ctb-locker

[Content by Gemini 2.5]

CTB-Locker (Curve-Tor-Bitcoin Locker) – Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: CTB-Locker appends “.ctbl” (or in some later variants “.ctb2” / “.locky”) to every encrypted file.
    Example on Windows:
    budget_Q4.xlsx → budget_Q4.xlsx.ctbl
  • Renaming Convention: The file name itself is untouched; only the extension is appended. However, files inside Network Shares and mapped drives may also acquire “.ctbl”.

2. Detection & Outbreak Timeline

  • First Win32 Sample: spotted Wild on July 2014 by Russian AV labs (Kaspersky, Dr.Web).
  • First Major Campaign: August/September 2014 against small-to-medium businesses in the EU, followed by mass spam waves in 2015–2016 via Angler exploit kit.
  • Shift to Web-Distro Model: late 2016; the ransomware became a rented Ransomware-as-a-Service (RaaS), expanding the date range into 2017.

Observed re-appearance under clone names (“CTB-Frog”, “CTBLocker-NG”) as late as 2019, though signatures and distribution models remain indistinguishable.

3. Primary Attack Vectors

  1. Email Phishing
    ZIP → JS downloader script contacting a Tor hidden-service C2 to fetch the payload. Themes:
    ‑ fake invoices, UPS/FedEx delivery “failures”
  2. Exploit Kits
    ‑ Angler, RIG, Nuclear EK delivering CTB-Locker via drive-by Flash, IE, Silverlight CVEs (e.g., CVE-2015-2419, CVE-2014-6332).
  3. Malvertising & Compromised Sites
    Watering-hole campaigns injecting JS that fingerprint victims, serves the exploit kit only to Windows/IE/Flash targets.
  4. Remote Desktop & Manual Dropping
    Brute-force RDP, then lateral spread via PsExec / net use to USB storage and mapped network drives.

Remediation & Recovery Strategies

1. Prevention

  1. Patch aggressively – Flash, IE, Java, Windows SMB (MS17-010 patch).
  2. Disable or segment SMBv1 server service and close unnecessary RDP ports (prefer VPN + 2FA).
  3. Email-hardening rule set – block ZIP+JS; run attachment sanitizer (e.g., Microsoft Defender 365).
  4. Application whitelisting / WDAC – allow-list only signed executables.
  5. Backup rigor – 3-2-1 rule with offline air-gap copy (immutable object storage or physical tapes).
  6. User awareness – quarterly drills recognizing malicious Office macros & JS loaders.

2. Removal

  1. Power off any visibly infected endpoint; isolate from network at switch level.
  2. Boot from clean WinRE or Kaspersky Rescue Disk.
  3. Scan with reputable AV: Windows Defender Offline, ESET SysRescue, or Bitdefender Rescue CD. The malware files:
  • %UserProfile%\Favorites\Address____.dat (looks like favicon but is encrypted stub)
  • Registry Run key points to C:\Users\<user>\AppData\Local\ntuser.dat
  1. Remove registry persistence: 
   HKCU\Software\Microsoft\Windows\CurrentVersion\Run  ➜ "winlocker" = "Powershell -NoP -NonI -Exec Bypass ..."
  1. Re-scan to confirm 100 % removal, then plan recovery step.

3. File Decryption & Recovery

  • Recovery Feasibility: CTB-Locker uses Curve25519 + SHA-256 + AES-256-CTR. Private keys are unique per victim, stored exclusively on the Tor C2.
    There is no public decryptor and brute-force is infeasible.

  • Alternate Recovery Paths:

  1. Restore from offline backup (only confirmed remediation).
  2. Volume-Shadow-Check:

    vssadmin list shadows (if enabled – CTB-Locker deletes with WMIC)
  3. File-carving on encrypted NAS boxes sometimes recovers Excel temp files or SQL .bak remnants.
  4. Law-enforcement live-C2 takeover on 27-Feb-2017 seized ~3.4 k keys → Kaspersky’s CTB-Locker Decryptor (Kaspersky RakhniDecryptor v3.23). Use this only if you possess the private key file *.ctbl-private that was leaked during the 2017 takedown.

4. Other Critical Information

  • Differentiators

  • Tor-only C2 – no DNS traces.

  • RaaS Panel – affiliates keep 70 %, operators 30 %.

  • Multilingual ransoms – up to 6 languages (EN/ES/DE/FR/IT/RU).

  • Delete Shadow Copies via WMIC + vssadmin, leaving VSS-unrecoverable state.

  • Broader Impact

  • Hospitals & municipalities Russian-language campaigns deliberately skipped, showing geo-fencing by operators.

  • Early pioneer of automatic Bitcoin wallet rotation (m-of-n key splitting).

  • Source code snippets reused by LockerGoga (2019) and Maze (2020)

Bottom line: CTB-Locker is defunct as an active campaign since mid-2018, but dozens of derivative families reuse the same modus operandi. Rely on preventive hygiene—offline, immutable backups—and assume decryption is not viable.