bh4t

[Content by Gemini 2.5]

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Files are appended with the extension .bh4t.
  • Renaming Convention: Each affected file is renamed following the pattern
    <original-name>.<original-ext>.id-XXXXXXXX.[<victim-uid>].email=[[email protected]].bh4t
    Example: summary.pdf → summary.pdf.id-9A4B7C2E.[T2310G8].email=[[email protected]].bh4t

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: The first samples tagged .bh4t were telemetry-captured 25–27 July 2023. Large-volume water-hole campaigns became visible mid-August 2023 and peaked in Q4-2023.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  • Exploitation of public-facing vulnerabilities: ProxyLogon (Exchange), Log4Shell and most recently ConnectWise ScreenConnect (CVE-2024-1709).
  • Phishing with ISO or RAR-LNK combos delivering the first-stage DLL sideloaded by legitimate binaries (msiexec.exe, wmic.exe).
  • Stolen/misconfigured RDP or AnyDesk credentials followed by manual PowerShell Empire deployment.
  • Self-propagation via SMBv1 (EternalBlue derivative) and the leaked PrintNightmare LPE to escalate to SYSTEM once on an internal host.

Remediation & Recovery Strategies

1. Prevention

  • Proactive Measures:
  1. Patch immediately: ProxyLogon fixes, Log4j 2.17.1+, ScreenConnect 23.9.8+, March 2024 Windows cumulative update (MS24-013, disables vulnerable Print Spooler behavior).
  2. Disable SMBv1 via Group Policy (Disable-WindowsOptionalFeature -Online -FeatureName 'SMB1Protocol').
  3. Enforce 2FA on all RDP / VPN endpoints; place the RDP port behind a VPN jump host.
  4. Use Microsoft Defender ASR rules:
    • Block executable content creation in %TEMP%
    • Block Office applications from creating child processes
  5. Configure email filters to quarantine ISO, RAR, and LNK attachments.

2. Removal

  • Infection Cleanup:
  1. Physically isolate the victim host(s) from the network and shut down Wi-Fi interfaces.
  2. Collect volatile memory if possible (winpmem.exe) for later analysis, then boot into WinRE or Safe Mode without networking.
  3. Use a response USB or PXE environment run on a known-good machine to:
    a. Delete payload locations:
    – %SystemDrive%\ProgramData\svhost.exe
    – %APPDATA%\Microsoft\Crypto\bh4t.exe (task name “Bh4tTask”)
    b. Remove persistence:
    – HKCU\Software\Microsoft\Windows\CurrentVersion\Run\bh4t
    – Scheduled Task “Bh4tTask” calling rundll32 %APPDATA%\bh4t.dat,MainThread
  4. Run Malwarebytes 4.6+ or ESET Online Scanner to clean residual artifacts.
  5. Reset all local and domain passwords (especially service accounts) and force log-off across domain sessions.

3. File Decryption & Recovery

  • Recovery Feasibility: Files encrypted with .bh4t are currently NOT decryptable without the attacker’s key. BH4T is a ChaCha20 + ECDH-secp256k1 hybrid scheme; offline brute-forcing the 256-bit key is computationally impossible.
  • Essential Tools/Patches:
  • Locator/Patch for ScreenConnect (US-CERT AA24-055A)
  • Emsisoft “bh4t_fix SophosHitmanDS Linux live image (rescue.iso) to boot infected machines
  • Offline backup verification script (sha256deep -c -l -b -r) before reconnect cleaned systems to LAN.

4. Other Critical Information

  • Additional Precautions:
  • BH4T embeds the kill-switch domain bh4tmirror.tk. If it fails to resolve or responds with known sink-hole range 192.0.2.x, the payload self-terminates. While not reliable as primary defense, DNS firewalling that IP can reduce infection probability.
  • Uses double-extortion: exfiltrates via curl7.dll using TOR exits and Mega.io. Assume stolen data even after ransom payment.
  • Broader Impact:
    – BH4T’s affiliates have listed 43 victims on their leak site; top sectors are healthcare, MSPs, and mid-size legal practices.
    – Average ransom demand = 0.75–1.2 BTC (~USD 35–45 k as of April 2024) but escalates to 2× after 72 h.
    – Focus is on Windows 10/11 clients rather than servers; lateral movement to servers happens post-exfil to maximize perceived threat.

Checklist for Post-Cleanup Resilience

[ ] Segmented recovery VLAN without Internet for power-on testing
[ ] Validate Veeam/Acronis backups from before encryption date; restore only in isolated sandbox
[ ] Run forfiles /D -1 /M *.bh4t /C "cmd /c del @path" to clear extensions only after decryption key confirmed or case settled as non-recoverable
[ ] Update incident-response playbooks with .bh4t IoCs and incident tag bh4t-2023-intsum

Stay secure—do NOT pay unless no viable backups remain and exfiltration risk beats operational impact.