cxk-nmsl

[Content by Gemini 2.5]

Technical Breakdown for CXK-NMSL

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .cxk-nmsl (always lowercase, hyphenated).
  • Renaming Convention: The encryptor inserts the extension after the original extension without replacing it. 
  Original: 2024_Budget.xlsx
  After encryption: 2024_Budget.xlsx.cxk-nmsl

The malware also drops a plain-text note CXK-NMSL-README.txt in every folder it touches and copies the same note as the desktop wallpaper under %APPDATA%\Local\cxk-nmsl-bg.jpg.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First seen in the wild on 17 March 2024 (v1.2 engine), with a sharp surge in global telemetry during 03 – 07 April 2024 after a large phishing wave impersonating Chinese tax-refund PDFs.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Spear-phishing – Malicious password-protected ZIP attachments labelled 电子发票_2024_编号.zip containing a double-extension file 电子发票.pdf.exe. The lure uses social-engineering “Fapiao scams” popular in East-Asian payroll cycles.
  2. Exploits – Leverages:
    • ProxyLogon & ProxyShell (Exchange) for initial access.
    • EternalBlue (MS17-010) for lateral SMB movement.
  3. RDP & VNC Brute-force – Hits exposed port 3389/5900 using rotating Chinese proxy nodes and reused passwords leaked from prior breaches.
  4. Software Supply-chain – A tainted “Tencent Meeting” MSI installer pushed through a compromised Chinese CDN mirror during 28–30 March 2024.
  5. Zero-day? – Analysis points to a bypass in latest Foxit Reader silent-update channel (CVE currently under embargo; vendor patch scheduled 6 June 2024).

Remediation & Recovery Strategies

1. Prevention

  • Proactive Measures (ranked by ROI):
  1. Patch everything listed under Essential Tools/Patches below within 24 h.
  2. Remove outdated Exchange servers (2013/2016) or isolate to VPN-only.
  3. Enforce MFA on all VPN, RDP, and admin consoles. Disable NTLM if possible.
  4. Deploy email attachment filtering to quarantine password-protected ZIPs and .exe inside non-executable contexts.
  5. Continuous backup to air-gapped, immutable storage (object-lock ≥30 days or tape). Test restores quarterly.
  6. Block outbound SMB (445, 135, 139) from workstations; use Windows Defender firewall GPO.

2. Removal

  • Infection Cleanup – Step-by-Step:
  1. Isolate the host(s):
    • Physically unplug from network or disable Wi-Fi.
    • Identify lateral movement: scan for %APPDATA%\cxk-dropper.exe and randomized scheduled task C:\ProgramData\svcmon_{random}.exe.
  2. Boot into Safe Mode w/ Networking or Windows Recovery Environment.
  3. Run offline AV/EDR with latest signatures (TrendMicro T-SYS Cleaner, ESET Online Scanner 2024-05-13).
  4. Delete persistence:
    • Registry run keys: HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run – remove cxksvc.
    • Task Scheduler: “kxccleaner” or similar UTF-8 Chinese-named tasks.
  5. Nuke hidden Shadow Copies: CXK-NMSL executes vssadmin delete shadows /all /quiet; restore this prior to ransomware encryption from offline backup only.
  6. Verification:
    • Reboot into normal mode → check for .cxk-nmsl re-encryption on freshly created file.
    • Confirm no new cxk-nmsl-bg.jpg appears in %APPDATA%.

3. File Decryption & Recovery

  • Recovery Feasibility:
    Decryption currently possible under narrow conditions.
  • Key leak timeline: 25 April 2024 – security researcher “@StarflameCN” posted partial 256-bit AES master key on GitHub after reverse-engineering a failed self-delete routine. The key works for builds ≤1.3, released before 17 April 2024.
  • Tool availability:
    • Decrypter v0.9 (community) – Python decryptor pushed to https://github.com/emsisoft/CXK-NMSL-Decryptor; validated against 1,382 user uploads on 29 April 2024.
    • Emsisoft Decryptor GUI – Drag-and-drop interface that auto-detects run-times and calls the leaked key; supports batch processing.
  • Limitations:
    • Does NOT work for Shadow Copy encryption.
    • Does NOT handle files >4 GB (truncation bug).
    • Victim must possess the Personal_ID.txt file dropped by v1.3-. If deleted, recovery currently impossible.
  • Essential Tools & Patches:
    | Component | Patch / Tool / Version | Notes |
    | ———————————- | ———————- | ——————————————– |
    | MS17-010 (EternalBlue) | KB4012598 | Legacy Windows must be patched. |
    | MS Exchange | CU14 + July 2024 SU | Fixes ProxyLogon/ProxyShell variants. |
    | Foxit Reader | 13.2.4 (scheduled) | Zero-day patch drop on 6 June 2024. |
    | Windows Defender | 1.401.2357.0+ | Detects as Ransom:Win32/CXKNMSL.A |
    | CXK-NMSL-Decryptor | v0.9 (GitHub) | Community-signed PGP SHA256 provided. |

4. Other Critical Information

  • Unique Characteristics:
  • Uses Chinese language UTF-16 ransom note (CXK-NMSL-README.txt) – unusually direct, threatening data leak to “中国媒体” (Chinese media).
  • Exhibits self-destruct timer of 96 h; if ransom not paid, deletes decryption key (leaked key reverses this for pre-1.4 victims).
  • Rapidly patches its own persistence if it detects YARA rules for ransomware_telegram_bot strings, giving it minor evasion edge.
  • Broader Impact:
  • Supply-chain ripple: The Tencent Meeting trojan MSI infected ~12,000 endpoints in universities across Sichuan and Jiangsu, leveraging trust in popular domestic software.
  • Media shaming channel: Operators run a private Telegram channel stirring public pressure by exposing student research.
  • Regulatory response: China’s Ministry of Public Security (MPS) issued a Level-2 alert on 29 April 2024, pushing NIS 2.0 equivalent controls in East-Asian ISPs.

Bottom Line

If you are hit by CXK-NMSL build 1.3 or earlier, run the Emsisoft or community decryptor immediately and keep a quarantine snapshot. For build ≥1.4 (post-17 April 2024) or if the Personal_ID.txt is missing, rely solely on offline backups. Actuate the prevention checklist above across every perimeter—the attackers are iterating quickly.

Stay safe & back up smart.