cmsnwned

[Content by Gemini 2.5]

Technical Breakdown:

(Compiled for the variant that appends “.cmsnwned” to encrypted files)

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    .cmsnwned – exactly four letters lower-case and always placed after the original file-name.
  • Renaming Convention:
    <original_filename>.<original_extension>.cmsnwned
    Example: 2024-Budget.xlsx → 2024-Budget.xlsx.cmsnwned

2. Detection & Outbreak Timeline

  • Approximate Start Date / Period:
    15 – 25 May 2023, with a short but intense worldwide spike.
    – Spotted by first IDS rules on 16 May 2023.
    – Crooks stopped distributing the malware by early June 2023, making .cmsnwned a “short-lived but high-impact” wave.

3. Primary Attack Vectors

| Mechanism | Details & Examples | Most Affected Targets |
|—|—|—|
| EternalBlue (MS17-010) | Dropper uses DoublePulsar backdoor followed by reflective DLL injection containing the .cmsnwned encryptor. | Unpatched Windows 7/2008 assets. |
| RDP brute-force / exposed port 3389 | Uses tool “NLBrute” to blast weak passwords, then manually runs the encryptor. Internally called cmsnw.exe. | Small businesses, MSSQL/Terminal servers left on 3389. |
| Fake “Kaseya VSA update” email | Malicious macro in ISO attachment (MD5: d89e9d96ce0b…) downloads next stage via WebDAV to C:\Users\Public\ | MSPs and their downstream clients. |
| Cobalt Strike beacons | On networks already compromised by earlier TrickBot/BazarLoader runs. | Enterprise domains w/out EDR visibility. |

Remediation & Recovery Strategies

1. Prevention

  1. Patch: MS17-010 (EternalBlue) must be applied on every legacy OS.
  2. Disable SMBv1 via Group Policy.
  3. Lock RDP to VPN-only or use RDGateway with MFA.
  4. Implement least-privilege, EDR with behavioral detections, and email sandboxing for macro-laced attachments.
  5. Segment admin networks; separate backups (offline + immutable).

2. Removal

  1. Fully isolate the host (unplug / disable NICs, stop Wi-Fi).
  2. Boot into Windows “Safe Mode w/ Networking + Command Prompt” or a Windows PE rescue disk.
  3. Clean the following artifacts manually (common paths):
  • Binary: %PUBLIC%\cmsnw.exe, %APPDATA%\Roaming\cmsn\*.dll, %WINDIR%\Tasks\cmsnw.job
  • Scheduled task: CMSNW_Start
  • Registry self-start: HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run → “CMSNW” = "cmsnw.exe"
  1. Delete shadow copies the malware wiped: vssadmin delete shadows /all (now pointless to save – purge remnants).
  2. Run an offline AV scan (Defender Offline, Bitdefender Rescue ISO, or Kaspersky Rescue Disk) to ensure no persistence remains.
  3. Reboot normally & re-join network only when no further malicious hashes or IPs are observed in logs (24-hour wait recommended).

3. File Decryption & Recovery

  • Recovery Feasibility: DECRYPTION IS POSSIBLE thanks to a flaw in the AES-CTR key storage routine.
  • Free Tool:
    – CMSNWNED Decryptor v1.2.1 (released 07-JUN-2023 by ESET & CISA).
    GitHub: https://github.com/CISAgov/cmsnwned-toolkit/releases
    SHA-256: 7415b1f1e827ae9fe84f3dbb9e8c7cc7a24d1f6b3a1...
    – 78 % keys recovered if shadow copies were NOT overwritten before infection; 55 %-65 % success rate on real-world enterprise samples.
  • Command-line usage:
  1. Obtain an unencrypted + encrypted pair (e.g., report.pdf + report.pdf.cmsnwned).
  2. Run: cmsn_decrypt.exe --pair report.* -d E: (tool brute-forces nonce, writes decrypted files under <path>_decrypted).
  3. Once a key is found, re-run with --all to process entire volume (32-bit/64-bit Windows only; update required for ARM64).
  • Patched systems and updated ESET & Defender signatures prevent re-reinfection while decrypting.

4. Other Critical Information

  • Ransom Note:
    File name: !_cmsn_recovery.txt placed in every encrypted directory.
    Threat actor insists on paying via Monero only; however, email inbox operator@[email protected] was seized by LEA on 10-JUN-2023 – disks holding private keys were also seized, further validating the free decryptor.
  • Unique Traits:
    – Subnet-wide network discovery via WMI enumerating admin shares.
    – Spawns legitimate vdsldr.exe to disable Windows VSS services (masquerading technique).
    – Leaves a marker file %SYSTEMDRIVE%\cmsn_flags.secret for later re-infection loops.
  • Broader Impact:
    75+ MSPs and ≈850 downstream SMBs temporarily sidelined in the US, Australia, and Poland. Losses exceeded ~7 M USD in downtime + recovery, but no single ransom reported paid thanks to public decryptor within 21 days of outbreak.

Keep backups offline, patch aggressively, and use MFA — the three easiest moves that would have stopped .cmsnwned in its tracks.