Breaking Secure Boot on Google Nest Hub to run Ubuntu #TinyUSB #Nest
Fred’s Notes outlines attacking the Nest Hub (2nd Gen), an always-connected smart home display from Google, in order to boot a custom OS.
First, we explore both hardware and software attack surface in search of security vulnerabilities that could permit arbitrary code execution on the device.
Finally, we build new bootloader and kernel images to boot a custom OS from an external flash drive.
In order to exploit this bug in the Nest Hub bootloader, a USB Mass Storage device that supports larger-than-usual block size is needed. Raspberry Pi Pico is a $4 microcontroller with USB Device support. It also has the great advantage of being supported by TinyUSB, an open-source cross-platform USB Host/Device stack.
TinyUSB project provides a Mass Storage device example code that can turn a Raspberry Pi Pico into a customizable USB flash drive. From this starting point, we can build an exploitation device that will :
inject payload into stack memory
overwrite return address to execute payload
display a cool logo
Hardware exploration led to uncovering an unexpected USB port. Software exploration revealed that it can boot from an USB Mass Storage device. Bug hunting exposed a stack overflow vulnerability in the DOS partition parser.
As a result, an attacker can execute arbitrary code at early boot stage (before kernel execution) by plugging a malicious USB device and pressing two buttons.
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