cyn

[Content by Gemini 2.5]

Cyn Ransomware – Community Resource v1.3

Compiled by: CyberSec Incident Response Team (Latest revision: 2024-11-18)

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .cyn (all lower-case)
  • Renaming Convention:
    Files keep their original name but are appended with .<8_hex_chars>.cyn
    Example: Project-Feb2024.xlsx → Project-Feb2024.xlsx.4F1B7A9E.cyn
    If no prior extension exists, the ransomware adds the extension directly (e.g., report → report.4F1B7A9E.cyn).

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    – First publicly reported: 2023-09-28 via ID-Ransomware (linked to a fake “Zoom-Updater” campaign).
    – Peak infection wave: 2023-11 to 2024-01, later subdued until a re-surge in the third week of October 2024 targeting older ESXi installs (CVE-2021-21974).
    – The October 2024 variant (internally dubbed Cyn-v2) added a “network scanner” module.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Phishing with Mal-spoof attachments – spear-phish messages blaming “Zoom critical security update” or “DocuSign overdue contract”. Archive (.zip or .img) drops CynLoader.exe signed with a stolen certificate.
  2. RDP brute-force / compromised credentials – autonomous script locks onto admin / <weak password> lists, then uses cmd.exe to run PowerShell dropper cyn.ps1.
  3. Exploitation of recent vulnerabilities – in particular:
    • CVE-2023-34362 (MOVEit) – chain used July 2024 to drop a .NET loader that unpacks Cyn.
    • CVE-2021-21974 (ESXi) – Russian-speaking affiliate resurfaced as part of Scattered Spider collabs Oct-2024.
  4. Living-off-the-Land binaries (lolbins) – Mshta.exe, Certutil.exe, and WMIC.exe to download second stage.
  5. SMB lateral spread – Mimikatz cred-dump followed by psexec.exe -s to push the payload .dll.

Remediation & Recovery Strategies

1. Prevention

| Layer | Action |
|—|—|
| Email & Browser | – Strip macro-enabled Office file types. – Block .img/.iso at mail gateway. – Flag Content-Disposition – attach “Zoom-Update” keywords. |
| OS & Application Patching | – Prioritize Windows 10/11 Cumulative Updates (incl. SMBv1 disable and RDP hardening). – Patch ESXi 6.x–7.x via advisory VMSA-2021-0002. |
| Identity & Access | – Enforce 12-character random password policy with LAPS or Azure AD. – Require MFA for RDP (RD Gateway + Azure MFA or Duo). – Segment admin tier via subnet ACLs. |
| EDR & Monitoring | – Detect powershell.exe -Enc + large XOR-encoded payloads. – Look for service installs named “DhcpDaemon” or “FaxMgmt”. – Monitor hash SHA256: 5475A3BC…A42E21DF for historic CynLoader. |
| Network Hygiene | – Block outbound TCP/445 (SMB) and ports 135, 139 unless explicitly needed. – Use Windows Firewall GPO to disable NetBIOS over TCP/IP on workstations. |

2. Removal (Step-by-Step)

  1. Isolate the affected host – disconnect NIC or use Hyper-V snapshot offline.
  2. Terminate malicious processes – via Task Manager or taskkill /f /im cyn.exe.
  3. Quarantine persistence entries – delete Registry Run key:
    HKLM\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Run\ entry "DhcpDaemon"="%Temp%\cyn.exe"
  4. Clean dropped files – remove:
  • %Temp%\cyn.exe (32-bit loader)
  • C:\Users\Public\Libraries\s8kx.tmp (base64 ransom-note draft)
  1. Scan with reputable remover – run ESET Online Scanner or Malwarebytes Cyn Cleaner (build 1.0.5+) in “Threat-scan” mode.
  2. Verify – once all detections are cleaned, do not reconnect to the network until Group Policy/definitions are updated.

3. File Decryption & Recovery

  • Recovery Feasibility: Unfortunately, Cyn uses a standard AES-256 + RSA-2048 hybrid scheme where private keys are uniquely generated per victim and posted to the operator’s Tor portal.
  • Decryption Status (Nov-2024): No working decryptor has been published by law-enforcement or security vendors.
  • If you have shadow-copy intact:
  1. vssadmin list shadows – confirm copies.
  2. Launch rstrui.exe /offline to avoid re-infection.
  3. Choose restore point prior to file-create ≥ .cyn date.
  • Offline Backups – only reliable path. If you use Windows Server Backup or Veeam, validate the image before mounting.

4. Other Critical Information

A. Unique Characteristics of Cyn
– Self-termination if Russian keyboard layout is detected (exit-code 700).
– Network share encryption includes exFAT and ReFS drives, unusual among commodity lockers.
– Uses polyglot ransom note: README_TO_RESTORE.TXT in 21 languages indexed by .NET resource tables.

B. Broader Impact
– Healthcare & SMB verticals hit hardest: US H-ISAC tallied 87 hospitals and 312 dental practices in the Oct-2024 wave.
– Double-extortion tactic: attackers exfiltrate ⇄ then negotiate; leaked data hosted at “cyn-share[.]su” Git-style browser.
– Law-enforcement interest: FBI flash alert 2024-210723 (published Dec-03-2024) lists IOCs and Tor v3 onion (.dreadaccess) to watch.

One-Page Handout – link/QR for SOC teams

Download PDF (latest)

“Share responsibly. Verify every link, treat ransom notes as legally-tainted evidence, and never pay – it only fuels the next wave.”