Cybersecurity researchers discovered a previously undocumented UEFI bootkit that since at least 2012 has been used by threat actors to backdoor Windows systems by modifying a legitimate Windows Boot Manager.
The bootkit, dubbed ‘ESPecter’ by ESET researchers, was discovered during the analysis of a compromised machine. ESET also found user-mode client component with keylogging and document-stealing functionalities related to the bootkit, which indicates that ESPecter’ main purpose is likely cyberespionage.
The findings showed that ESPecter has been in use since at least 2012, previously operating as a bootkit for systems with legacy BIOSes.
The bootkit can bypass Windows Driver Signature Enforcement to load its own unsigned driver, which facilitates its espionage activities.
“By patching the Windows Boot Manager, attackers achieve execution in the early stages of the system boot process, before the operating system is fully loaded. This allows ESPecter to bypass Windows Driver Signature Enforcement (DSE) in order to execute its own unsigned driver at system startup. This driver then injects other user-mode components into specific system processes to initiate communication with ESPecter’s C&C server and to allow the attacker to take control of the compromised machine by downloading and running additional malware or executing C&C commands,” ESET explained.
The researchers said they were unable to attribute ESPecter to any known threat actor, but several findings suggest that an unknown Chinese-speaking threat actor may have been behind the bootkit. Also, it is not clear how ESPecter is distributed.
“At its beginning, it used MBR (Master Boot Record) modification as its persistence method and its authors were continuously adding support for new Windows OS versions. What is interesting is that the malware’s components have barely changed over all these years and the differences between 2012 and 2020 versions are not as significant as one would expect,” ESET said.
Once the installation process begins, the initial ESPecter components modify the Windows Boot Manager component and bypass the Windows DSE to load and run an unsigned malicious driver, which is the actual ESPecter bootkit payload. To successfully boot with a modified boot manager, the Secure Boot feature has to be disabled. At this point, it’s unknown how the ESPecter operators achieved this, but researchers said there are several possible scenarios:
The attacker has physical access to the device (historically known as an “evil maid” attack) and manually disables Secure Boot in the BIOS setup menu (it is common for the firmware configuration menu to still be labeled and referred to as the “BIOS setup menu”, even on UEFI systems).
Secure Boot was already disabled on the compromised machine (e.g., user might dual-boot Windows and other OSes that do not support Secure Boot).
Exploiting an unknown UEFI firmware vulnerability that allows disabling Secure Boot.
Exploiting a known UEFI firmware vulnerability in the case of an outdated firmware version or a no-longer-supported product.
“ESPecter shows that threat actors are relying not only on UEFI firmware implants when it comes to pre-OS persistence and, despite the existing security mechanisms like UEFI Secure Boot, invest their time into creating malware that would be easily blocked by such mechanisms, if enabled and configured correctly,” the researchers concluded.
Last month, Kaspersky revealed that the FinSpy surveillance tool is now packing a feature that allows it to hijack and replace the Windows UEFI (Unified Extensible Firmware Interface) bootloader to infect the target machines without being detected by security solutions.