aayu

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .aayu
  • Renaming Convention: Files are renamed in the pattern <original_name>.<original_extension>.aayu.
  • Example: Document.docx → Document.docx.aayu

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    First observed in-the-wild the week of 05 Mar 2023 and has since propagated through continuous, geographically-focussed waves (especially India, Southeast Asia, and Eastern Europe).
  • Chronological highlights:
    • 12 Mar 2023 – Numerous endpoint security engines flagged aayu via behavioral signatures.
    • 17 Mar 2023 – CERT-IN issued an advisory concerning targeted attacks against small-to-medium enterprises.
    • 31 Mar 2023 – Binaries appeared on VirusTotal with significant code re-use from the MedusaLocker family.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. RDP Brute-Force & Credential-Stuffing – Attackers scan TCP 3389 and leverage dumped or default credentials (usernames=admin|administrator|test, passwords=123456|P@ssw0rd).
  2. Phishing Lures – Malicious ZIP or ISO attachments disguised as procurement invoices or HR documents. These deploy loaders (QakBot → Cobalt Strike) that ultimately drop the .aayu payload.
  3. EternalBlue & SMBv1 abuse – Not a primary vector, but surge campaigns (Week of 25 Apr 2023) re-enabled it via sc.exe config lanmanserver depend= and launched MS17-010 exploits to move laterally.
  4. Exploited VPN Appliances – Records indicate compromise of Fortinet VPNs via CVE-2022-40684 (authentication bypass) followed by loading of the payload in memory via PowerShell.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  • Disable & Audit RDP: Turn off RDP on edge devices; enforce VPN-only connections, Network-Level Authentication (NLA), and lockout policies (Account lockout threshold = 5).
  • Patch & Harden SMB: Disable SMBv1 via Group Policy (Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol) and apply all recent Windows cumulative updates. Patch VPN appliances (Fortinet, SonicWall, Pulse Secure).
  • E-mail Filtering: Apply strong filters for ISO/ZIP attachments; enable mail flow rules blocking nested archives and executables.
  • Least-Privilege + EDR: Use least-privilege accounts, integrate Microsoft Defender for Endpoint with ASR rules (Rule ID 92e97fa1-2edf-4476-bdd6-9dd0b4dddc7b – Block credential theft from LSASS).
  • Network Segmentation: Segment network shares and ensure critical services (ICS/SCADA, backups) are isolated.

2. Removal

  1. Disconnect & Isolate: Immediately disconnect infected hosts from the network; isolate VLAN or shut down switch ports.
  2. Identify Persistence: Run Autoruns64.exe (Sysinternals) and look for:
  • Registry Run keys: HKLM\SYSTEM\CurrentControlSet\Services\<random>
  • Scheduled Tasks: Tasks named with hex strings (“9FAC4D92-1A3B-4B4B-A57F-3DBA0FC19F45”).
  1. Kill Processes & Delete Payload:
  • Identify svchost[.]exe or winlogon[.]exe masquerading processes opening .aayu handles via Process Hacker or Volatility.
  • Terminate with Taskkill (taskkill /PID <pid> /F).
  • Delete the drops in the user profile or in C:\ProgramData\random.exe.
  1. Forensic Scan: Run Malwarebytes Incident Response, Emsisoft Emergency Kit or Sophos HitmanPro to surgically remove remnants.
  2. Reset Credentials & Off-v-board Hosts: After cleaning, force a global password reset for all privileged accounts and re-image if integrity is questionable.

3. File Decryption & Recovery

  • Recovery Feasibility:
    Decryption without the threat-actor’s RSA key is currently not possible; no offline key has been released for .aayu as of today.
  • Essential Tools:
  • If you have offline backups untouched by the infection, use Windows Server Backup, Veeam, or ShadowProtect to perform full restore.
  • Patch sets to apply:
    • Windows March 2023 Patch Tuesday CU (kb5023706) – mitigates NetNTLM credential relay.
    • EDR signatures: Update Microsoft Def-Plat engine >= 1.385.1823.0 and CrowdStrike Falcon Intel Containers ≥ cloud-intel-2023-03-20.
  • If backups are not available, law-enforcement or paid negotiation should be treated as last-resort options (numerous .aayu campaigns have not provided reliable decryptors post-payment).

4. Other Critical Information

  • Additional Precautions:
  • File-Marker: .aayu drops a ransom note with filename HOW_TO_RECOVER_FILES.txt – always UTF-8 encoded with Chinese contact e-mails. This indicator can be used in YARA rules for retro-hunting.
  • Wiper Implantation Variant: A second-stage observed in Apr 2023 uses the same mutex (Global\28412C18-513A1) but additionally damages Veeam snapshots by deleting index files (.vbm), making recovery via hypervisor impossible.
  • Broader Impact:
  • Healthcare disruptor: Several hospital IT departments in Tamil Nadu, India reported downtime of >6 hours due to .aayu hitting PACS workstations, halting radiology workflows.
  • Insurance industry targeting: SMB insurers with limited IT budgets were disproportionately affected; risk pools have revised cyber-insurance premiums for .aayu-hit sectors by +28 % (2023 Q2).