Implementing And Detecting a PCI Rootkit [ PDF ]

In February 2006, the author presented a means of persisting a rootkit in the system BIOS via the Advanced Configuration and Power Interface (ACPI). It was demonstrated that the ACPI tables within the BIOS could be modified to contain malicious ACPI Machine Language (AML) instructions that interacted with system memory and the I/O space, allowing the rootkit bootstrap code to overwrite kernel code and data structures as a means of deployment.

Whilst using ACPI as a means of persisting a rootkit in the system BIOS has numerous advantages for the rootkit writer over "traditional" means of persistence (that include storing the rootkit on disk and loading it as a device driver), there are several technologies that are designed to mitigate this threat. Both Intel SecureFlash and Phoenix TrustedCore motherboards prevent the system BIOS from being overwritten with unsigned updates.

This paper discusses means of persisting a rootkit on a PCI device containing a flashable expansion ROM. Previous work in the Trusted Computing field has noted the feasibility of expansion ROM attacks (which is in part the problem that this field has set out to solve), however the practicalities of implementing such attacks has not been discussed in detail. Furthermore, there is little knowledge of how to detect and prevent such attacks on systems that do not contain a Trusted Platform Module (TPM). Whilst the discussion mainly focuses on the Microsoft Windows platform, it should be noted that the techniques are equally likely to apply to other operating systems.