Unlocking the Silicon - Hardware Reversing an ATmega16A CTF Board
Introduction
I recently tackled a custom CTF board built around the classic Atmel ATmega16A 8-bit microcontroller from Defcon 33 Bug Bounty Village. This challenge beautifully demonstrated the intersection of hardware interfacing and software reverse engineering.
Challenge Link: https://www.bugbountydefcon.com/bbv-badge-2025
Github Link https://github.com/dhaval17/BBV-CTF
Objective
Bring us a Badge with all four LEDs lit
The rules stated that participants could hack the hardware and turn all LEDs on - making reverse engineering a viable approach.
Step 1: Hardware Reconnaissance and Interfacing
The first rule of hardware hacking: look for developer interfaces. Inspecting the PCB, I spotted a cluster of exposed test pads: MOSI, MISO, SCK, RESET, +, and -.
Using a multimeter in continuity mode, I confirmed the pads were pointing at pins 1 to 4 on the SoC - this matched the standard In-System Programming (ISP) interface for AVR chips, which relies on the SPI protocol to read and write to the chip’s internal Flash and EEPROM memory.
The Interface
The exposed pads provided:
- MOSI (Master Out Slave In)
- MISO (Master In Slave Out)
- SCK (Serial Clock)
- RESET
- VCC (+)
- GND (-)
To interface with it, I used a standard USBasp programmer. A critical safety check was required: since the CTF board was self-powered via its own supply, I avoided connecting the VCC pin to prevent conflicts between power supplies.
Step 2: The Firmware Heist
In commercial products, developers set “Lock Bits” to permanently disable the ISP read functionality, protecting their intellectual property. However, CTF authors sometimes leave these unlocked to make the challenge solvable through multiple approaches.
Using Linux and avrdude, I attempted to read the firmware:
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avrdude -c usbasp -p m16 -U flash:r:firmware_dump.hex:i
avrdude -c usbasp -p m16 -U eeprom:r:eeprom_dump.hex:i
Success! The lock bits were unconfigured (Mode 1), and avrdude successfully pulled the raw machine code from the ATmega16A’s 16KB flash memory. The firmware and EEPROM were saved in Intel HEX format for further analysis.
Step 3: Static Analysis with Ghidra
I imported the firmware into Ghidra, the open-source reverse engineering suite. After setting the architecture to AVR8, Ghidra’s decompiler translated the assembly back into approximate C code.
Hunting for the logic that handled button presses, two critical functions emerged:
The Payload Routine
An Interrupt Service Routine (ISR) triggered by a hardware timer that checked the state of several boolean flags in the SRAM. If the flags matched expected values, the code would trigger hardware interactions like rotating a stepper motor.
The Initializer (Reset)
Looking at the boot sequence, I noticed a loop executing immediately upon power-on. The code was iterating through a specific region of non-volatile Flash memory starting at address 0x154E, comparing sequences and setting corresponding flags.
The lock wasn’t checking a single password - it was loading a master array of five distinct 10-bit sequences.
Step 4: Extracting the Raw Flags
With the exact memory address identified, I didn’t need to decompile any more logic. The verification engine was performing a raw, unencrypted byte-to-byte comparison.
I opened the .hex file in a raw text editor and navigated directly to the memory boundary containing 0x154E. By chunking the data starting at 0x154E into 10-byte blocks, the five physical flag sequences were revealed.
Result: Punching these exact binary sequences into the physical board successfully triggered the hardware, rotating the stepper motor and solving the challenge!
The Flags
There were 5 flags total:
- 4 flags unlocked LEDs at the bottom
- 1 flag unlocked a secret LED animation
Key Insights
Why This Worked
- Exposed Interfaces - The ISP pads were easily accessible without disassembly
- Unconfigured Lock Bits - No firmware protection was enabled
- No Encryption - The flag sequences were stored unencrypted in memory
- Direct Comparison - The verification logic used simple byte-to-byte comparison
Defense Lessons
For hardware developers:
- Always configure AVR lock bits in production
- Encrypt sensitive data stored in EEPROM/Flash
- Consider using one-time programmable (OTP) fuses
- Disable ISP in production or use a key-based authentication scheme
- Add tamper detection and epoxy coat exposed interfaces
Tools Used
- USBasp Programmer - For ISP interface communication
- avrdude - For reading flash and EEPROM
- Ghidra - For firmware analysis and decompilation
- Multimeter - For pin identification and voltage measurement
- Text Editor - For hex file analysis
Conclusion
This challenge demonstrated that hardware security requires multiple layers of protection. Even without sophisticated exploitation techniques, the combination of accessible interfaces and improper security configuration made the challenge straightforward to solve through reverse engineering.
Hardware hacking is a fascinating intersection of electrical engineering, systems design, and software analysis - and it’s a reminder that security must be implemented at all levels of a system.