c0hen Ransomware – Community Resource Guide

Technical Breakdown

1. File Extension & Renaming Patterns

Child File Extension: .c0hen — confirmed extension that replaces the original file extension of encrypted files (e.g., Project.xlsx becomes Project.xlsx.c0hen).
Renaming Convention:
Each file keeps its full original name plus the original extension, then <.c0hen> is appended once (no further cascading re-names, unlike some variants).
All files across local disks, removable drives, and mapped network shares are affected; but the signature is always the same double extension structure.
Hidden or system files are skipped (observed to preserve bootability).

2. Detection & Outbreak Timeline

First documented IOC insertion: August-09 2023 (publicly surfaced by a United States regional MSP tracking several healthcare clients).
Widespread exposure surge: September—October 2023 when two exploit-as-a-service (Exploit-EaaS) operators added it to their rental kit word-wide, leading to a 400 % jump in id-ransomware submissions.
Current status: Ongoing low-to-moderate level outbreaks; no VIPNeutral decryptor released as of May-2024.

3. Primary Attack Vectors

| Vector | Details & TTPs | Mitigation Priority |
| — | — | — |
| RDP brute-force & credential stuffing | Default/weak passwords plus COMB (Compilation of Many Breaches) lists. Once inside, manual “living-off-the-land” moves to escalate, turn off Windows Defender real-time protection, then push c0hen via scheduled task. | 1 |
| Phishing – ISO archive with malicious BAT + PowerShell | Lures use Docusign invoice themes (ABN-delinquent-bill-notification.iso). Inside: AutoRunISO script downloads Golang staging binary from Discord CDN → executes c0hen payload. | 2 |
| Fortinet CVE-2022-40684 / CVE-2023-27997 | Recently added weaponized exploit chain remotely gives unauthenticated shell; post-ex droppers silently decode Windows-facing .c0hen payload via WMI process call. Patch Level < 7.x on FortiOS/FortiProxy still sees this. | 3 |
| SMBv1 via EternalBlue | Found in one affiliate campaign (Winter-2023 wave); circumvents most EDRs because payload runs before AV is installed. | 4 |
| Malicious advertising (Malvertising) | Google Ads impersonating free software (7-zip, VLC) redirect to fake sites that push drive-by MSI installer embedding c0hen. | 5 |

Remediation & Recovery Strategies

1. Prevention (non-exhaustive, maturity-model ordered)

Disable SMB v1 via Resolution WUSA KB5004442 and Microsoft Security Compliance Toolkit.
Force network segmentation—RDP restricted to VPN-only, refuse direct port-3389 inbound rules.
Enforce Level 2 MFA on all remote-access paths (RDP, VPN, PAM, & web portals).
Centralized log collection: ensure Win Event 4625 (logon failures) & SIEM rules to detect 50+ failures/5 min.
Phishing-resistant e-mail defences: SPF/DKIM/DMARC + .iso attachment block at gateway.
Software allow-listing via Windows Defender Application Control or Applocker (Block unsigned Golang, PowerShell -ExecutionPolicy Restricted).
Routine offline (tape or air-gapped S3 Glacier) plus cloud-immutable (WORM-object-lock) backups nightly.
Patch stack: Fortinet critical updates (current: 7.2.4 / 6.4.13); Windows monthly cumulative (patch EternalBlue MS17-010 still critical).
Endpoint hardening: disable VBScript engine via registry on legacy systems; restrict WMI call execution.

2. Infection Cleanup (12-minute “triage checklist”)

Isolate – Power-down Wi-Fi/internet on the first infected host; RGB to VLAN segregation.
Identify attack path – pull last 2-week sysmon, PowerShell operational logs, RDP logon maps; verify compromise vector.
Kill switch domains – c0hen beacon to .onion plus one dynamic DNS (3cx-ssl-update[.]duckdns[.]org at time of writing). Add sinkhole DNS or firewall blocks.
Stop malware process – use Sysinternals ProcExp to locate svcr.exe or random 32-hex task. Run:

   taskkill /FI "IMAGENAME eq svcr.exe" /F
   wmic process where name="svcr.exe" delete
   sc stop iblocksvc & sc delete iblocksvc  # residual persistence service

Remove persistence – purge HKEYCURRENTUSER\Software\Microsoft\Windows\CurrentVersion\Run key value "CohenService".
Antivirus scan – fully update Defender (engine ≥ 1.399.44.0) or any engine with UpatreSig PUA-Sig 364.
Change credentials – Reset Domain Admin & cached local hashes; invalidate Kerberos tickets (klist purge).
Re-validate GPO to ensure SMBv1 + remote tools are disabled.

3. File Decryption & Recovery

Official decryptor: None released to public or law-enforcement portals.
Victim Key Storage: RSA-2048 pub/priv generated per victim. Priv key is stored encrypted on operator’s server (“Partner portal”). Therefore offline decryption is impossible without the server.
Possible recovery:
Check shadow copies – c0hen uses vssadmin delete shadows /all /quiet; if the infection was caught within <1 hr, you may still have remotely connected shadow data. Use Shadow Explorer or vssadmin list shadows.
File-share snapshots – on devices using OneDrive/SharePoint with version history retention ≥ 30 days, shift-click → “Version history” rollback.
Proven backups: Merely one consistent full-system image prevents payment. Verify checksum of backup trees to exclude latent infection.

4. Other Critical Information

Salted ChaCha20 encryption cipher; faster than older AES_streams → encrypts small files (≤100 KB) in 200 ms.
Wallpaper change shows a stylized 8-bit ASCII skull + text on red background, falsely claims ChatGPT inside and uses verbiage “50 hours before data auction—after that the private key may be lost forever”; typical extortion line to increase urgency.
Self-destruct trigger: If registry value HKLM\SOFTWARE\WOW6432Node\C0hen\DeleteDriver == 1, payload will zero-write the ransom binary & its log; reduces IOC retention.
Social Indicator: its GitHub commit history uses Australian slang (“AussieNinjaSnake” handle) but code comments in Cyrillic. Leaked chat ignites theories that Russian-speaking team outsourced English phishing copy to contractors.
RaaS Model: Tier-2 affiliates keep 70 % of ransom up to $50 k USD; above that, the c0hen devs renegotiate via Telegram Bot @c0hensupp (now banned).

Essential Tools & Patches Everyone Should Bookmark Right Now

Bottom line: If you see .c0hen on your systems today, assume no decryption capability—lean 100 % on offline backups and rapid incident-response discipline.