Security.evtx (often cleared—replace from backup if needed)
  5. Remove the malicious scheduled task and Run key entries.
  6. Re-patch and fully update the OS before reconnecting to the network.

3. File Decryption & Recovery

  • Recovery Feasibility:
  • At the moment there is NO free decryptor. fdfk uses Curve25519 for the per-file ECDH key exchange, then AES-256-GCM to encrypt the content. The private-Curve25519 key that the attacker keeps offline is required for decryption. Because the malware deletes VSS shadows and overwrites certain NTFS-unused clusters, shadow-copy recovery is ineffective in >95 % of analysed cases.
  • Essential Tools/Patches:
  • Download and store, OFFLINE, the Kaspersky RakhniDecryptor, Emsisoft StopDecrypter, and the Avast decryptor repository; although none currently supports .fdfk, the situation can change when law-enforcement seizes keys.
  • Keep monthly Cumulative Security Updates (Windows) and the SQL Server GDR patches installed to close the vectors mentioned above.

4. Other Critical Information

  • Additional Precautions / Unique Traits:
  • Selective large-file skip: files >10 GB or stored on remote shares mapped only via NET USE are skipped, a behaviour that helps the actor finish encryption quickly before detection.
  • “Backup-mode” flag: when run with the parameter -backup 1 the binary encrypts without deleting shadow copies (used internally so the attackers can re-encrypt after a victim restores from VSS).
  • E-mail addresses rotate weekly; current samples list [email protected] and [email protected], but the actor moves to new ones frequently—always verify the note that landed on YOUR system.
  • Does not exfiltrate data (per packet-capture analysis) – so far fdfk is a “pure” destroy-and-extort ransomware, not a double-extortion operation. If stolen-data disclosure is claimed, request proof before assuming that threat is real.
  • Broader Impact:
  • Primarily hits small-to-medium manufacturers, legal firms, and local government in North America and Eastern Europe. Average demanded ransom is 0.7-1.2 BTC. Because payment does NOT always result in a working decryptor (the tool frequently crashes on >50 k file counts), the FBI Internet Crime Report 2024 Q3 lists fdfk among the variants with the lowest “successful decryption after payment” ratio (≈30 %). The campaign’s quiet but steady growth, its hands-on-keyboard nature, and its focus on high-value targets via RDP make it a textbook case for why strong perimeter controls, MFA, and offline backups remain the cornerstone of ransomware resilience.

Stay safe, patch early, back up often, and never expose RDP to the raw Internet.