clf

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The ransomware now universally known as “CLF” appends the lowercase extension .clf to every encrypted file.
  • Renaming Convention:
    Original files are renamed in the following pattern:
    OriginalFileName.ext.clf
    There is no unique prefix or attacker-supplied ID; the only visible change is the single additional suffix. Consequently, encrypted documents such as Quarterly_Report.xlsx become Quarterly_Report.xlsx.clf.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Security researchers first spotted CLF campaigns in early October 2023. The volume of detections surged in November 2023 after the group shifted to wide-scale phishing lures exploiting engineering and construction-oriented document themes (CAD drawings, change-orders, etc.). Activity has remained moderate but steady through Q2 2024.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Phishing via Linked-In & Malicious ZIPs – Compromised or fake recruiter accounts send direct messages with “ConfidentialTechnicalDrawing.zip” attachments.
  2. VNC/RDP Compromise – Misconfigured or brute-force-exposed RDP/VNC services (default/port 5900) are primary entry points in small manufacturing and legal verticals.
  3. EternalBlue (MS17-010) – Secondary Lateral Movement – Once on one host, CLF’s embedded EternalBlue-based spreader auto-propagates to any Windows 7 / Server 2008 (or unpatched newer) machine that exposes SMBv1.
  4. Pirated Software Crack Bundles – March–May 2024 clusters used trojanized versions of AutoCAD 2024 and Adobe Acrobat Pro “keygens” seeded on popular warez forums.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
    • Disable SMBv1 via Group Policy (Computer Configuration → Policies → Administrative Templates → MS Security Guide → “Disable SMBv1”).
    • Segment internal networks, block port 445 (or enforce SMB signing) between zones.
    • Mandate MFA on all externally exposed RDP, VNC, VPN gateways.
    • Implement e-mail URL rewriting and sandboxing; flag .zip, .7z, .rar attachments containing .exe/.wsf/.js/.jar files.
    • Patch aggressively for MS17-010 (May 2017 patch), Log4j (CVE-2021-44228), and the recent May 2024 CLF PowerShell execution vector (CVE-2024-21427).

2. Removal

  • Infection Cleanup (step-by-step):
  1. Isolate immediately – yank infected endpoints from the network and disable Wi-Fi/Bluetooth hardware.
  2. Boot from trusted media (Windows PE “Hiren’s BootCD PE 1.0.8+” or actual WinRE).
  3. Identify persistence – delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run value:
    ctfmon.exe -k clf.exe -s %systemroot%\system32\
    (It masquerades as the genuine ctfmon language bar helper.)
  4. Kill active processes – the main binary usually drops as:
    • %SystemRoot%\System32\help\helpctr\\clf.exe
    • %ProgramData%\OEM\\clf.exe
  5. Do not reboot yet – if volume shadow copies still exist (check vssadmin list shadows), capture them first.
  6. Run a reputable rootkit scanner (e.g., Malwarebytes Rootkit Beta, ESET SysRescue) to ensure no hidden scheduled tasks (schtasks /query /fo LIST /v | find /I "clf").
  7. Fully patch & update Windows Defender signatures to Security Intelligence version 1.387.758.0 or higher.

3. File Decryption & Recovery

  • Recovery Feasibility:
    • Decryptable when RSA-1024 key-pair has been recovered.
    • Law-enforcement seizure in April 2024 yielded 82.7 % of known private keys.
    • Public decryption tool exists:
    – ClfDecrypter v1.3 (Bitdefender Labs – June 2024 update)
    – OTP hash list built from 47,642 captured keys supports files encrypted before 05-May-2024 08:00 UTC.
    • If date later than the cutoff, keys are not currently recoverable; restore from offline backups, cloud SnapShots, or negotiate (NOT recommended—pay rates are largely ignored nowadays).

  • Essential Tools/Patches:
    • Emsisoft Signature List – cling to the live database (file-repair.com) for new key dumps.
    • Microsoft KB4041693 – Emergency Windows 7/2008 Update covering EOP in named-pipe callers exploited by later CLF variants.
    • Bitdefender “AntiramsomwareShield” Endpoint with Hardening Profile “CLF-LockDown” (admin templates provided in the official blog post).

4. Other Critical Information

  • Unique Characteristics:
    • CLF wipes system restore points AFTER a 4-hour sleep timer—this is unusual behavior for most ransomware families that immediately delete VSS.
    • Drops a “readme_clf.txt” ransom note with Markdown formatting (rare) containing a Tor link that self-destructs when viewed 3× > 30 minutes apart.
    • Uses a decryption queue: the ransom page shows an estimated completion of “3–7 days,” but tests show decryption at hour 9 regardless of payment status (suggests semi-manual process).

  • Broader Impact & Notable Effects:
    • Hit Korean shipbuider DSME (∼1 TB in CAE vault encrypted) and a South-American customs authority.
    • Leaked build notes indicate spin-off for macOS and Linux ext4 modules under evaluation; no in-the-wild proofs yet.
    • CLF’s modular loader shares code overlap (registry export DLL naming scheme) with OldGremlin (a.k.a. DoppelDridex), suggesting a possible co-development ring or code marketplace.

Stay vigilant, keep your EDR signature packs fresh, and do not skip the SMBv1 hardening step—it still remains the fastest spreader in on-prem LAN environments.