ESET researchers investigated the misuse of vulnerable kernel drivers in depth saying "Software" drivers are among the different types of kernel drivers that provide particular, non-hardware-related capabilities such as software debugging and diagnostics, as well as system analysis. These have the potential to greatly increase the attack surface.
Although it is no longer possible to directly load a malicious, unsigned driver in current versions of Windows, and kernel rootkits are deemed obsolete, there are still ways to load malicious code into the kernel, particularly through manipulating legal, signed drivers. There are many drivers available from a variety of hardware and software suppliers that allow you to completely access the kernel with minimal effort.
The most common vulnerabilities detected in-kernel drivers:
- Checks that restrict read and write access to critical model-specific registers are disabled (MSRs).
- Exposing the ability to read and write from physical memory in user mode.
- The ability to read and write to virtual kernel memory from user mode is now enabled.
"When malware actors need to run malicious code in the Windows kernel on x64 systems with driver signature enforcement in place, carrying a vulnerable signed kernel driver seems to be a viable option for doing so," says Peter Kálnai, Senior Malware Researcher at ESET and one of the report's co-investigators.
Bring Your Own Vulnerable Driver, or BYOVD, is a technique that has been observed in the wild by both high-profile APT actors and commodity malware, such as the RobbinHood ransomware, which, as commodity malware, aims to reach as many people as possible. As a result, seeing it use a BYOVD approach is uncommon but significant.
Mitigation strategies that work :
- Virtualization-based security is a Windows 10 feature that uses hardware virtualization to place the kernel in a sandbox, safeguarding the operating system with various protections.
- Drivers in recent Windows systems have a valid signature based on an "acceptable" certificate, which can be revoked. Revocation of a vulnerable driver's certificate would be a simple approach to "disarm" it and render it useless.
- When the most notoriously susceptible drivers are detected on a system, Microsoft and numerous third-party security product suppliers, including ESET, use driver blocklisting to detect and eliminate them.
Vulnerable drivers have been exploited by both game cheaters and malware producers, and while significant progress has been made to reduce the impacts, the fight continues. The people responsible for the problem want to remedy it — the vendors who were contacted were quite proactive during the disclosure process, eager to repair the flaws that were discovered.
