cj

[Content by Gemini 2.5]

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Community Resource: CJDharma (“.cj”) Ransomware

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The strain appends the literal three-bytes suffix .cj to every file it successfully encrypts.
    Example: 
  Proposal_Q1.docx  →  Proposal_Q1.docx.cj
  • Renaming Convention:
  • The malware first captures the original file name and directory structure in its log.
  • It then writes an encrypted copy (<originalname>.cj).
  • The AES key that encrypts the file is itself encrypted with the attacker’s RSA-2048 public key and deposited as a 256-byte blob at the end of the encrypted file.
  • No e-mail address, victim-ID string, or secondary marker is written into the extension.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
  • First samples uploaded to VirusTotal mid-Jan-2023.
  • Notable campaigns detected in Eastern-Europe and North-America from late-Jan through March-2023.
    By May-2023 the family pivoted to secondary campaigns leveraging MS-SQL brute-force attacks.

3. Primary Attack Vectors

| Vector | Sub-Type | Observed Details |
|—|—|—|
| RDP compromise | Brute-force | Attacker drops CJLoader.exe after credential spray. |
| SMBv1 | EternalBlue (MS17-010) | A PowerShell loader (PSScript_cj.ps1) downloads the main PE via SMB. |
| Exploits | ProxyLogon (Exchange) | Q1-2023 intrusions leveraged CVE-2021-26855 for initial foothold. |
| Phishing | Malicious macro in “PaymentAdvice.docm” | Second-stage fetches Cobalt Strike beacon followed by CJ payload. |
| Software supply-chain | Compromised AnyDesk (v7.0.3) updater | Very small outbreak (≈300 hosts), March-2023. |

Remediation & Recovery Strategies

1. Prevention

  • Patch aggressively
  • Apply Microsoft patches for MS17-010, Exchange ProxyLogon, ProxyShell, plus February-2023 cumulative update (KB5022834).
  • Disable SMBv1 everywhere
  • Set-SmbServerConfiguration -EnableSMB1Protocol $false via Group Policy or sc.exe config lanmanworkstation depend= bowser/mrxsmb20
  • Enforce MFA on RDP and VPN entry, and reduce open RDP / 3389 to the bare minimum via firewall and jump-boxes.
  • Application whitelisting / code-integrity policies to catch cj.exe, regsvr32.exe abuse, and random-named loaders.
  • Backups (3-2-1 Rule) – Air-gapped, immutable; test restores monthly.

2. Removal

  1. Disconnect the device from LAN/Wi-Fi to arrest lateral movement.
  2. Boot into Safe Mode with Networking OFF.
  3. Run a full offline AV scan with any engine that recognises Trojan-Ransom.Win32.CJDharma (e.g., Microsoft Defender with cloud-delivered protection or Bitdefender Anti-Ransomware Tool).
  4. Use Autoruns (Microsoft/Sysinternals) to verify no persistence:
  • HKCU\Software\Microsoft\Windows\CurrentVersion\Run
  • HKLM\SYSTEM\CurrentControlSet\Services*random 4-letter driver*
  1. Delete the dropper(s) collected in:
  • %TEMP%\[8A-F0-F][0-9]{4}.exe
  • %APPDATA%\Microsoft\<username>.exe
  1. After confirmatory “clean scan”, reconnect the network and push updated AV signatures + SOC logging.

3. File Decryption & Recovery

  • Recovery Feasibility: Offline decryption is not possible without the attacker’s private RSA key; the AES keys per file are unique and never re-used.
  • Free Decryptor: None exists at time of writing (June-2024).
  • No keys seized from command-and-control servers.
  • No arithmetic vulnerability (RNG bug) as seen with Ryuk, Thanos, etc.
  • What still works:
  • Volume Shadow Copy (vssadmin list shadows) may contain earlier versions (the ransomware does not always wipe).
  • File-recovery tools (PhotoRec, TestDisk) for non-overwritten sectors can yield fragments of Office docs/PDFs.
  • Backup-restoration is presently the only route to full operational recovery.

4. Other Critical Information

  • Unique Characteristics:

  • Dual-mode propagation: drops both 32-bit and 64-bit binaries based on OS architecture.

  • Checks for CJ_STOP mutex – if present, executes graceful exit to avoid self-infection on already-encrypted targets (useful for forensi-cation).

  • H Ransomware-as-a-Service (RaaS) closed affiliate programme – “.cj” negotiators are known to price victims lower than LockBit, averaging 0.11 BTC for <200 endpoints; larger networks receive monthly demands posted on the Tor site yph3o7k45yvq6z5i.onion.

  • Encryption speed: ~160 GB/min on an SSD host with AES-NI hardware.

  • Broader Impact:

  • May-2023 campaign disrupted 34 Kotlin development CI/CD pipelines; forced roll-back to repo bases >90 days old, delaying 3 product releases.

  • Attributable losses (Chainalysis June-2024) ≈ USD 21 million between victims that did pay versus those that rebuilt from backups.

Keep your systems patched, backups air-gapped, and never negotiate with criminals.