Crafting an EFI Emulator and Interactive Debugger

In 2016 I reversed Apple’s EFI firmware password reset scheme using SCBO files. There was an old rumor that these files were able to unlock firmware password locked Macs (and even a sketchy video about a universal SCBO able to unlock any Mac). That post is available at Apple EFI firmware passwords and the SCBO myth.

All the interesting computing action happened at the EFI execution level. I made good reversing progress with static analysis, but dynamic analysis with a debugger would make the job much easier. I love debuggers because they allow you to quickly test ideas and cut corners while reversing a target. Reading disassembly listings for long periods is tiring. (U)EFI debuggers can be found in the market but they are usually quite expensive (a couple thousand USD).

My solution was to create an emulator and debugger based on Unicorn. At the time I was working a lot with Unicorn so it was natural to use it to solve this problem (“if all you have is a hammer, everything looks like a nail”). After I wrote the blogpost some people directed me to some emulators (TianoCore EmulatorPkg and efiperun). I never tried them to see if they contained an interactive debugger like I wanted. The pain wasn’t big since this was a couple of days project and it was quite fun to write.

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Apple EFI firmware passwords and the SCBO myth

My original goal when I started poking around Apple’s EFI implementation was to find a way to reset a MacBook’s firmware password. My preliminary research found references to a “magical” SCBO file that could be loaded onto a USB flash drive and booted to remove the password. The normal process workflow is to first contact Apple support. Since I don’t have the original sales receipt of this specific Mac, I assume this option isn’t possible, since anyone with a stolen Mac could get the password reset. Things got more interesting when I found a website that allegedly sold the SCBO files – just send them the necessary hash (more on this later), pay USD100, and get a working SCBO file in return. There are videos (in Portuguese but you can watch the whole process) of people claiming this works, and even some claims about an universal SCBO that unlocks multiple Macs.

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London and Asia EFI monsters tour!

Finally back home from China and Japan tour, so it’s time to finally release the updated slides about EFI Monsters. After Secuinside I updated them a bit, fixing stuff I wasn’t happy with and adding some new content. The updated version was first presented at 44CON London. I had serious reservations about going to the UK (not even in transit!) but Steve Lord and Adrian charm convinced me to give it a try. [Read More]

Reversing Prince Harming’s kiss of death

The suspend/resume vulnerability disclosed a few weeks ago (named Prince Harming by Katie Moussouris) turned out to be a zero day. While (I believe) its real world impact is small, it is nonetheless a critical vulnerability and (another) spectacular failure from Apple. It must be noticed that firmware issues are not Apple exclusive. For example, Gigabyte ships their UEFI with the flash always unlocked and other vendors also suffer from all kinds of firmware vulnerabilities. [Read More]