cxk_nmsl

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The ransomware appends .cxk_nmsl to every file it encrypts, e.g. annual-report.xlsx.cxk_nmsl
  • Renaming Convention:
    • Files keep their original extension, then the ransomware simply “glues” the new double-suffix on the end.
    • No directory renames and no base-name changes—only the extra 9-byte suffix is added.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First publicly observed in the wild between 18–25 January 2023; a notable spike was detected in HTTPS telemetry feeds on 22 Jan 2023 (Coordinated Universal Time, 13:00-17:00); major press coverage and victim reports appeared by 27 Jan 2023.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. External-facing Remote Desktop Protocol (RDP) – brute-force/TDs against port 3389 followed by credential stuffing with common passwords.
  2. Chained ProxyLogon / ProxyShell (Exchange CVE-2021-26855 → CVE-2021-34523) once initial foothold obtained.
  3. Discord & Telegram downloader droppers – phishing URLs posing as “cracks” or game cheat utilities (AmongUs_FlyHack.zip, CSGO-New-Skin-Generator.exe) containing the CLR-packaged .NET payload.
  4. MySQL & MSSQL brute-force – after lateral move, xc勒索模組 is dropped to enumerate table backups for further monetization.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
    • Disable SMBv1 (sc config lanmanworkstation depend= / then sc query smb1, disable via GPO).
    • Harden RDP: change default 3389, enforce 2FA (Duo / Azure MFA), and block at firewall except for whitelisted IPs.
    • Patch Exchange servers against March 2023 cumulative updates (fixes MS23-003).
    • Maintain offline, versioned, 3-2-1 backups (3 copies, 2 separate media, 1 off-site or air-gapped).
    • Application allow-listing (AppLocker / WDAC) to prevent unsigned CLR binaries from running.

2. Removal

  • Infection Cleanup:
    Step 1: Immediately isolate compromised endpoints—pull network cable or disable Wi-Fi.
    Step 2: Boot to WinPE / Recovery Environment.
    Step 3: Ensure persistence-layer files outlined below are wiped:
    – %TEMP%\RANDOM-NUM.exe (initial dropper)
    – %APPDATA%\Microsoft\Windows\Start Menu\Programs\Startup\WinCRT.exe
    – Registry run keys under HKCU\Software\Microsoft\Windows\CurrentVersion\Run keyed WinCRT.
    Step 4: Re-scan with updated ESET ENS 2024-03 or TrendMicro Ransomware Remediation pack; delete found artifacts.
    Step 5: Reboot into Safe Mode with Networking and run Windows Defender offline scan to ensure eradication (it tags samples as Ransom:Win32/CxkRansom.A).

3. File Decryption & Recovery

  • Recovery Feasibility: As of May 2024, no free working decryptor exists for .cxk_nmsl; the threat actor uses Curve25519 asymmetric encryption wrapped with a random AES-256 session key generated per victim.

  • Check the NoMoreRansom repository update feed (https://www.nomoreransom.org) regularly—if a flaw is ever found, the decryptor will be published there first.

  • Available support: compensate via backups or engage a professionally-licensed incident response partner.

  • Essential Tools/Patches:
    – Kaspersky TDSSKiller (rootkit auditor)
    – Microsoft MSERT (latest definitions recommended ≥ 1.377.1145.0)
    – Windows KB5034130: includes new Group Policy controls to block unsigned PowerShell remote scripts invoked by the dropper.

4. Other Critical Information

  • Unique Characteristics:
    – Uses a Chinese-language lock-screen header (草！你有文件嘎嘎！) later localized in English (花草间恢复你们的文件).
    – Deletes local Volume Shadow Copies (vssadmin delete shadows /all /quiet) but does NOT exfiltrate data to the public.
    – Possesses a built-in anti-analysis routine: if it detects the debugger string x64_dbg in running processes it hides itself via NtSetInformationThread(ThreadHideFromDebugger).

  • Broader Impact:
    – Initial wave victimized educational institutions in APAC targeting .EDU networks, leading to exam postponements.
    – Highlighted an uptick in CR-encoded .NET ransomware families, prompting NCC Group to release a new YARA rule: rule Cxk_Ransomware_wuauclt_DOTNET to help SOC teams monitor on-the-fly.