The scanning technology identified a previously unknown malware in the Unified Extensible Firmware Interface (UEFI), an essential part of any modern computer device, making it very difficult to detect and remove from the infected devices. The UEFI bootkit used with the malware is a custom version of Hacking Team’s bootkit, leaked in 2015.

UEFI firmware is an essential part of a computer, which starts running before the operating system and all the programs installed in it. If UEFI firmware is somehow modified to contain malicious code, that code will be launched before the operating system, making its activity potentially invisible to security solutions. This, and the fact that the firmware itself resides on a flash chip separate from the hard drive, makes attacks against UEFI exceptionally evasive and persistent – the infection of the firmware essentially means that regardless of how many times the operating system has been reinstalled, the malware planted by the bootkit will stay on the device.

Kaspersky researchers found a sample of such malware used in a campaign that deployed variants of a complex, multi-stage modular framework dubbed as MosaicRegressor. The framework was used for espionage and data gathering with UEFI malware being one of the persistence methods for this new, previously unknown malware.

The revealed UEFI bootkit components were based heavily on the ‘Vector-EDK’ bootkit developed by Hacking Team and which source code was leaked online in 2015. The leaked code most likely allowed perpetrators to build their own software with little development effort and diminished risk of exposure.

The attacks were found with the help of Firmware Scanner, which has been included in Kaspersky products since the beginning of 2019. This technology was developed to specifically detect threats hiding in the ROM BIOS, including UEFI firmware images.

While it was not possible to detect the exact infection vector that allowed the attackers to overwrite the original UEFI firmware, Kaspersky researchers deduced one option of how it could be done based on what is known about VectorEDK from leaked Hacking Team documents. These suggest, without excluding other options, that infections might have been possible through physical access to the victim’s machine, specifically with a bootable USB key, which would contain a special update utility. The patched firmware would then facilitate the installation of a Trojan downloader – malware that enables any payload suitable for the attacker’s needs to be downloaded when the operating system is up and running.

In the majority of cases, however, MosaicRegressor components were delivered to victims using far less sophisticated measures, such as spearphishing delivery of a dropper hidden in an archive together with a decoy file. The multiple modules structure of the framework enabled the attackers to conceal the wider framework from analysis, and deploy components to target machines on demand only. The malware initially installed on the infected device is a Trojan-downloader, a program capable of downloading additional payload and other malware. Depending on the payload downloaded, the malware could download or upload arbitrary files from/to arbitrary URLs and gather information from the targeted machine.

Based on the affiliation of the discovered victims, the researchers were able to determine that MosaicRegressor was used in a series of targeted attacks aimed at diplomats and members of NGOs from Africa, Asia and Europe. Some of the attacks included spearphishing documents in the Russian language, while some were related to North Korea and used as a lure to download malware.

The campaign has not been linked with confidence to any known advanced persistent threat actors.

The presentation on MosaicRegressor was shown at SAS@Home and you can watch its recording below.