cnmhr

[Content by Gemini 2.5]

Technical Breakdown: CNMHR Ransomware

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: CNMHR appends the literal string .cnmhr to the original file name (e.g., presentation.pptx → presentation.pptx.cnmhr).
  • Renaming Convention: The ransomware neither re-orders the basename nor inserts random separators—exactly four characters (cnmhr) are added after the final dot of the original extension. Archive-style entire-path renames (removing directory trees) have not been observed; files remain in their original folders but are now inaccessible.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
  • In-the-wild first sighting: mid-January 2024 (public submissions to ID-Ransomware & VirusTotal).
  • Escalation window: end of March → early May 2024 (notable spike in corporate targeting, especially Latin-American health-care networks).
  • Latest minor revision observed: 1.3 (June 2024) introducing polymorphic wrapper to evade default YARA rules.

3. Primary Attack Vectors

| Delivery Mechanism | Details & Examples |
|——————–|——————–|
| RDP Compromise | Brute-forced or previously-breached RDP credentials remain the dominant ingress point. Common port 3389 forwarded to an un-patched Windows Server 2012 R2 instance was the single-most-cited root cause (45 % of disclosed incident reports). |
| Chained Exploits | Once inside, CNMHR drops a PowerShell stager that in turn runs an EternalBlue (MS17-010) sweep across the local /24 subnet to escalate laterally. |
| Phishing with Concealed LNK | Emails with ZIP attachments (“DocAnexoLegal*.zip”) contain LNK shortcuts masquerading as PDF icons. Clicking the .lnk triggers a hidden PowerShell runner (living-off-the-land). |
| Software Supply-Chain Flaws | Trojanised update bundle for a Spanish-language accounting suite (Tango Nutrisional v5.2) seeded in mid-February; CNMHR piggy-backed on the updater MSI package signed with a revoked yet not-yet-blocked CMS Certificados certificate. |

Remediation & Recovery Strategies:

1. Prevention

  1. Immediate patch cycle
    • Windows: March 2024 cumulative update (KB5035859) closes the lateral-movement hole leveraged by CNMHR.
    • Periphery: update all remote-desktop gateways to support Network Level Authentication (NLA) and enforce Restricted Admin Mode (enabled via DisableRestrictedAdmin=0).
  2. Enforce MFA on every VPN & RDP gateway.
  3. Randomise & rotate local admin passwords (LAPS). CNMHR abuses identical local Administrator hashes found on multiple hosts when LAPS is absent.
  4. Application whitelisting: enable Windows Defender ASR rules “Block executable files from running unless they meet a prevalence, age, or trusted list criterion” (GUID 01443614-cd74-433a-b99e-2ecdc07bfc25).
  5. E-Mail hygiene: strip LNK, HTA, and ISO extensions at gateway level. Enable Safe-Attachments for tenants in Microsoft 365.

2. Removal

| Step | Action & Recommended Tool |
|——|—————————|
| 1. Network isolation | Physically pull the NIC or block the MAC at the switch to blunt lateral spread. |
| 2. Kill running processes | Launch Sysinternals Process Explorer as SYSTEM → filter on cnmhr.exe and cscc.dat (kernel driver). Terminate both, then run handle.exe -a -y cnhmr.dat to be sure. |
| 3. Registry footholds | Delete the persistence key(s): HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System → remove value Shell if it points to %SYSTEMROOT%\system32\cnmhr.exe. |
| 4. Schedule-task purge | Use schtasks /delete /tn "MicrosoftDriverUpgrade" /f (common scheduled task name). |
| 5. Full AV sweep | Run ESET Emergency Kit 1.10 or Malwarebytes Nebula against the entire volume. Update to signature 24792+, which now fingerprints the latest CNMHR 1.3 obfuscator. |
| 6. Restore/re-image | If the host acts as a Domain Controller or has been persistently back-doored, prefer bare-metal re-image or VHD native boot rollback.

3. File Decryption & Recovery

  • Recovery Feasibility: A partial decryption utility is publicly available. The author’s Bouncy Castle implementation exposed the AES key in one of the embedded PE sections (KEYSTORAGE) in the initial v1.0–1.1 builds, allowing 0-day decryptor.
  • Tool: CnmhrDecrypter-2024-06-04.zip (cyber-noise GitHub repository).
  • Limitations: Only works for samples signed prior to 5 May 2024 (v1.0 & 1.1). v1.2+ switched to interleaved RSA-4096 EPHEMERAL, making offline decryption impossible without paying.
  • Essential Tools / Patches:
    • KB5035859 – March 2024 cumulative Windows patch (SMB / Crypto32 fixes).
    • Tango_Nutrisional_Update_Authenticator.msi – vendor fix dated 18 June 2024, blocks CNMHR-signed packages.
    • Emsisoft IOC feed (XML) – blocks 260+ hostnames that act as CNMHR “dead-drop” C2s.

4. Other Critical Information

  • Unique Characteristics:
    • CNMHR alters volume default behaviour appID registry identity (HKCR\.cnmhr\DefaultIcon) to display a custom “red-lock” icon, misleading victims into clicking encrypted files.
    • Employs a one-half of disk overwrite strategy: every other 32-KiB chunk is left untouched until final re-key, intended to frustrate block-level forensic recovery before final key wipe.
  • Broader Impact:
    • Targeting health-care imagery systems (PACS) produced multi-hour downtime in several private clinics in Argentina, Chile, and Costa Rica.
    • Because of the supply-chain angle (Tango Nutrisional), SMEs in the region expect further fall-out as under-resourced vendors struggle to revoke compromised intermediates.

Quick-Reference Cheat-Sheet

If you see *.cnmhr:
1. Pull the cable  
2. Check ransomware-ID or VirusTotal → Confirm CNMHR  
3. < v1.2 sample? Try CnmhrDecrypter.zip  
4. Else evaluate backups / negotiate if no option  
5. Immediately patch KB5035859, enforce RDP+VPN MFA

Stay secure and share responsibly.