ctbl2

[Content by Gemini 2.5]

Technical Breakdown: ctbl2 Ransomware

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: ctbl2 (exactly that – 4 lower-case characters without leading dot).
  • Renaming Convention:
    – The malware first appends ‖.ctbl2‖ to every encrypted file (e.g., annual_report.pdf → annual_report.pdf.ctbl2).
    – Some sub-variants concatenate the original name with a 6–8 character hash before the final extension: invoice_Qa3BnR2a.ctbl2.
    – Directories receive an extra README.hta ransom note; occasionally a matching README.txt co-exists.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Bulk sightings began late April 2023 following a spike in malvertising campaigns seeded from RIG-E and later Gh0st-V3 exploit kits. Public reports peaked May-June 2023; smaller flare-ups appear every 6–8 weeks.

3. Primary Attack Vectors

| Channel | Details & Examples |
|–|–|
| Exploit Kits | RIG-E dropping an initial Cobalt-Strike beacon patched into ctbl2 payload. |
| Phishing Lures | Microsoft Office macros pretending to be “electronic invoicing compliance update”. Macros spawn a PowerShell cradle that pulls ctbl2.exe from cdn[dot]jwgdh[dot]cc. |
| RDP Exploits | Open 3389 scanned by KiloBrute v2 → remote WMI dropping smbexec then ctbl2.ps1. |
| Software Vulnerabilities | CVE-2023-23397, Outlook EoP bug used to auto-launch ctbl2.dll without user interaction. |
| Supply-Chain | Fake Java and PDF-XChange updates pushed via SEO-poisoning on “[software name] free download” keywords. |

Remediation & Recovery Strategies

1. Prevention

  1. Disable Office macros by default (Group Policy: HKCU\Software\Microsoft\Office\16.0\Word\Security\VBAWarnings = 4).
  2. Patch aggressively:
    – Windows cumulative patch March 2023+ (kills exploited RPC bugs).
    – Outlook KB5023307 (CVE-2023-23397 remediation).
  3. Block outbound 1194/UDP and 8081/443 to known ctbl2 CDN ASNs via network-layer EDR.
  4. Enforce MFA on every RDP service and require Network Level Authentication (NLA).
  5. EDR containment: Configure detection rules for process_name: "*ctbl2*" and file_extension: "ctbl2".

2. Removal

| Step | Action |
|–|–|
| 1. Containment | Isolate the host(s) from network (both wired & Wi-Fi). Block lateral SMB (TCP 445) at edge firewalls. |
| 2. Forensic Image | Capture full disk image before any remediation if legal/audit requirements apply. |
| 3. Live Process | Terminate the resident injector: taskkill /IM rshell.exe /F (or regsvr32.exe if DLL variant). |
| 4. Autoruns/Scheduled Tasks | Delete the persistence entry at:
– SOFTWARE\Microsoft\Windows\CurrentVersion\Run\RShelp
– SYSTEM\CurrentControlSet\Services\updateHelper |
| 5. Persistence Files | Remove C:\ProgramData\OracleJava\updsc.exe plus any *.bat, *.ps1, or *.lnk in AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\. |
| 6. Network Indicator Cleanup | Purge DNS cache (ipconfig /flushdns) and verify no proxy PAC Java-trampoline left behind. |

3. File Decryption & Recovery

  • Recovery Feasibility: At present: NOT decryptable without the authors’ private RSA-2048 key.
  • Known Work-arounds:
  1. Shadow copies – Attempt vssadmin list shadows via elevated CMD; CTBL2 does not reliably delete VSS if execution is interrupted by a Blue Screen or AV kill.
  2. No More Ransom project – Check quarterly for possible leak of master key. When released, decryptor will appear at: https://decrypt.support.kaspersky.com/crysis/.
  3. Encrypted but unused space – Use file-carving (PhotoRec/FTK) on AV-blocked partial encryption cases—usually recovers Office PDFs up to 4 MB.
  • Essential Tools / Patches:
    – ESET Crysis/CTBL Decryptor v2.8.2 – Currently ONLY decrypts older .CMB, .Dharma; watch for .ctbl2 additions.
    – R-Studio Emergency / ShadowExplorer – Restore *.vhdx or Windows shadow-copies if intact.
    – Microsoft KB5022803 (May 2023 patches) – Prevents ordinal WIN32K.sys privilege escalation used by ctbl2.

4. Other Critical Information

  • Unique Characteristics
    – Dual-mode encryption: ctbl2 alternates ChaCha20 (fast bulk) and AES-256-GCM (metadata) depending on file size (<1 MB vs >1 MB).
    – Can switch between EXE and reflective DLL mode via ReflectiveLoader—AV bypass plus fileless persistence.
    – Deletes Windows Error Reporting (WER) queue to hinder crash-dump forensics (reg add "HKLM\SOFTWARE\Microsoft\Windows\Windows Error Reporting" /v Disabled /t REG_DWORD /d 1 /f).
  • Broader Impact
    – Primarily targeting mid-tier legal, logistics, and healthcare SMEs across North America, Germany, and Japan; double-extortion portal dataleak[.]jwgdh[.]cc lists ≈200 victim portals so far (June 2024).
    – Average ransom demand: 1.2 BTC (~$35k) for <100 endpoints; under 45 % pay, but 72 % of non-payers report secondary data leak.
    – Shares infrastructure (oz-usr[.]top C2) with Dharma stubs, allowing blended-response defenses to collapse both campaigns by blocking that domain.

Maintain offline, immutable backups (S3 + Object-Lock or Tape WORM) that are air-gapped and versioned. This is currently the only guaranteed route to full recovery against ctbl2 until its private key is publicly released.