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OpenOCD

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Theory

OpenOCD (Open On-Chip Debugger) is an open-source debugging tool designed primarily for embedded systems. It is widely used by hardware developers and penetration testers alike to communicate with and control the internals of microcontrollers (MCUs) and System-on-Chips (SoCs). It provides debugging, in-system programming, and boundary-scan testing functionalities. OpenOCD connects to hardware using various communication interfaces such as JTAG (Joint Test Action Group), SWD (Serial Wire Debug), or similar protocols, which are typically used for debugging and flashing firmware on microcontrollers.

With OpenOCD, a user can interact with a target device at a low level, controlling registers, memory, and other essential hardware features. This can be useful in hardware hacking or penetration testing environments, where attackers are trying to reverse engineer or modify embedded systems to find vulnerabilities or access sensitive information.

Commonly, OpenOCD is paired with GDB (GNU Debugger) to provide a rich environment for debugging embedded applications. The debugger is capable of setting breakpoints, examining memory, and stepping through code execution, enabling precise control over what is happening on the device.

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Cheat Sheet

# Install OpenOCD on Linux
sudo apt-get install openocd

# Start OpenOCD with JLink debugger and STM32 target configuration
openocd -f interface/jlink.cfg -f stm32h7x.cfg

# Connect to OpenOCD via telnet
telnet 127.0.0.1 4444

#Connect to OpenOCD via gdb
gdb
(gdb) target extended-remote localhost:3333
(gdb) monitor reset halt
(gdb) load
(gdb) continue

# Halt the CPU
halt

# Reset and initialize the CPU
reset init

# Get flash memory information
halt; flash info 0

# Dump the flash memory to a file
halt; dump_image flashdump.bin 0x00000000 0xF90600

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Usage

An example of using OpenOCD is dumping the firmware of a microcontroller using a JTAG interface:

  • Connect a JTAG programmer to the target device

    • Ensure proper pin alignment for TCK (Test Clock), TMS (Test Mode Select), TDI (Test Data In), TDO (Test Data Out), and GND

  • Install OpenOCD on your system

    • On a Linux system, use the following command to install it:

  • Create or download a configuration file for your target MCU or SoC

    • This file contains the specific instructions to communicate with the target device

  • Start OpenOCD with the configuration file (-f) for the interface you are using and the taregt

    • Example command for Jlink Debugger and a STM32 taregt:

      If everything is correct you should see output like this:

  • Open another terminal and connect with GDB to control the debugging session

    • Example command to connect GDB to OpenOCD:

    • Use GDB commands to control the microcontroller

      • Set a breakpoint:

  • To dump memory or read registers etc. we can use the telnet port

  • Example commands:

The dumped firmware can then be analyzed.

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