cryptowall 4.0

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: CryptoWall 4.0 does not add a brand-new extension like many contemporary strains; instead it produces enciphered files whose names remain unchanged at the byte level, but the inside of every file is unreadable. Observers sometimes see a “.CRYPT” or appended numeric string (*.id-<8-hex>.crypt) if earlier campaigns leaked in, but the v4.0 mainstream wave typically retains original names and extensions (.docx, .pdf, etc.) after encryption.
  • Renaming Convention: After encryption, CryptoWall 4.0 replaces the first 512–1024 bytes of each victim file with statically-encrypted ciphertext blocks. When browsing directories, the only immediate surface change is that each encrypted file has no readable header (e.g., a Word file no longer opens as a DOCX even if the extension is unchanged). The ransomware compensates for this by writing accompanying “DECRYPTINSTRUCTION.TXT,” “DECRYPTINSTRUCTION.HTML,” and sometimes “DECRYPT_INSTRUCTION.PNG” files in every affected folder. Those note files contain the ransom message and sometimes carry a three-character “affiliate snippet” at the bottom (e.g., /![345] ) that helps the operators split payments.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: v4.0 began to appear in the wild around 31 October 2015 and peaked through Q4 2015 – Q2 2016. Security researchers at the Dell SecureWorks Counter Threat Unit designated it “CTU CryptoWall 4.0” in the 12 November 2015 advisory.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  • Spam campaigns distributing ZIP/JS attachments via the Cutwail / Pushdo botnet. Lure themes revolved around “fax from HP,” “invoice,” and “DHL shipping notice.”
  • Exploit Kit drive-bys – Angler, Nuclear, and RIG delivered the ransomware payload via browser/Java/Flash exploits.
  • RDP brute-force & credential stuffing targeting weak RDP passwords; once inside, the attackers ran the installer manually.
  • Legacy SMBv1 pass-the-hash (less common than for SamSam or WannaCry) was seen when lateral movement was needed inside SMB-accessible networks.
  • File-less stage delivered directly by PowerShell reflective DLL injection, which helped the loader evade signature-based AV detection.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  1. Disable/delete open SMBv1 (protocol type 1), or segment legacy hosts behind strict VPN.
  2. Enforce application whitelisting (AppLocker / WDAC). Block unsigned executables in %AppData%, %Temp%, and %UserProfile%.
  3. Mandate MFA on all RDP logins; close RDP to the Internet on port 3389/TCP and instead use VPN + jump boxes.
  4. Phishing-resistant email gateway: sandbox-enabled filtering for JS/WScript/PowerShell attachments, domain impersonation checks.
  5. Deploy Web-filtering appliances or DNS services that sinkhole known Cutwail/Angler intermediary domains.
  6. Patch Java, Flash and MS Office – most 2015-era Angler pages exploited (CVE-2015-0096, CVE-2014-6332, CVE-2015-5122).
  7. Harden PowerShell through Constrained Language Mode and blank command-line logging.

2. Removal – Step-by-Step

  1. Physical / network isolation: Immediately power-off, disconnect LAN & Wi-Fi, but keep one healthy admin workstation online for downloads.
  2. Purge persistency keys: 
   HKCU\Software\Microsoft\Windows\CurrentVersion\RunOnce
      szName = “System”  
      Data  = C:\Users\<user>\AppData\Roaming\<random>.exe

and also 

   HKCU\Software\Microsoft\Windows\CurrentVersion\Run\CryptoWall
  1. Endpoint forensics – Copy the infection artifact to a forensics share (.exe and any “_locky” temp folders in %Temp%) before deleting.
  2. User credential reset – All human and service accounts that accessed the compromised host.
  3. Full scan – Boot into WinRE or use an offline AV tool to hunt for remnants; CryptoWall 4.0 files typically have no legitimate signature.
  4. Verify hash changes: compare SHA-256 sums for OS binaries against good-baseline to quickly spot backdoors the actor may have left behind.

3. File Decryption & Recovery

  • Recovery Feasibility: No public decryption tool exists for CryptoWall 4.0; the ransomware secured its RSA-2048 server-side private key for File Encryption Key (FEK) wrapping. Barring an extortion-payment (which the FBI explicitly discourages), the only reliable route is restoration from immutable backups.
  • Essential Tools / Patches:
  • Offline backup orchestration: Veeam immutable repositories, or Wasabi/OCI Object Storage with WORM policies.
  • Patch bundle against the MS15-132 / MS15-100 cluster (includes the Angler Flash exploit) released 08 December 2015 applies to IE.
  • Java 8u74+ or simply uninstall Java < 8 entirely for low-demand endpoints.
  • NTLM hardening (KB5004442) to curb pass-the-hash.

4. Other Critical Information

  • Unique Characteristics:
  • CryptoWall 4.0 introduced nested file-name stripping: it deletes original names in memory and writes random 3-to-15-letter alphabetical names to disk (only accessible via forensic carving). It therefore makes manual identification of original directories nearly impossible without the ransom note decryption service UI.
  • Language-awareness: ransom notes were translated in 14 languages dynamically based on victim’s browser/keyboard layout.
  • No Command-and-Control chatter once encryption is completed – it “silently” self-destructs payloads and installs no callback timers, which complicates takedown.
  • Broader Impact:
  • Dec 2015 – Apr 2016 accounting later showed > US $18 million in paid ransoms CTU-wide.
  • The campaign extinguished Shadow-Copy with vssadmin delete shadows /all, destroying Volume-Based recovery. This drove a governance-wide change requiring air-gapped or immutable backup architectures that still stand today as best practice.