MicroPython's Reverse Engineering Challenges Discussed at DEF CON 32
/ 4 min read
Quick take - MicroPython, a firmware environment for microcontrollers, is increasingly utilized across various sectors for its ease of learning and rapid prototyping, but it presents unique challenges for reverse engineering due to its distinct bytecode and the use of “frozen” modules, as discussed by Wesley McGrew during his DEF CON 32 presentation.
Fast Facts
- MicroPython is a firmware environment for microcontrollers, gaining popularity in industrial, scientific, and educational sectors, particularly in DEF CON badge projects.
- It features a unique bytecode that complicates reverse engineering, as traditional tools cannot decode it, creating barriers for analysts.
- A key feature is the ability to “freeze” modules into firmware, enhancing performance but posing challenges for extraction and analysis.
- Wesley McGrew presented methods for reverse engineering frozen modules using the Raspberry Pi Pico, highlighting tools like mpy-tool for compiling and disassembling .mpy files.
- McGrew emphasized that while frozen modules provide obfuscation, they do not ensure security, and reverse engineering can still uncover their contents.
MicroPython: A Growing Firmware Environment
MicroPython, a firmware environment designed for microcontroller systems, is gaining traction in various fields, including industrial, scientific, and educational sectors. It is particularly noted for its use in DEF CON badge projects. MicroPython is recognized for its ease of learning and rapid prototyping capabilities. It serves as a practical implementation of CPython but uses its own compiled bytecode language. This unique bytecode complicates reverse engineering efforts, as traditional reverse engineering tools are not equipped to decode MicroPython’s bytecode, creating a barrier for analysts attempting to reverse engineer it.
Features and Challenges of MicroPython
A significant feature of MicroPython is the ability to “freeze” modules, which are compiled directly into the microcontroller firmware. This process enhances performance by eliminating the need for separate file loading; however, it also poses challenges for extraction and analysis. Wesley McGrew, a Senior Cyber Fellow with Martin Federal, addressed these issues during his DEF CON 32 presentation, focusing on methods for reverse engineering frozen modules using the Raspberry Pi Pico as a demonstration platform.
Frozen modules are often used in Capture The Flag (CTF) events to obscure code and conceal sensitive information. They reduce metadata and complicate disassembly. While tools exist for handling pre-compiled modules, specific solutions for frozen modules are limited. McGrew emphasized that frozen modules offer obfuscation but do not provide true security, as reverse engineering can still reveal their contents.
Reverse Engineering Process
The reverse engineering process begins with obtaining a firmware image, which can be examined using the picotool command to identify frozen modules on the Raspberry Pi Pico. Some firmware builds may directly enumerate frozen modules, simplifying the analysis. Access to the REPL (Read-Eval-Print Loop) can allow users to list frozen modules if available. During his presentation, McGrew demonstrated the process using a basic “Hello World” Python module, which is compiled and frozen in MicroPython.
The mpy-tool utility is crucial in this process, enabling the compilation of Python code into .mpy files, which are unique to MicroPython. Unlike CPython bytecode, MicroPython’s bytecode format is distinct, and tools designed for CPython are incompatible with it. The mpy-tool can also disassemble .mpy files, revealing essential components such as string tables, object tables, and bytecode.
In reverse engineering efforts, locating the starting address of frozen strings is facilitated by identifying the MP_qstr_frozen_const_pool, which links hashes, lengths, and string data. The structure of the .mpy file includes a header with metadata and supports variable length integer encoding for data structures, optimizing memory usage. The use of “interned strings” and QStrings further enhances efficiency by pooling unique string data.
McGrew noted that parsing nested bytecode can be intricate, especially due to undefined lengths for bytecode blocks. Ghidra, a popular reverse engineering tool, does not natively support MicroPython bytecode. Creating a Unix build of MicroPython can simplify debugging and analysis; however, certain functions may not operate correctly without mock functions in this environment.
To facilitate a deeper understanding of MicroPython bytecode, McGrew recommended a thorough review of the mpy-tool and the vm.c code files. He also suggested studying “The Ghidra Book” by Chris Eagle and K. Karson, which includes contributions from Karen Nance, an expert in MicroPython. McGrew concluded by advocating for the ethical application of the knowledge shared during his talk, emphasizing the importance of responsible use in reverse engineering practices.
Original Source: Read the Full Article Here
