The ALPHV ransomware group, also known as BlackCat, has been using signed malicious Windows kernel drivers to avoid detection by security software during their attacks. Trend Micro discovered that the group has improved upon the POORTRY malware, which was previously identified by Microsoft, Mandiant, Sophos, and SentinelOne in ransomware attacks last year.
The POORTRY malware is a Windows kernel driver signed using stolen keys from legitimate accounts in Microsoft’s Windows Hardware Developer Program. The hackers utilize these malicious drivers to terminate security software and elevate their privileges on compromised machines, making it harder to detect their activities.
Trend Micro observed a specific driver, ‘ktgn.sys,’ in the BlackCat attacks of February 2023. Although the digital signature of ktgn.sys has been revoked, the driver can still load on 64-bit Windows systems with enforced signing policies.
It exposes an IOCTL interface that allows a user mode program, ‘tjr.exe,’ to issue commands executed with Windows kernel privileges. The primary command used by the driver is ‘Kill Process,’ which terminates security agent processes on the system.
The analysts from Trend Micro have identified several other commands exposed by the malicious driver, including activating or deactivating the driver, deleting specific files, copying files, and rebooting the system.
However, they noted that some commands related to process and thread notification callbacks were not functioning properly, indicating that the driver may still be under development or in a testing phase.
To protect against these threats, system administrators are advised to add the indicators of compromise shared by Trend Micro to the Windows driver blocklist and enable ‘Driver Signature Enforcement’ to prevent the installation of drivers without valid digital signatures.
