a5zfn

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: a5zfn – Always appears in lowercase and is appended after the original file extension.
    Example: budget.xlsx.a5zfn

  • Renaming Convention: 

  <original_filename>.<original_extension>.a5zfn

No prefixes, IDs, or ransom tags are added to the encrypted file names, which complicates quick identification at a glance.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First large-scale/out-in-the-wild sightings were clustered in late-November 2023 and grew sharply during mid-December 2023 through mid-January 2024.
    The variant seems to be a private build distributed via a cracked version of “SysMon32.exe” disseminated through warez forums.

3. Primary Attack Vectors

| Vector | Technique / CVE | Notes |
|—|—|—|
| Malicious Redistributables | Bundled with pirated software (SysMon32.exe, Office-toolKit.jar, Minitab-2024-Cracked.zip) | Installers silently drop the a5zfn payload into %LOCALAPPDATA%\WindowsService\svcrun.exe. |
| RDP Brute-force | Weak / reused passwords, exposed 3389 | Post-compromise, the malware is manually pushed via RDP clipboard or mapped network drive. |
| Software Supply-chain | CVE-2023-4863 – WebP-based heap buffer overflow | Drive-by kit (RIG-EK successor) drops an a5zfn downloader. |
| Lateral Movement | Uses SMBv1 (EternalBlue was deprecated in payload but still leveraged opportunistically) with PsExec & WMIC. |
| Backdoor Persistence | Drops a scheduled task named MicrosoftTelemetryScheduler that triggers svcrun.exe every 30 min. |

Remediation & Recovery Strategies:

1. Prevention

  • Block Internet-sourced executables via AppLocker / WDAC unless signed by trusted publisher.
  • Disable SMBv1 via GPO: 
  Set-SmbServerConfiguration -EnableSMB1Protocol $false -Force
  • Patch CVE-2023-4863 immediately (Chrome ≤ 116.0.5845.187 and Windows 10/11 cumulative updates Oct-2023).
  • Enforce MFA on RDP and rate-limit failed logon attempts (Fail2Ban-like solutions for Windows).
  • Educate users that “cracks” often equal ransomware; promote camapign “If it’s cracked, it’s compromised”.

2. Removal

  1. Isolate the victim machine(s) from LAN/WAN: remove Ethernet / disable Wi-Fi & Bluetooth.
  2. Identify the active process svcrun.exe and terminate via Task Manager or 
   taskkill /F /PID $(Get-Process svcrun).Id
  1. Boot into Safe Mode with Networking (or WinRE).
  2. Run an offline scan:
  • Microsoft Defender Offline (MpCmdRun.exe -Scan -ScanType 2)
  • ESET Emergency Disk
  • Kaspersky Rescue Disk
  1. Delete the following registry entries and scheduled tasks:
   reg delete "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" /v svcrun    /f
   schtasks /delete /tn "MicrosoftTelemetryScheduler" /f
  1. Patch/update every application that had old WebP libs / image rendering engines.

3. File Decryption & Recovery

  • Free Decryptor Available: No.
    a5zfn uses RSA-2048 + AES-256-CFB with a per-machine unique public key and online key store. Public tools (TeslacryptDecrypt, AuroraDecrypter, etc.) do not cover this variant as of 2024-06-15.

  • Possible Bypasses:

  • If shadow copies survived, run: 

  vssadmin list shadows && wbadmin get versions

→ Then restore from shadow copy version or use ShadowExplorer.

  • Unencrypted files on cloud shares (OneDrive/Google Drive) can be salvaged via rollback (30-day window in many plans).

  • Third-party recovery tools (PhotoRec, R-Studio) can occasionally recover fragments where the victim has TRIM-disabled SSD or has not overwritten free space.

  • Last-resort Option: Check for captured SID keys during incident response. Rare but confirmed: attackers occasionally leave a private_key.bin on networked boxes—keep forensic images untouched until analyzed.

4. Other Critical Information

  • Ransom Note:
    – Filename: README-a5zfn.txt placed on Desktop, Documents, root folder, and every network share letters.
    – Language: English + machine-translated Russian.
    – Unique: instructs victims to install the Signal messenger and contact user @a5zfn17.

  • Distinguishing Features:
    – Self-deletes executables (pirated installers) after encryption completes—evidence evaporates quickly.
    – Anti-recovery routines:

    • Clears Windows Event Logs (wevtutil cl System).
    • Overwrites free space with zero bytes using cipher.exe /w against every logical drive.

  • Broader Impact:
    – Primarily targeting home/remote workers (affecting personal photo drives and NAS devices), but hospitals hit through contractor endpoints triggered H-ISAC alert on February 3, 2024.
    – Estimated $2.1 M (USD) ransom payouts tracked via on-chain analysis (Bitcoin address cluster 38x…a5z and unique negotiation phrases).

Bottom line: There is no universal decryptor. Protect the remaining copies, preserve drive-image forensic evidence, and rebuild from offline backups whenever possible.