I recently worked on a project that involved embedded systems and reverse engineering. This sort of territory can be a little hairy the first few times out. I ran into some interesting challenges and discoveries along the way which I thought might be worth writing a little bit about. I can’t tell you what the target was. But, it was important. And, we beat the crap out of it. So instead, I’ll tell you what I wish it was: a networked 4-slot toaster.

Now… to make things interesting; Early on, I’d discovered a vulnerability in the toaster that allowed any attacker to load their own firmware on the device. Ouch! My toast! My beautiful toast!

In order to drive home the risk (mostly to the vendor) of the firmware loading vulnerability, I was asked by my customer (also the vendor’s customer) to demonstrate the attack by actually loading malicious firmware onto the device and getting it to run.

Mind you, the request to prove this is actually pretty sane. I had little knowledge of the boot loader, or even of the firmware image format. I couldn’t say for sure that there wasn’t a code-signing feature, which would prevent the toaster from loading any image that wasn’t cryptographically signed by the vendor. That would have rendered the firmware loading attack impotent. To make things worse, the vendor was being pretty light on details. Can’t say I blame them.