BlackCat Ransomware Deploys New Signed Kernel Driver

silversurfer

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Aug 17, 2014
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Executive Summary​


In late December 2022, Mandiant, Sophos and Sentinel One, via a coordinated disclosure, reported malicious kernel drivers being signed through several Microsoft hardware developer accounts (certified by Microsoft’s Windows Hardware Developer Program). These profiles had been used in a number of cyberattacks that included ransomware-based incidents. Microsoft subsequently revoked several Microsoft hardware developer accounts that were abused in these attacks.

In this blog post, we will provide details on a BlackCat ransomware incident that occurred in February 2023, where we observed a new capability, mainly used for the defense evasion phase, that overlaps with the earlier malicious drivers disclosed by the three vendors. BlackCat affiliates have been known to use multiple techniques during the defense evasion phase, impairing defenses by disabling and modifying tools or using techniques as safe mode boot.

Our analysis sheds light on this new capability, which involves the use of a signed kernel driver for evasion. We believe that this new kernel driver is an updated version that inherited the main functionality from the samples disclosed in previous research. The driver was used with a separate user client executable in an attempt to control, pause, and kill various processes on the target endpoints related to the security agents deployed on the protected machines.

Malicious actors use different approaches to sign their malicious kernel drivers: Typically by abusing Microsoft signing portals, using leaked and stolen certificates, or using underground services. In our case, the attackers tried to deploy the old driver disclosed by Mandiant, which is signed through Microsoft (SHA256: b2f955b3e6107f831ebe67997f8586d4fe9f3e98). Since this driver has already been previously known and detected, the malicious actors deployed another kernel driver signed by a stolen or leaked cross-signing certificate. Trend Micro continues to monitor the abuse of any signed drivers and the related tools, tactics, and procedures (TTPs) associated with this attack surface.
 

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