Introduction
Rootkits are malware implants which burrow themselves in the deepest corners of the operating system. Although on paper they may seem attractive to attackers, creating them poses significant technical challenges and the slightest programming error has the potential to completely crash the victim machine. In our APT predictions for 2022, we noted that despite these risks, we expected more attackers to reach the sophistication level required to develop such tools. One of the main draws towards malware nested in such low levels of the operating system is that it is extremely difficult to detect and, in the case of firmware rootkits, will ensure a computer remains in an infected state even if the operating system is reinstalled or the user replaces the machine’s hard drive entirely.
In this report, we present a UEFI firmware rootkit that we called CosmicStrand and attribute to an unknown Chinese-speaking threat actor. One of our industry partners, Qihoo360, published a blog post about an early variant of this malware family in 2017.
Affected devices
Although we were unable to discover how the victim machines were infected initially, an analysis of their hardware sheds light on the devices that CosmicStrand can infect. The rootkit is located in the firmware images of Gigabyte or ASUS motherboards, and we noticed that all these images are related to designs using the H81 chipset. This suggests that a common vulnerability may exist that allowed the attackers to inject their rootkit into the firmware’s image.
In these firmware images, modifications have been introduced into the CSMCORE DXE driver, whose entry point has been patched to redirect to code added in the .reloc section. This code, executed during system startup, triggers a long execution chain which results in the download and deployment of a malicious component inside Windows.
Looking at the various firmware images we were able to obtain, we assess that the modifications may have been performed with an automated patcher. If so, it would follow that the attackers had prior access to the victim’s computer in order to extract, modify and overwrite the motherboard’s firmware. This could be achieved through a precursor malware implant already deployed on the computer or physical access (i.e., an evil maid attack scenario). Qihoo’s initial report indicates that a buyer might have received a backdoored motherboard after placing an order at a second-hand reseller. We were unable to confirm this information.
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CosmicStrand: the discovery of a sophisticated UEFI firmware rootkit
In this report, we present a UEFI firmware rootkit that we called CosmicStrand and attribute to an unknown Chinese-speaking threat actor.
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