VU#718077: UEFI Shell module embedded in SPI Flash can be used to bypass Secure Boot

The UEFI Shell program may expose raw memory access capabilities that, if present in platform firmware for debugging or advanced support use cases, could be abused to undermine UEFI Secure Boot protections. When the UEFI Shell is included in SPI flash, an attacker with the ability to modify UEFI boot configuration may be able to create multiple boot option entries and bypass controls intended to prevent the UEFI Shell from launching while Secure Boot is enabled. This could allow an attacker to modify the pre-boot environment and execute unauthorized software during system startup.

The Unified Extensible Firmware Interface (UEFI) is a firmware specification that defines the interface between a computing platform's hardware and operating system (OS) during the early boot process before the operating system is loaded. UEFI Secure Boot helps ensure that only trusted and digitally signed software is executed during these early stages of platform initialization.

The TianoCore EDK II project provides an open-source reference implementation of the UEFI and Platform Initialization (PI) specifications. The project includes the UEFI Shell, which provides command-line utilities for debugging, diagnostics, and advanced platform management. Many OEM and Independent BIOS Vendor (IBV) firmware implementations include the UEFI Shell in SPI flash for service and support purposes. Because the shell executes in the pre-boot environment, it provides powerful commands such as dmem (display memory) and mm (memory modify) that can access physical memory. Many implementations include a boot entry for the UEFI Shell but remove or suppress it when Secure Boot is enabled to reduce the risk of misuse.