avos2

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: avos2
  • Renaming Convention:
    Files are renamed to:
    <original_filename>.[<payment_email>].[<victim_ID>].avos2
    Example:
    Report_2024.xlsx.[[email protected]].ABCD1234.avos2

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First broadly reported to threat sharing feeds in June 2022. Active campaigns peaked in Q3-Q4 2022, with resurgences observed through 2023. The variant continues in smaller-volume operations as affiliates recycle the builder.

3. Primary Attack Vectors

  • Propagation Mechanisms:
    • Remote Desktop Protocol (RDP) brute-force – scans for Internet-exposed TCP/3389 with weak credentials.
    • Phishing – ZIP or ISO attachments containing malicious .lnk or .exe droppers.
    • Software supply-chain abuse – several documented hits occurred via trojanised pirated game or productivity installers (especially Russian-language forums).
    • EternalBlue (CVE-2017-0144) and BlueKeep (CVE-2019-0708) – in older, unpatched Windows 7 / Server 2008 environments.
    • Credential stuffing – leverages previously breached credential dumps to move laterally via PsExec, PowerShell remoting, and WMI.

Remediation & Recovery Strategies:

1. Prevention

| Measure | Detail |
|—|—|
| Disable/restrict RDP | Close port 3389 on perimeter firewalls or implement VPN-only access with MFA. |
| Network segmentation | Isolate critical servers from end-user VLANs; use ACLs on switches/Next-Gen Firewalls. |
| Patch & inventory | Apply cumulative Windows updates to close EternalBlue, BlueKeep, PrintNightmare, etc. |
| Application control | Enforce Microsoft Defender Application Control (WDAC) or AppLocker blocking policy mode. |
| Mail-filtering & macro-control | Strip ISO/IMG/HTA attachments at gateway; block Office macros from the Internet zone. |
| Offline & cloud-immunised backups | 3-2-1 rule: 3 copies, 2 media, 1 off-line/off-site. Immutable cloud backups (SOB, Wasabi S3-Object Lock). |
| Privileged Access Management (PAM) | Require jump-hardened admin hosts, unique local admin passwords (LAPS), tiered accounts. |

2. Removal

Stage 1 – Incident Triage

  1. Immediately isolate the machine—disable all NICs or unplug Ethernet/Wi-Fi.
  2. Collect volatile artefacts for IR if applicable (memory dump, active connections).
  3. Boot from an offline recovery media (Windows PE, ESET SysRescue Live, Kaspersky Rescue Disk) to prevent encryption continuation.

Stage 2 – Eradication

  1. Delete the following registry Run keys added by avos2:
    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\
    and any RunOnce entries pointing to %APPDATA%\Roaming\Sys.dll.
  2. Locate and remove:
    • %APPDATA%\Roaming\Sys.dll (primary payload)
    • %TEMP%\*.bat, %TEMP%\*.ps1 used for lateral movement restart.
    • Scheduled Task named “UpdateTask” created under \Microsoft\Windows\UpdateOrchestrator.
  3. Use Malwarebytes 4.5+, ESET Online Scanner, or Windows Defender Offline to quarantine detected remnants.
  4. Change every password (local, service, cached domain) on affected and neighbouring systems.
  5. Patch any exploited vector before returning the machine to the network.

3. File Decryption & Recovery

  • Recovery Feasibility: As of June 2024 no publicly free decryptor exists for avos2: it uses ChaCha20+RSA-2048 hybrid encryption, with the private key stored only on the attacker’s side.
  • Exceptions:
    • If avos2 crashed mid-encryption, partially-encrypted files saved under *.avos2~ can sometimes have their header rolled back using PhotoRec to strip 1.5 MB off the file. Efficacy is file-type dependent.
    • Brute-forcing the 2048-bit RSA key is currently infeasible.
  • Imminent hope: Previous Avos-family counterparts (e.g., AvosLocker) had a master key leaked in July 2023 (Avaddon precedent). If researchers manage to obtain similar keys for avos2, decryptors would appear on NoMoreRansom.org and Emsisoft’s website.

Essential Tools/Patches (Prevention & Remediation)
| Tool/Patch | Use-case | URL |
|—|—|—|
| Patch Tuesday (Windows cumulative) | Closes SMB & RDP exploits | search “Windows Security Update Guide” |
| SMBv1 Disable script | Prevents EternalBlue | Microsoft KB 2696547 |
| NGAF/NGFW RDP brute-force rules (FortiGate, Palo Alto) | Block scan/attack traffic | Vendor IPS signatures |
| KB5004442 – RDP CredSSP hard patch | Strengthen CredSSP exception handling | Microsoft Support |
| LAPS | Randomises local admin passwords on every workstation | Microsoft Docs |
| Veeam Immutable Backups (v11+) or AWS S3 Object Lock | Protect backup repositories | Vendor documentation |

4. Other Critical Information

  • Double-extortion playbook: Like its predecessor AvosLocker, avos2 exfiltrates up to 1 TB of sensitive data to Mega / AnonFiles before encryption. The affiliate will threaten publication if ransom is unpaid.
  • Anti-detection tricks: Uses Bring Your Own Vulnerable Driver (BYOVD)—drops the legitimate but old RTCore64.sys to kernel-mode-open memory, disabling EDR hooks and Windows Defender once elevated. Renewed versions are signed with stolen Entrust certificates.
  • Geographic prevalence: First discovered for English-speaking victims, but operations shifted profusely into Asia-Pacific and LATAM during late-2022 and 2023. Affiliate recruitment posts appear on Russian-speaking criminal forums.
  • Typical ransom note: GET_YOUR_FILES_BACK.txt or RESTORE_FILES_INFO.hta placed on every encrypted drive—demand usually starts at 2 BTC or 50,000 USD in Monero (for faster settlement). Escalation countdown is 72–96 h.

Final Guidance:
Do not pay the ransom—payments only embolden attackers and may still result in leaked data. Focus your incident response on contained system rebuilding, patched infrastructure, and proven offline backups. Monitor NoMoreRansom.org, BleepingComputer, and the Avast ThreatLabz blog for breaking news on future decryptor releases.