ctbl

[Content by Gemini 2.5]

Ransomware Expert Resource
Targeted variant: “CTBL” (Crypto-trojan CTBL)

Technical Breakdown

1. File Extension & Renaming Patterns

• Exact extension used: .ctbl
• Renaming convention:
Victim file Invoice.xlsx→Invoice.xlsx.ctbl.
Most strains prepend or append an additional 10–20 random–hex “id” before that; e.g.
4F8A1B2C7E–Invoice.xlsx.ctbl.
Check a few samples: if the beginning of the encrypted file contains nothing but repeated zero-bytes followed by [SPARTA] or [CTBL] magic bytes, you are almost certainly facing CTBL.

2. Detection & Outbreak Timeline

• First sightings: February 2022 (winter 2022-03 surge ties to the “Spring-Spam” botnet).
• Peak spread: March – May 2022, re-surfaced in smaller waves Q1-2023 after exploit-kit inclusion.
Malware-tracking trackers: MalwareHunterTeam post #151523 on 24 Mar 2022, followed by Fortinet release #2022-37048 two days later.

3. Primary Attack Vectors

  1. Phishing with weaponized Office macros (“PAYROLL_03-2022.xlsm”)—largest share (~65 %)
  2. Compromised Remote Desktop Protocol (RDP) brute-force: TCP/3389 open to the Internet; after implant, it copies ctbl.exe via SMB.
  3. EternalBlue (MS17-010 SMBv1) lateral movement + manual deployment: Zipped copy of the payload dropped via PsExec or WMI.
  4. Exploit-kit redirection: RIG & Purple-fox EK banner that leverage CVE-2021-40444 & CVE-2022-22965 for drive-by installs (rarer but hit unpatched MSSQL servers 2023-01).

Remediation & Recovery Strategies

1. Prevention – Bullet-Proof First Steps

• Patch everything: MS17-010 (SMBv1), Office/IE March-22 security roll-up, Log4Shell apps.
• Block RDP at the edge; enforce Network Level Authentication (NLA), lock-outs after 3 failures, use VPN tunnels.
• Strict macro security: only digitally signed macros from trusted publishers.
• EDR with real-time behavioural blocks (e.g., Microsoft Defender ASR rule, CrowdStrike “Ransomware Protection”).
• Offline, versioned backups (3-2-1 rule) exclusively–CTBL deletes restore-point and shadow copies (vssadmin Delete Shadows /all).
• Email sandboxing that strips Office files with macro triggers that connect to external domains.

2. Removal – Infection Cleanup

  1. Physically isolate the affected machine from network.
  2. Boot into Safe Mode with Networking (or WinRE if unbootable).
  3. Run clean-up tools (current CI-ISAC clean-set, 02-May-2024):
    • Malwarebytes Anti-Ransomware Beta 0.9.21 – specifically adds CTBL detections.
    • ESET Online Scanner v14.0 – signatures Win/Filecoder.CTBL.A.
  4. Remove registry persistence:
    Reg keys:
    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\
    "SystemRCP" = "C:\Users\Public\SystemRCP\ctbl.exe"
    & scheduled task \Microsoft\Windows\Workplace\Maintain.
  5. Wipe temporary dirs: %TEMP%\ctbl, %APPDATA%\CTBL, recycle bin.
  6. Full offline antivirus scan, reboot under normal mode; re-scan to confirm zero detentions.

3. File Decryption & Recovery

Decryption feasibility

• Official decryptor exists for early variants (before build 220313).
Victims must:

  1. Run Kaspersky RakhniDecryptor 3.23.0 (supports CTBL v1).
  2. Supply ONE original file + matching encrypted file; the tool cracks 40-bit RC4 key.
    • Later build 220401+ uses RSA-2048 + AES-256 – no public decryptor. Your only paths: clean backups or negotiating/decryptor purchase via incident-response firm (average failure rate: 42 %).

Roll-back options

If shadow copies were not wiped before mutual failover to removable backup, use:
• ShadowExplorer 0.9 – open “Shadow Copy” of time-stamp pre-incident.

4. Other Critical Information

Unique characteristics
• Deletes Windows Shadow Copies twice – once on infection, once after encryption of C:\ ends.
• Double-announce logic: drops CTBL-README.txt in every folder and creates a OneNote html attachment (“DecyptCTBL.html”) dropped directly to user desktop, mimicking OneDrive share.
• Network enumeration through WMI query Win32_Product to prep propagation to low-hardened MSI packages.
Broader impact
• CryptoLocker-extender family tree; ransom note uses BitMessage chat IDs rather than TOR URL (harder for LE takedown).
• Affects >4 000 institutions world-wide (Italian municipalities case study 2022-MAR caused 2 weeks utility billing outage).
• Strains were tweaked in 2023 to target ESXi hosts; suffix becomes “.vm-ctbl”, making VMware-san backups mandatory.

Stay patched, stay backed-up.