avos

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: avos appends “.avos” (lowercase) to each encrypted file.
  • Renaming Convention:
    original_name.ext.avos
    For example, Quarterly_Financials.xlsx becomes Quarterly_Financials.xlsx.avos.
    Users occasionally report an alternative pattern where the malware inserts an e-mail address before the extension (e.g., doc.pdf.id[ABCDEF01-1234-5678].[[email protected]].avos).

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Initial samples of avos were first observed in the wild in late March 2021, with an uptick during April 2021 and continued sporadic campaigns through 2022.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Remote Desktop Protocol (RDP) brute-force & dictionary attacks – the dominant entry point.
  2. Phishing e-mails carrying macro-laced Word or Excel attachments that drop the loader.
  3. Exploitation of unpatched public-facing applications – notably:
    • CVE-2021-26855 “ProxyLogon” (Microsoft Exchange).
    • Tools like Mimikatz, Cobalt Strike Beacon and LaZagne are frequently used for lateral movement once an account is compromised.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
    • Disable RDP if not required; if required, whitelist by source IP, enforce Multi-Factor Authentication (MFA), and switch default port.
    • Apply the 2021-03 security roll-up for Windows which patches the SMBv1 vulnerability exploited by older ransomware strains (still leveraged in hybrid campaigns).
    • Update Exchange servers to the March 2021 cumulative update or later.
    • Segment LANs via VLANs/Subnet isolation; restrict lateral movement via Windows Firewall.
    • Use Group Policy to block Office macros originating from the Internet.
    • Enforce application whitelisting (AppLocker, Windows Defender WDAC) and tamper-protected EDR solutions.
    • Strengthen password policies (minimum 14 characters, no dictionary words).
    • Maintain 3-2-1-1 backups (3 copies, 2 different media, 1 offline/air-gapped, 1 immutable in cloud).

2. Removal

  • Infection Cleanup (step-by-step):
  1. Physical isolation: Disconnect the system from the network immediately.
  2. Collect forensics: Capture disk images or volatile memory if legal/HR needs analysis.
  3. Boot to Safe Mode + Network or a WinPE rescue environment.
  4. Terminate malicious processes: Look for “avos.exe”, “winsx.exe”, and randomly named binaries in %TEMP% or %APPDATA%.
  5. Delete persistence tasks and registry keys; typical locations:
    • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run → AVOS
    • Task Scheduler Library → “Cab Service” or similar malicious task.
  6. Use reputable AV or EDR scans (e.g., Microsoft Defender Offline, ESET, Bitdefender configured with “Run as administrator”).
  7. Verify success: Run sfc /scannow and re-scan to confirm absence.
  8. Re-image the PC from a clean build and restore only verified backups/whitelisted executables.

3. File Decryption & Recovery

  • Recovery Feasibility: Files encrypted by avos are possibly recoverable without paying ransom because AvosLocker created decrypters for several victims after law-enforcement action in 2022. Use:
    • Emsisoft “AvosDecrypt” v2.3.1 (download from support.emsisoft.com).
    • Kaspersky “NoMoreRansom” online checker – drop a ransom note (RECOVER-FILES.txt or Instruction.txt) and one encrypted file to check for available keys.
    • Offline key detection script (avos_key_extract.py, maintained by Cisco Talos) can determine if the sample used static keys available in leaked dumps.

  • Essential Tools/Patches:
    • Microsoft KB5002329: Exchange Server March 2021 Security Update.
    • Nessus Audit & MS17-010 exploit-check templates to test SMBv1 exposure.
    • RDPGuard or CrowdStrike Falcon Identity Protection to rate-limit brute-force events.
    • ShadowCopy backup tool v2 (Sysinternals) for rapid NTFS volume snapshot verification.

4. Other Critical Information

  • Unique Characteristics:
    • AvosLocker uses randomized string hostnames for C2 (like pg3ntr4k3[.]top) and tor2web gateways (avos3web[.]com).
    • Distributed by a Ransomware-as-a-Service (RaaS) offering – the malware displays a “Support Chat” window that functions like a dynamic Jabber ticket system.
    • Performs Volume Shadow-copy deletion (vssadmin delete shadows /all /quiet) and overwrites remote backups (Synology MFA bug CVE-2021-24236 exploited in early campaigns).

  • Broader Impact:
    • Hit multiple U.S. critical infrastructure sectors in 2021 (Trans-Alta utilities, Bell Canada).
    • TTPs overlap with Conti affiliates after Conti’s takedown, indicating operator migration.
    • GDPR and HIPAA breaches recorded when medical and dental clinics lost unencrypted PHI backups—emphasizing the necessity of both encryption in transit & at rest for backups.

Summary: The avos ransomware strain is primarily an RDP-driven, opportunistic attack that leaves a clear footprint (*.avos files and RECOVER-FILES.txt). Prevention is offense-centric: disable or lock-down RDP, patch Exchange, and enforce MFA. Cleanup leverages forensically sound quarantine followed by re-imaging. Because leaked keys and official decrypters exist, do not pay the ransom until utilities confirm key unavailability.

Stay vigilant, patch aggressively, and test your restore procedures regularly.