Lessons Learned from Building Custom Hacker Hardware

Lessons Learned from Building Custom Hacker Hardware

Speakers: c4m0ufl4g3 (Jonathan Fischer)

Conference: BSides SF

YouTube: https://www.youtube.com/watch?v=qaYVBg8tNec

Overview

In this insightful talk, Jonathan Fischer, known as c4m0ufl4g3, delves not into the capabilities of a specific piece of offensive hardware, but rather into the arduous and often unpredictable journey of bringing such a device from a mere concept to a deployable product. Co-created with his colleague Jeremy Miller, their project, dubbed "Injectal and Hide," was released at Defcon 30 and BSides Las Vegas. However, the focus here is on the invaluable lessons learned throughout the development process, offering a rare glimpse into the practical realities, setbacks, and triumphs of custom hardware engineering within the cybersecurity domain.

The talk serves as a practical guide and inspiration for aspiring hardware hackers and security researchers. Fischer meticulously documents the iterative steps, from initial proof-of-concept to final PCB design, highlighting critical decisions, unexpected challenges like component shortages and obscure hardware quirks, and the methodologies employed to overcome them. By sharing their experiences, Fischer aims to equip others with the knowledge and confidence to embark on their own hardware projects, fostering a new generation of innovators capable of building robust and covert security tools.

This presentation is particularly significant because it addresses a common gap in the security community: while many talks showcase the "what" of a project, few illuminate the "how" – the nitty-gritty details of development that often determine success or failure. For offensive practitioners, understanding these development nuances can lead to more effective and resilient tools. For defenders, it provides crucial insights into the sophistication and adaptability of custom implants, informing better detection and mitigation strategies.

Background

▶ Watch: Why share the journey of building custom hacker hardware (0:25)

The genesis of Injectal and Hide emerged from a unique research opportunity granted to Jonathan Fischer and Jeremy Miller, both with a strong hardware focus. This period coincided with a heightened awareness of hardware implants following the public release of the NSA ANT catalog and the widespread popularity of devices like the Rubber Ducky, OMG Cable, and the Demon Seed. While commercial off-the-shelf (COTS) devices offered compelling capabilities, Fischer and Miller quickly identified limitations when assessing their suitability for immediate and long-term red team operations.

Their primary concern was the lack of covertness and adaptability. Many commercial devices, despite their effectiveness, were becoming victims of their own success. For instance, while an OMG cable might be indistinguishable from a legitimate charging cable at first glance, the increasing awareness within the security community meant that a random charging cable might be viewed with suspicion. Furthermore, existing solutions often lacked the flexibility for deep customization, such as adding new radio interfaces or altering form factors beyond their original design. Crucially, concerns over complete ownership of the firmware were paramount, especially when handling sensitive client data (PII or PHI). Many "open source" claims pertained only to payloads, with core firmware remaining proprietary, raising auditability and trust issues.

With these limitations in mind, they established a clear set of goals for their custom hardware project to avoid scope creep:

• Use less common protocols: Moving beyond ubiquitous options like hidden Wi-Fi APs to enhance stealth.
• Covertness and difficult identification: Designing the implant to be visually distinct from known commercial devices and adaptable to various form factors (e.g., keyboards, mice, conference speakers) to surprise targets and maintain longevity.
• Complete ownership: Full access and auditability of the entire firmware stack to ensure data handling security and enable deep customization.
• Custom features: The ability to integrate specific functionalities, such as additional radio types, not possible with COTS products.
• Scalability: A unique ambition to extend the impact from a single PC to an entire floor or building through mesh networking, drastically increasing the chances of operational success.

These ambitious goals laid the groundwork for a hardware development journey that would challenge their assumptions and push their engineering capabilities.

Key Findings

▶ Watch: Deciding to build custom hardware over commercial devices (2:50)

The development of Injectal and Hide, as chronicled by Fischer, was a testament to iterative design and problem-solving, yielding several critical findings:

1. Scope Definition is Paramount: Early on, the speakers emphasized the necessity of clearly defined goals to prevent scope creep, a common pitfall in complex projects. This disciplined approach allowed them to manage expectations and maintain focus throughout development.
2. Unexpected Hardware Quirks are Inevitable: One of the most striking discoveries involved a SparkFun XB3 Thing Plus board that, despite advertising UART (Universal Asynchronous Receiver-Transmitter) support in its product literature, did not have it implemented. The workaround, accidentally discovered by Jeremy Miller, involved grounding the clock pin to force a UART fallback – a solution neither the developers nor SparkFun engineers had anticipated. This highlighted the importance of hands-on testing and the potential for undocumented behaviors in hardware.
3. Performance Bottlenecks Require Iteration: An initial 0.5-second lag in keystroke relay, while seemingly minor, was identified as a critical showstopper. This performance issue, initially attributed to a single board handling multiple radio protocols, necessitated a redesign to offload processing, ultimately leading to a more robust architecture.
4. Software-Defined Interfaces are Key for Flexibility: As the design evolved and dedicated hardware serial lines became scarce, the team successfully implemented software-defined UART (Sircom) interfaces. This demonstrated that with careful attention to variant files in the Arduino environment, software can compensate for hardware limitations, offering greater flexibility in pin assignments.
5. Chip Shortages Demand Creative Solutions: The global chip shortage significantly impacted their ability to source raw SAMD21 microcontrollers. Their solutions included sacrificing existing, readily available Adafruit Trinket M0 boards for their chips, purchasing dev boards from eBay, and even inadvertently establishing a relationship with a vendor through eBay that eventually produced their boards for Defcon. This underlines the resilience and resourcefulness required in hardware development, especially in volatile supply chain environments.
6. PCB Design is a Learning Curve: Neither Fischer nor Miller had prior experience designing custom PCBs. They embraced free tools like EasyEDA and KiCad, learning about footprints (TQFP, QFN), multi-layer boards, power regulation (LDO regulators), and Serial Wire Debugging (SWD) interfaces. The initial "ugly" design, with chips on both sides, led to practical lessons about heat management during soldering.
7. Specialized Tools are Essential for Miniaturization: Working with tiny Surface Mount Technology (SMT) components, such as 0402-sized resistors, proved challenging for hand soldering. The acquisition of a $100 digital microscope dramatically improved visibility and soldering precision, illustrating that appropriate tooling is not a luxury but a necessity for fine-pitch work.
8. Modularity Can Be an Accidental Advantage: Forced by the chip shortage to switch to a header-style radio connector, the team inadvertently created a modular design. This "happy accident" allowed for field-swappable radio "hats," enabling rapid adaptation to different RF environments or protocols without redesigning the entire board.

These findings collectively underscore that building custom hacker hardware is a complex, multi-disciplinary endeavor demanding technical prowess, persistence, and an openness to unexpected challenges and creative problem-solving.

Technical Deep Dive

▶ Watch: Project goals: covertness, difficult identification, and longevity (3:18)

The technical journey of Injectal and Hide began with a Proof of Concept (PoC), leveraging Commercial Off-The-Shelf (COTS) components to validate core functionalities economically and quickly.

The initial PoC involved three main boards:

1. Adafruit Trinket M0: A tiny, $10 board, approximately the size of a quarter, chosen for its small footprint and the ability to flash it with the Arduino bootloader. It features a dedicated USB serial line, making it ideal for intercepting keyboard data. It runs on the Microchip ATmega SAMD21 chip.
2. Arduino Nano 33 IoT: A slightly larger, $25 board, also running the SAMD21 chipset and compatible with the Arduino IDE. Its key advantage was multiple exposed serial lines for peripherals and integrated BLE (Bluetooth Low Energy) functionality for wireless communication. This board was envisioned as the central processing unit for the implant.
3. SparkFun XB3 Thing Plus: Priced at $60, this board was crucial for its XB Digime Mesh radio. This proprietary protocol, similar to ZigBee, was chosen for its mesh networking capabilities, enabling scalability up to 1,000 nodes and advertised line-of-sight range of 300 feet (100 feet in an office environment). It also offered built-in encryption. The board itself was programmed in MicroPython.

The SAMD21 chipset was a common thread across the M0 and Nano boards. Its widespread use in Arduino boards meant stability, mature libraries for Human Interface Device (HID) emulation, and readily available supply (initially 80 cents per chip, though later rising to $4 due to shortages). It operates at a low power of 3.3 volts, making it suitable for USB bus power.

The initial architecture for the PoC was: Keystrokes -> Trinket M0 -> Arduino Nano 33 IoT (handling BLE and XB) -> PC. The Nano would also relay data over Bluetooth to an "extender" (another Nano/XB setup) which would then mesh out to other extenders and the Python 3 Command and Control (C2) server.

A critical issue emerged during PoC testing: a noticeable 0.5-second lag in keystroke display. This was deemed a showstopper, compromising covertness. The hypothesis was that the single Arduino Nano 33 IoT board was overloaded processing both BLE and XB communications.

This led to Prototype 1, aimed at eliminating the lag. A second Arduino Nano 33 IoT board was added to the extender, specifically to offload BLE processing. The architecture became: Trinket M0 -> Arduino Nano (XB) -> PC, with the extender now consisting of two Nano 33 IoT boards (one for BLE, one for XB) and two SparkFun XB3 boards. It was during this phase that the infamous SparkFun XB3 UART bug was discovered. Despite the product literature, UART communication with the SparkFun board was unreliable. Jeremy Miller's accidental grounding of the clock pin on the UART interface miraculously enabled it to work, forcing a fallback mechanism that SparkFun engineers themselves were unaware of. While a clever hack, this highlighted the unreliability of the SparkFun board for their project.

For Prototype 2, the SparkFun XB3 Thing Plus boards were replaced with just the XB radio modules, directly communicating with the Arduino Nano boards via UART, as the radio modules themselves reliably supported the protocol. This simplification eliminated the need for the problematic SparkFun board, making the extender smaller and cheaper. This iteration successfully eliminated the keystroke lag, confirming the project's viability and meeting their initial software operational goals.

Prototype 3 focused on refining the protocol stack and adding persistent storage. The BLE interface was dropped because it was a common protocol, easily detectable by smartphones. To ensure data persistence and allow for longer deployment times (e.g., a week without collection), a micro SD card was integrated for local storage of keystrokes. The architecture solidified into: Trinket M0 -> Arduino Nano (with SD card and XB radio) -> PC. This required implementing a software-defined UART (Sircom) interface on the Arduino Nano due to a lack of available dedicated hardware serial lines. Fischer stressed the importance of carefully consulting the variant file in the Arduino environment to correctly map software pin numbers to hardware pads.

Before committing to a custom Printed Circuit Board (PCB), Fischer validated working with raw SAMD21 chips. Using SparkFun dev boards and later a test socket for individual chips, he confirmed that while the SparkFun bootloader worked without additional components, the Arduino bootloader required at least two specific capacitors as part of the recommended power circuit for the SAMD21. He ultimately adopted the full recommended circuit for maximum reliability, even if it added minor cost.

The transition to a custom PCB involved navigating design suites like EasyEDA (free, lower learning curve) and KiCad (industry standard, more features, steeper learning curve). The design process started with adapting the open-source Arduino Nano schematic, removing unused components, and understanding the purpose of others. Key PCB design considerations included:

• Power: Utilizing the USB 5V DC supply and regulating it down to 3.3V for the SAMD21 chips using a Low Dropout (LDO) regulator capable of supplying up to 1 Amp, ensuring stable power even if the USB bus was overloaded (spec: 500mA for USB 2.0).
• Connections: Deciding on USB connectors, wire leads, solder pads, or header pins for modularity.
• Component Footprints: Understanding TQFP (Thin Quad Flat Package), suitable for hand soldering but fragile, versus QFN (Quad Flat No-leads), smaller but requiring solder paste. Awareness of grounding pads and copper keep-out areas for RF components was crucial.
• Mounting: Choosing between through-hole (easy to solder, obstructs traces) and SMT (Surface Mount Technology) for compact designs.
• Debugging/Programming: Incorporating an SWD (Serial Wire Debugging) interface – a 2-wire stripped-down JTAG for ARM controllers – for flashing bootloaders and firmware using tools like the Sega JLink or Allax J-Tech adapter with Microchip Atmel Studio.

The chip shortage significantly impacted the custom PCB phase, forcing the team to source chips creatively (eBay, sacrificing COTS boards) and pivot designs. An initial PCB design with chips on both sides proved problematic for soldering due to heat transfer. Subsequent iterations moved all components to one side and corrected a polarity error in the serial communication lines between the microcontroller and radio. The final PCB design, refined for ease of manufacturing (e.g., JLCPCB), 3D-printed cases, and improved mounting, incorporated a header-style connector for the radio. This accidental design choice, born out of necessity, transformed into a feature allowing for field-swappable radio "hats," demonstrating the adaptive nature of hardware development.

Demo / Proof of Concept

▶ Watch: Project goal: unique scalability for broader impact (5:50)

This particular talk is not a live demonstration of the "Injectal and Hide" hardware in action, but rather a detailed retrospective on its development journey. The speaker, Jonathan Fischer, explicitly states at the outset that he has presented on the capabilities of the device in prior conferences. Instead, this presentation focuses on the "lessons learned" during the creation of the hardware.

However, the talk does include visual aids and descriptions of the various developmental stages, which serve as a form of "proof of concept" for the design process itself. Fischer displays images of the early "rat's nest" prototypes on breadboards, illustrating the complexity of wiring multiple COTS components together. He also presents the progressive evolution of their custom Printed Circuit Board (PCB) designs, from initial "ugly" layouts to the final, more professional version released at Defcon. These visual examples and the detailed technical explanations of each iteration effectively demonstrate the feasibility and challenges of their custom hardware build.

Defensive Implications

▶ Watch: Beginning the proof of concept with commercial off-the-shelf components (6:35)

While the talk focuses on the offensive journey of building custom hardware, the insights shared by Jonathan Fischer carry significant defensive implications for organizations and security professionals:

1. Awareness of Custom Implants: The primary takeaway for defenders is that adversaries are building highly customized hardware implants specifically designed to evade detection. Relying solely on signatures for known commercial devices (like OMG cables or Rubber Duckies) is insufficient, as custom solutions can appear in novel form factors and utilize less common protocols.
2. Enhanced Physical Security Scrutiny: The emphasis on covertness, adaptability, and changing form factors (e.g., implants in keyboards, mice, conference room speakers) means defenders must elevate their physical security posture. Any unknown or suspicious peripheral, even seemingly innocuous ones, should be treated with extreme caution. Regular audits of physical IT assets, especially in sensitive areas, are crucial.
3. Beyond Wi-Fi and Bluetooth Monitoring: The project's goal to "use less common protocols" and its adoption of Digime Mesh radio highlights the need for broader RF spectrum monitoring. Defenders should not limit their wireless intrusion detection to standard Wi-Fi or common Bluetooth channels. Understanding and monitoring for less common or proprietary mesh networking protocols could be vital for detecting covert communications.
4. Supply Chain Integrity: The challenges faced by the team during the chip shortage (sacrificing COTS boards, sourcing from eBay, nurturing vendor relationships) reveal the lengths to which anyone, including adversaries, might go to acquire components. This underscores the importance of robust supply chain security, verifying the authenticity and origin of hardware components, especially for critical infrastructure or highly secure environments.
5. Understanding Hardware Design Vulnerabilities: By understanding the intricacies of hardware development (e.g., the need for SWD interfaces, the use of LDO regulators, component footprints), defenders can better assess potential attack vectors. For example, exposed SWD pins on a device could be an access point for firmware manipulation. Knowledge of common power requirements (e.g., 3.3V) can help identify devices that might be drawing unusual power from a USB bus.
6. Firmware Auditing and Trust: The speaker's emphasis on "complete ownership" of firmware for security and auditability is a strong message for defenders. Organizations should be wary of closed-source hardware, especially those handling sensitive data. Where possible, source hardware with auditable firmware or implement rigorous internal testing to validate device integrity.
7. Performance Anomaly Detection: The "0.5-second lag" issue illustrates that even subtle performance anomalies can be indicators of tampering. While difficult to detect at scale, monitoring for unusual latency in human interface devices could be a niche defensive technique.

In essence, Fischer's talk is a call for defenders to think like hardware hackers, anticipating the ingenuity and persistence required to build and deploy custom implants, and adapting their defensive strategies accordingly.

Key Takeaways

• Hardware development is an iterative, problem-solving journey: Expect and embrace multiple prototypes, unexpected roadblocks, and continuous refinement from concept to product.
• Define project scope rigorously to prevent creep: Clear goals are essential to guide development and maintain focus, especially in complex, multi-disciplinary projects.
• Commercial off-the-shelf solutions have limitations for red teaming: Custom hardware offers unparalleled covertness, adaptability, complete firmware ownership, and the ability to integrate unique features like scalable mesh networking.
• Unexpected hardware quirks and bugs are inevitable: Be prepared for undocumented behaviors, test thoroughly, and be resourceful in finding workarounds (e.g., the SparkFun UART bug workaround).
• Chip shortages and supply chain issues demand creativity and persistence: Adapt designs, source components resourcefully (even from unconventional places like eBay), and be willing to sacrifice existing hardware to keep development moving.
• Invest in appropriate tools and knowledge for PCB design and assembly: Tools like digital microscopes are crucial for working with small SMT components, and a solid understanding of electrical theory and PCB design principles (even without an EE background) is vital.
• Modularity can be an accidental but powerful advantage: Designing for interchangeable components, even if forced by external factors, can significantly enhance the adaptability and longevity of a hardware implant.

About the Speaker(s)

Jonathan Fischer, known by his handle c4m0ufl4g3, is a seasoned professional with over nine years of experience in the information security industry, exclusively focused on the offensive side of cybersecurity. His expertise in offensive security is complemented by a decade of prior experience designing, implementing, and programming electrical control systems for operational technologies (OT), including factory automation and machine controls for off-highway vehicles.

This unique blend of industrial control system engineering and offensive security has provided him with a deep understanding of hardware, embedded systems, and radio frequency (RF) communications. During his time in OT, he gained practical experience with radio controls, custom wire harnesses, embedded programming, and specifying complete sensor systems. This background naturally led to his interest in hardware, IoT, and RF security.

Fischer is an independent researcher, and the work presented in this talk, including the development of "Injectal and Hide" with his colleague Jeremy Miller, is his own. He is dedicated to sharing his "lessons learned" to inspire and educate the security community, fostering the development of new custom hacker hardware.