In a newly detected attack campaign, the attackers behind RobbinHood ransomware deploy legitimate, digitally signed hardware drivers to delete security tools on target machines before they encrypt files.
These attacks exploit known vulnerability CVE-2019-19320, report Sophos researchers who investigated two attacks employing this technique. The flaw exists in a signed driver that is part of a now-deprecated software package published by Taiwanese motherboard manufacturer Gigabyte. When it was patched with proof-of-concept code in 2018, Gigabyte said its products weren't affected by the flaws. While the company later rescinded its statement, it continued using the vulnerable driver, which is still a threat.
The code-signing mechanism used to digitally sign the driver comes from Verisign, which has not revoked the signing certificate, Sophos' Andrew Brandt and Mark Loman write in a blog post on their findings. As a result, the Authenticode signature for the driver remains valid.
The attackers use the Gigabyte driver as a wedge to load a second, unsigned driver onto a Windows machine. This second driver then has the freedom to kill processes and files belonging to security tools and bypass tamper protection so the ransomware can continue to spread. This technique has been used to subvert a setting in kernel memory in Windows 7, 8, and 10.
"The malicious driver contains only code to kill, nothing else," Loman, director of engineering for Sophos, said in an email to Dark Reading. "So even if you have a fully patched Windows computer with no known vulnerabilities, the ransomware provides the attackers with one that lets them destroy your defenses as a precursor to the ransomware attacks."
This marks the first time Sophos has seen ransomware deploy its own legitimately signed – albeit vulnerable – third-party driver to assume control over a device and use it to deactivate its installed security software, evading the capabilities built to block this kind of malicious activity. By disabling the protection, attackers pave the way for their malware to install and execute ransomware.
Over the course of their investigation, researchers detected several traits indicating these campaigns have the same author as RobbinHood, the same ransomware that struck the city of Baltimore last May.
Loman recommends what he calls a "three-pronged approach" to defend against this type of attack. For starters, because today's attacks use myriad techniques, defenders should adopt different technologies to disrupt multiple stages of an attack, integrate the public cloud into their security strategy, and enable key functionalities like tamper protection in their endpoint protection software.
Second, he suggests adopting strong security practices including multi-factor authentication, complex passwords, limited access rights, regular patching, and backups. And finally, Loman encourages organizations to continue investing in employee security training.