Defeating Signed BIOS Enforcement

In this paper we evaluate the security mechanisms used to implement signed BIOS enforcement on an Intel system. We then analyze the attack surface presented by those security mechanisms. Intel provides several registers in its chipset relevant to locking down the SPI flash chip that contains the BIOS in order to prevent arbitrary writes. It is the responsibility of the BIOS to configure these SPI flash protection registers correctly during power on. Furthermore, the OEM must implement a BIOS update routine in conjunction with the Intel SPI flash protection mechanisms. The BIOS update routine must be able to perform a firmware update in a secure manner at the request of the user. It follows that the primary attack surfaces against signed BIOS enforcement are the Intel protection mechanisms and the OEM implementation of a signed BIOS update routine. In this paper we present an attack on both of these primary attack vectors; an exploit that targets a vulnerability in the Dell BIOS update routine, and a direct attack on the Intel protection mechanisms. Both of these attacks allow arbitrary writes to the BIOS despite the presence of signed BIOS enforcement on certain systems.