DEF CON CTF forensics: finding a key inside a router firmware image
This note revisits a firmware-forensics style challenge from a DEF CON CTF qualifier, where competitors were given a binary firmware image and asked to recover a hidden flag.
Identifying the target
The first step in any firmware-forensics challenge is working out what the image actually is before trying to unpack it. A quick look at the binary's structure and embedded signature strings pointed to firmware for a consumer router, in this case a D-Link DIR-815. Knowing the device model matters because it tells you which filesystem layout and compression scheme to expect, rather than guessing blindly.
Unpacking the image
Firmware images for small office and home routers are usually a bootloader plus one or more compressed filesystem partitions, and this one turned out to use SquashFS compressed with LZMA, a very common combination for embedded Linux devices of that generation. Generic firmware-extraction tooling of the time could split the image into its component partitions and then decompress the filesystem automatically once the right structure was recognised, producing a browsable root filesystem on disk.
Finding the flag
Once the filesystem was extracted, the challenge became ordinary file-system reconnaissance: walking the directory tree looking for anything that did not belong in stock router firmware. A non-standard directory under the home path held a text file containing the flag, a reminder that CTF organisers often hide their answer in the most mundane possible location rather than somewhere technically exotic.
Why this pattern recurs in real audits
The same workflow shows up in legitimate IoT and embedded-device security assessments: identify the device and its firmware format, extract the filesystem rather than trying to reverse-engineer the compressed blob directly, and then look for configuration files, credentials, or debug artefacts left in the image by the vendor. Firmware forensics challenges like this one are good practice for that real-world skill, because the tooling and reasoning transfer directly.
Takeaway
Correct identification of the firmware format upfront saves far more time than jumping straight to a disassembler, and a methodical filesystem walk often finds what a CTF or a real audit is looking for.
Key takeaways
- Identify the firmware's device and compression format before attempting extraction.
- SquashFS plus LZMA was a very common embedded-Linux firmware layout in this era.
- Extracting the filesystem and walking it by hand often reveals hidden files faster than binary reverse engineering.
- The same workflow used in this CTF applies directly to real IoT firmware security assessments.
This note has been rewritten and checked for this edition; it is not a copy of the original post.