Bypass !!top!! | Hvci

Modern CPUs use hardware-based shadow stacks to prevent ROP attacks.

This article explores what HVCI is, why it is a high-value target for attackers, and the common techniques used to circumvent these protections. What is HVCI?

Instead of writing new code, an attacker uses a BYOVD vulnerability to overwrite system configurations, tokens, or flags stored in data pages. For example, they might modify the token of a user-mode process to escalate privileges to NT AUTHORITY\SYSTEM , or manipulate process structures to hide malware from the task manager. The hypervisor allows this because no code permissions are being altered. 3. Return-Oriented Programming (ROP) and JOP in the Kernel

: Use a driver with a known "arbitrary write" vulnerability to modify kernel data structures (like process tokens or security callbacks) rather than trying to execute new code. Hvci Bypass

To counter BYOVD attacks, Windows implements an automated, cloud-updated driver blocklist. When a signed driver is found to possess vulnerabilities that facilitate an HVCI bypass, its certificate hash is added to the blocklist. Windows Defender Application Control (WDAC) dynamically blocks these drivers from initializing, rendering the BYOVD vector ineffective for known vulnerable assets. 2. Kernel Data Protection (KDP)

Historically, certain third-party software suites or poorly implemented virtual machine software allocated persistent RWXcap R cap W cap X

Attackers drop a legitimate, cryptographically signed driver (often a legacy hardware utility or anti-cheat driver) that contains a known security flaw, such as an arbitrary memory read/write vulnerability. Modern CPUs use hardware-based shadow stacks to prevent

Microsoft actively maintains a built-in driver blocklist in Windows. When a signed driver is found to have vulnerabilities that enable BYOVD attacks, its certificate hash is added to the blocklist, preventing it from being loaded even if it possesses a valid signature.

HVCI has successfully forced a paradigm shift in Windows kernel security. By decoupling code integrity verification from the standard kernel and placing it into a hypervisor-protected vault, it has eradicated traditional code-injection methods.

Tools like KVC demonstrate how to use a legitimate, signed driver to patch kernel callbacks (like CiValidateImageHeader ) in memory temporarily to load an unsigned target driver. Mitigation and Defense Instead of writing new code, an attacker uses

: Regularly update the operating system and drivers to patch known vulnerabilities.

To fully appreciate HVCI bypass techniques, it's essential to understand what HVCI is and how it protects the Windows kernel.

HVCI stops this by separating the operating system into Virtual Trust Levels (VTLs) using a hypervisor (Hyper-V):