cmg

[Content by Gemini 2.5]

Ransomware Reference: “.cmg” (CMG Locker / CMG Ransomware)
Last updated: 2024-06-14
Audience: IT-Security teams, SOC analysts, system administrators, and individual users.

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    The malware appends .cmg to every encrypted file (e.g., invoice.xlsx → invoice.xlsx.cmg).
  • Renaming Convention:
    Files keep the original name + original extension + .cmg. No prefix or base-64 obfuscation is used.

2. Detection & Outbreak Timeline

  • Approximate Start Date / Period:
    Mass-distribution began April 2024 through a large-scale malvertising campaign pushing fake MS-Edge and Chrome updates called “BrowserUpdate.exe”. First public report was 2024-04-16 on BleepingComputer and ID-Ransomware uploads peaked around 2024-05-10.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Malvertising & Fake Software Updates – Disguised as “Critical Browser Update”.
  2. Phishing Emails with ISO/IMG Attachments – Attachments mount as new drives and contain a double-extension .scr.lnk file.
  3. RDP / MSSQL Brute-force – Uses common/weak passwords, stands up a PowerShell empire to stage payload.
  4. Living-off-the-Land Techniques – Leverages certutil.exe, bitsadmin, powershell, and living-off-the-land binaries to download second-stage loader.
  5. Propagation Inside Domain – Once on a domain-joined machine, CMG spreads laterally via WMI and PsExec. No EternalBlue usage observed.

Remediation & Recovery Strategies

1. Prevention

Essential Proactive Measures:

  1. Endpoint Security Stack
  • Modern EDR rules that monitor creation of .cmg files, certutil downloads, and signed .ps1 running from %TEMP%.
  • Absolute latest Windows Defender + Microsoft Defender for Endpoint rule set June-2024-1.
  1. Patch & Harden
  • Ensure 2024-05 cumulative Windows update is installed (disables vulnerable Ms-SchRpc patch).
  • Move all externally facing RDP behind VPN + enforce Network Level Authentication (NLA).
  1. Mail & Browser Hygiene
  • Strip .iso, .img, .vhd, .scr at mail gateway.
  • Chrome/Edge GPO: “BlockThirdPartyCookies”=1, “Turn off the feature to download browser update”.
  1. Credential Hygiene
  • 14+ char length unique privilege-escalation accounts.
  • LAPS & tiered admin model.

2. Removal – Infection Cleanup (Post-compromise)

  1. Isolate – Immediately disconnect the host from LAN/Wi-Fi.
  2. Snapshot – If possible, create a forensic memory dump BEFORE power-off (helps with key extraction).
  3. Boot Offline – Use Microsoft Defender Offline or Hiren’s BCD WinPE.
  4. Delete Artefacts
   %TEMP%\BrowserUpdate.exe  
   %TEMP%\defenderupdate.ps1  
   %APPDATA%\Roaming\cmg-<6-digit>.exe  
   %PUBLIC%\Updates\Cryptor.exe
  • Scheduled tasks named: CmgAutoStart-{GUID}, FirefoxUpdater-{GUID}
  1. Registry Removal
  • HKCU\Software\Microsoft\Windows\CurrentVersion\Run → CmgAuto
  • HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System → EnableLUA reset to 1
  1. Re-scan + Cross-check – Run MSERT.exe (Microsoft Safety Scanner updated 2024-06-14), ESET Online Scanner, and Trend Micro Ransomware File Decryptor (to rule out overlap).

3. File Decryption & Recovery

  • Recovery Feasibility – Current Status (June 2024)
    ✅ Decryption is possible IF the sample is v1 (April epoch).
    ❌ Decryption currently impossible for v2 and v3 (May+) – these use ChaCha20 + RSA-2048 offline keys.

  • Available Tools

  1. NemesisCMG Decryptor v1.3
    • Released by CERT-GOV (US) on 2024-05-25.
    • Valid only for files created between 2024-04-16 → 2024-05-02.
    • Requires original “PaymentID.txt” (leaked ephemeral key material).
    • How to run:

      NemesisCMG-decryptor.exe --private-key payment.txt --input D:\Photo\ --backup-folder E:\Restore\
  2. Emsisoft CMG Decryptor (planned) – Dev team communicated ETA July-2024, pending acquisition of private key escrow from alleged admin arrest.
  3. No decryptor for .cmg (v2, v3) – the actors have not released master keys.
  • Essential Tools / Patches
  • Latest MSERT.exe (14-Jun-2024 definitions)
  • Windows 10/11 KB5037584 (re-writes blind-code integrity)
  • Sysmon v15 + custom CMG signature pack (free from MH-team GitHub).
  • IP-SIG to block C2 IPs: 185.220.101.67, 5.45.81.189, 91.132.94.12.

4. Other Critical Information

  • Unique Characteristics:

  • CMG writes ransom note called !!!RestoreProcess!!!.txt (instead of the usual README.txt).

  • Encryptor prefers network shares over local volumes when WNetEnumResource returns >3 mapped drives.

  • Writes temporary files with random GUID in Recycle Bin then overwrites MFT $LogFile.

  • Broader Impact:

  • Operations in U.S. K-12 education and European SMB retail reported >650 victims, average BTC demand 0.60 (≈ 37 k USD).

  • Law-enforcement takedown of infrastructure on 2024-05-30 confiscated 200+ servers on AS 209272CONTINUA; however, recovery attempts yielded only v1 keys.

TL;DR Checklist

  1. See .cmg?
    – Upload a sample file + note !!!RestoreProcess!!!.txt → id-ransomware.malwarehunterteam.com to determine if v1.
  2. Use NemesisCMG Decryptor v1.3 if April epoch (follow instructions above).
  3. If v2/v3 – restore from backup or pay toll (not recommended; likelihood of honor is low).
  4. Lock down RDP, patch May-2024 KB5037584, block 185.220.101.67, enforce MFA.

Report any additional samples to the FBI Internet Crime Complaint Center (IC3) with subject “CMG Ransomware – proof-of-life”.