Security researcher Jonathan Brossard created a proof-of-concept hardware backdoor called Rakshasa that replaces a computer’s BIOS (Basic Input Output System) and can compromise the operating system at boot time without leaving traces on the hard drive.
Brossard, who is CEO and security research engineer at French security company Toucan System, demonstrated how the malware works at the Defcon hacker conference on Saturday, after also presenting it at the Black Hat security conference on Thursday.
Rakshasa, named after a demon from the Hindu mythology, is not the first malware to target the BIOS — the low-level motherboard firmware that initialises other hardware components. However, it differentiates itself from similar threats by using new tricks to achieve persistency and evade detection.
Rakshasa replaces the motherboard BIOS, but can also infect the PCI firmware of other peripheral devices like network cards or CD-ROMs, in order to achieve a high degree of redundancy.
Rakshasa was built with open source software. It replaces the vendor-supplied BIOS with a combination of Coreboot and SeaBIOS, alternatives that work on a variety of motherboards from different manufacturers, and also writes an open source network boot firmware called iPXE to the computer’s network card.
All of these components have been modified so they don’t display anything that could give their presence away during the booting process. Coreboot even supports custom splashscreens that can mimic the ones of the replaced BIOSes.
Existent computer architecture gives every peripheral device equal access to RAM (random access memory), Brossard said. “The CD-ROM drive can very well control the network card.”
This means that even if someone were to restore the original BIOS, rogue firmware located on the network card or the CD-ROM could be used to reflash the rogue one, Brossard said.
The only way to get rid of the malware is to shut down the computer and manually reflash every peripheral, a method that is impractical for most users because it requires specialised equipment and advanced knowledge.
Brossard created Rakshasa to prove that hardware backdooring is practical and can be done somewhere in the supply chain, before a computer is delivered to the end user. He pointed out that most computers, including Macs, come from China.
However, if an attacker would gain system privileges on a computer through a different malware infection or an exploit, they could also theoretically flash the BIOS in order to deploy Rakshasa.
The remote attack method wouldn’t work in all cases, because some PCI devices have a physical switch that needs to be moved in order to flash a new firmware and some BIOSes have digital signatures, Brossard said.
However, Coreboot has the ability to load a PCI extension firmware that takes precedence before the one written on the network card, therefore bypassing the physical switch problem.
The attack “totally works when you have physical access, but remotely it only works 99 percent of the time,” Brossard said.
The iPXE firmware that runs on the network card is configured to load a bootkit — malicious code that gets executed pior to the operating system and can infect it before any security products start.
Some known malware programs store the bootkit code inside the Master Boot Record (MBR) of the hard disk drive. This makes it easy for computer forensics specialists and antivirus products to find and remove.
Rakshasa is different because it uses the iPXE firmware to download the bootkit from a remote location and load it into RAM every time the computer boots.
“We never touch the file system,” Brossard said. If you send the hard drive to a company and ask them to analyse it for malware they won’t be able to find it, he said.
In addition, after the bootkit has done its job, which is to perform malicious modifications of the kernel — the highest-privileged part of the operating system — it can be unloaded from memory. This means that a live analysis of the computer’s RAM won’t be able to find it either.
Detecting this type of compromise is very hard because programs that run inside the operating system get their information from the kernel. The bootkit could very well fake this information, Brossard said.
The iPXE firmware is capable of communicating over Ethernet, Wi-Fi or Wimax and supports a variety of protocols including HTTP, HTTPS and FTP. This gives potential attackers a lot of options.
For example, Rakshasa can download the bootkit from a random blog as a file with a .pdf extension. It can also send the IP addresses and other network information of the infected computers to a predefined email address.
The attacker can push configuration updates or a new version of the malware over an encrypted HTTPS connection by communicating directly with the network card firmware and the command and control server can be rotated among different websites to make it harder for law enforcement or security researchers to take it down.
