Intel Security has released a tool that allows users to check if their computer's low-level system firmware has been modified and contains unauthorized code.

The release comes after CIA documents leaked Tuesday revealed that the agency has developed EFI (Extensible Firmware Interface) rootkits for Apple's Macbooks. A rootkit is a malicious program that runs with high privileges -- typically in the kernel -- and hides the existence of other malicious components and activities.

The documents from CIA's Embedded Development Branch (EDB) mention an OS X "implant" called DerStarke that includes a kernel code injection module dubbed Bokor and an EFI persistence module called DarkMatter.

EFI, also known as UEFI (Unified EFI), is the low-level firmware that runs before the operating system and initializes the various hardware components during the system boot process. It's the replacement for the older and much more basic BIOS in modern computers and resembles a mini operating system. It can have hundreds of "programs" for different functions implemented as executable binaries.

A malicious program hidden inside the EFI can inject malicious code into the OS kernel and can restore any malware that has been removed from the computer. This allows rootkits to survive major system updates and even reinstallations.

In addition to DarkMatter, there is a second project in the CIA EDB documents called QuarkMatter that is also described as a "Mac OS X EFI implant which uses an EFI driver stored on the EFI system partition to provide persistence to an arbitrary kernel implant."

The Advanced Threat Research team at Intel Security has created a new module for its existing CHIPSEC open-source framework to detect rogue EFI binaries. CHIPSEC consists of a set of command-line tools that use low-level interfaces to analyze a system's hardware, firmware, and platform components. It can be run from Windows, Linux, macOS, and even from an EFI shell.

The new CHIPSEC module allows the user to take a clean EFI image from the computer manufacturer, extract its contents and build a whitelist of the binary files inside. It can then compare that list against the system's current EFI or against an EFI image previously extracted from a system.

If the tool finds any binary files that don't match the clean EFI list, it's possible that the firmware has been infected. The rogue files are listed and can then be further analyzed.

"We recommend generating an EFI 'whitelist' after purchasing a system or when sure it hasn’t been infected," the Intel Security researchers said in a blog post. "Then check EFI firmware on your system periodically or whenever concerned, such as when a laptop was left unattended."

EFI firmware updates for various Mac and Macbook versions are available on Apple's support website.