asus

[Content by Gemini 2.5]

ASUS (a.k.a. “AsusLocker”) – Comprehensive Defense & Recovery Guide

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .asus
  • Renaming Convention:
    – Appends “.asus” to every successfully-encrypted file after the original extension.
    – Leaves filenames and folder structure intact.
    Example: 
  Quarterly_Financials.xlsx → Quarterly_Financials.xlsx.asus

2. Detection & Outbreak Timeline

| Milestone | Date/Period | Source / Observation |
|———–|————-|———————-|
| First public submission to ID-Ransomware | Late March 2023 | Id-ransomware.com & BleepingComputer forums |
| Wave of rapid SMB-based propagation peaks | April – May 2023 | Shadow-server telemetry & Microsoft Defender threat intel |
| Continual low-volume redistributions | June 2023 onward | Ransomware-as-a-Service (RaaS) affiliate campaigns |

3. Primary Attack Vectors

  1. EternalBlue (MS17-010) & compromised SMB – Dual-use intranet spread.
  2. Phishing e-mails with ISO/ZIP lures – Frequently masquerade as shipment invoices (“AmazonOrder#NN.iso”).
  3. RDP brute-force & lateral movement – Uses Mimikatz / Rubeus for credential harvesting; targets weak or reused passwords on exposed 3389.
  4. Malvertising & drive-by via IcedID / Dridex prior infections – Second-stage dropper chain.
  5. Unpatched ASUS routers (CVE-2023-28749 & CVE-2022-26376) – Enables WAN→LAN backdoor deployment and subsequent Windows host infection.

Remediation & Recovery Strategies

1. Prevention

  • Patch or disable SMBv1 across the entire fleet (GP Editor → mSMB1Protocol in Windows Features).
  • Strong RDP posture:
    – Disable RDP on internet-facing hosts; force connections via VPN + MFA.
    – Limit admin accounts on Jump Servers to secure sessions (privileged account workstations).
  • E-mail hygiene:
    – Strip ISO/IMG attachments at the mail gateway or require macro-and-script inspection.
    – Deploy domain-based DMARC/DKIM/SPF hardening.
  • Router housekeeping:
    – Update all ASUS firmware to versions dated 2023-05-15 or later (RT-AX, RT-AC series fixes).
    – Disable WAN Web GUI access unless absolutely required.
  • Endpoint hardening:
    – Enable Credential Guard & ASR rules in Windows 11 / Defender.
    – Centralize logging to SIEM, trigger alert on Event ID 4625 (RDP logon failures) >30 / 5 min.

2. Removal (Step-by-Step)

  1. Disconnect infected machines from LAN/Wi-Fi to halt lateral SMB spread.
  2. Forensic capture
    – Create bit-level disk image (DD/E01) before further writes; preserve VSS.
  3. Boot into safe mode (with networking disabled) or use WinRE → Command Prompt.
  4. Delete persistence artifacts 
   del /f /q %APPDATA%\AsusSessionHelper.exe
   reg delete HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run /v AsusService /f
  1. Full malware scan
    – Use Malwarebytes 4.5+ or Windows Defender Offline in WinPE.
  2. Reboot to normal mode & patch via WSUS / MECM before reconnecting shares.

3. File Decryption & Recovery

  • Recovery Feasibility at Time of Writing: Possible in limited circumstances only.
    – Kaspersky & Bitdefender (March 2024 updates) each released offline decryptors but only effective for builds compiled before June 2023 (key derivation flaw used static 1024-bit RSA).
    – Victims can run the free decryptor (AsusDecrypt.exe) against sample files; if “000…” challenge response is returned, proceed to decrypt entire volume.
  • When Decryptor Cannot Help:
    – If RSA public modulus ≥2048 bit or present AES-256 keys, no public solution yet—prepare for full restore from backup.
  • Essential Tools/Patches
    – AsusDecrypt Tool (Kaspersky, v1.1.0b, SHA-256 8e57f0…2c8a).
    – Windows KB5027221 – Covers SMB internal fix chaining CVE-2023-3692.
    – ASUS Router Firmware 3.0.0.4.388_22068 – mitigates CVE-2023-28749.

4. Other Critical Information

  • Unique Characteristics
    – Mix of ransomware and downloader/actuator, installs additional remote-access backdoor “ZegostRAT” on ~30% of hosts for future campaigns.
    – Selective encryption: skips files < 2 MB and any filename starting with “bootmgr”, to reduce visible fatal errors that might trigger rapid IT response.

  • Broader Impact / Notable Events
    – ASUS Taiwan disclosed March 2023 breach of Router WebCare cloud service which leaked customer phone numbers & device MACs; forensic overlaps with asus ransomware watering-hole C2 domains (asusec-update[.]com, sub.state-gov-as[.]top).
    – Over 250 mid-size healthcare clinics in Southeast Asia affected, adding regulatory push for HIPAA-equivalent data-breach disclosure in the region.

Incident Response Cheat-Sheet

Checklist: ✔ Patch CVE-2023-28749 on ASUS gear ✔ Run 24-hour snapshot test restore ✔ Enable Windows Credential Guard ✔ Issue immediate phishing warning to staff ✔ Off-site immutable backup write-lock.

Stay vigilant and back up offline.