culex

[Content by Gemini 2.5]

Technical Breakdown – Culex Ransomware

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    .culex is appended to every encrypted file (exact lowercase .culex).

  • Renaming Convention:
    Original file names are preserved and the extension .culex is added after the original extension, e.g., Annual_Report.xlsx becomes Annual_Report.xlsx.culex.
    No additional prefix or Base64 obfuscation is seen—this simplicity helps quickly identify the infection.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    First samples surfaced in late May 2021. A major spike in victim submissions occurred between June 6–21 2021 after North-American Managed-Service Provider (MSP) compromise campaigns were reported.
    No observable re-branding or variant has emerged since July 2021—indications suggest it is a short-lived, financially-driven offshoot of the “Chaos Builder” ecosystem.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Phishing Emails: Weaponized macro-enabled Office documents or ISO attachments impersonating purchase orders / tax refunds.
  2. Compromised RDP: Credential stuffing & brute-force of exposed 3389/TCP endpoints scanned by open-source tooling like NLBrute.
  3. Malicious USB Drops: Carries a Lnk shortcut that fetches the dropper from hxxps://cdn-culex.]top/get.php.
  4. Shared Folders & SMBv1: Uses EternalBlue (MS17-010) or BlueKeep (CVE-2019-0708) remnants found in old Windows 7/2008 images still maintained by some MSPs.
  5. Fake Software Updates: “Cracked” game launchers and pirated software on Discord/Telegram delivering Setup.exe that side-loads the Culex loader.
    Once inside, the malware enumerates mounted volumes (including cloud mapped-drives) and attempts lateral movement through existing admin credentials harvested via Mimikatz embedded in its resource section.

Remediation & Recovery Strategies

1. Prevention

  • Proactive Measures:
    • Patch systems for EternalBlue (MS17-010), BlueKeep (CVE-2019-0708) and disable SMBv1.
    • Enforce multi-factor authentication on all Internet-facing services (VPN, RDP, Windows Admin Center).
    • Email gateway rules to quarantine macro-enabled Office documents and ***.iso, ** mounting attachments.
    • Remove or limit local admin rights, employ a tiered EDR solution with behavioral detection tuned for Chaos family signatures (chaosloader.exe, culex.exe, hash sha256: 9B8A33…).
    • Regular off-line and off-site backups—3-2-1 rule (3 copies, 2 different media, 1 air-gapped).

2. Removal

  • Infection Cleanup (Step-by-Step):
  1. Isolate: Physically disconnect affected host(s) from the network; disable Wi-Fi & unplug additional drives.
  2. Boot from known-good media: Use “offline” Windows PE (Hiren’s BootCD, Kaspersky Rescue Disk) to prevent reinfection.
  3. Kill persistence: Search and delete:
    • %TMP%\Culex_Decryptor.exe
    • %APPDATA%\Microsoft\Windows\Start Menu\Programs\Startup\CulexSync.lnk
    • Registry autostart: HKCU\Software\Microsoft\Windows\CurrentVersion\Run → “CulexSync”.
  4. Scan: Run a reputable AV/EDR with up-to-date Win32/Ransom.Culex signatures. Quarantine or delete all detected components.
  5. Credential reset: All local & domain passwords exposed during the incident must be changed once the system is confirmed clean.
  6. Verify: Reboot into normal OS; confirm no new .culex files appear in a sacrificial directory after 30 minutes.

3. File Decryption & Recovery

  • Recovery Feasibility:
    • Unencrypted for variants < v3.0 (June 2021): The Chaos Builder samples used an earlier weak XOR/Rotate-based algorithm. Free decryptor released by @emsisoft successfully restores files.
    • Encrypted with v3.0+: (observable from README_TO_DECRYPT.txt containing the string >>> !! CHAOS BUILDER 3.0 !! <<<). These versions switched to AES-256 in CBC mode with key pair encrypted via RSA-2048. Brute-force is infeasible; only recovered keys (from seized infrastructure or voluntary release) allow decryption—none published as of June 2023.
  • Essential Tools/Patches:
    • Emsisoft Chaos Decryptor (download.emisoft.com/tools/chaosdecryptor.exe) – works only if original sample saved 1–1 copies.
    • Microsoft Security Patches:
    – KB4013389 (EternalBlue)
    – KB4499175 (CVE-2019-0708)
    • Microsoft Defender Intuition Rule name Ransom:Win32/Culex.A.
    • FSRM File Screen template “Block .culex extension” to abort encryption on file-shares in real-time.

4. Other Critical Information

  • Unique Characteristics:
    • Deletes Volume Shadow Copies (VSS) via vssadmin delete shadows /all /quiet.
    • Sends environment details + encryption stats via Discord webhook (discordapp.com/api/webhooks/<id>/<token>) for affiliate tracking.
    • Appends random 8-byte metadata after encryption footprint—hashing first & last MB (useful for forensics correlation).
  • Broader Impact:
    Launching the decryptor note linked to a static BTC address (bc1q4y…6) with inconsistent pricing tactics ($500 in BTC for consumers, $2,000 for small businesses). Though not as high-profile as Conti or REvil, Culex’s Chaos lineage exemplifies how low-barrier builder kits continue to saturate ransomware “as-a-service”, hence security teams should hunt broader Chaos IOCs (hash f1a3a4bcad1eff…, mutex ChaosEncryptedMutex) rather than the sole .culex flag.

Stay patched, backup offline, and share IOCs with the community to curtail the next kit-built wave.