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A Microsoft Windows policy loophole has been observed being exploited primarily by native Chinese-speaking threat actors to forge signatures on kernel-mode drivers.

“Actors are leveraging multiple open-source tools that alter the signing date of kernel mode drivers to load malicious and unverified drivers signed with expired certificates,” Cisco Talos said in an exhaustive two-part report shared with The Hacker News. “This is a major threat, as access to the kernel provides complete access to a system, and therefore total compromise.”

Following responsible disclosure, Microsoft said it has taken steps to block all certificates to mitigate the threat. It further stated that its investigation found “the activity was limited to the abuse of several developer program accounts and that no Microsoft account compromise has been identified.”

The tech giant, besides suspending developer program accounts involved in the incident, emphasized that the threat actors had already gained administrative privileges on compromised systems prior to use of the drivers.

It’s worth pointing out that the Windows maker had rolled out similar blocking protections in December 2022 to prevent ransomware attackers from using Microsoft-signed drivers for post-exploitation activity.

Driver signature enforcement, which requires kernel-mode drivers to be digitally signed with a certificate from Microsoft’s Dev Portal, is a crucial line of defense against malicious drivers, which could be potentially weaponized to evade security solutions, tamper with system processes, and maintain persistence. The policy change was introduced with the debut of Windows Vista.

The new weakness discovered by Cisco Talos makes it possible to forge signatures on kernel-mode drivers, thereby allowing Windows certificate policies to be bypassed.

This is made possible due to an exception carved out by Microsoft to maintain compatibility, which permits cross-signed drivers if the computer was upgraded from an earlier release of Windows to Windows 10, version 1607; Secure Boot is off in the BIOS; and the drivers were “signed with an end-entity certificate issued prior to July 29th 2015 that chains to a supported cross-signed [certificate authority].”

images from Hacker News