Exploit Development: No Code Execution? No Problem! Living The Age of VBS, HVCI, and Kernel CFG | Connor McGarr’s Blog

Introduction

I firmly believe there is nothing in life that is more satisfying than wielding the ability to execute unsigned-shellcode. Forcing an application to execute some kind of code the developer of the vulnerable application never intended is what first got me hooked on memory corruption. However, as we saw in my last blog series on browser exploitation, this is already something that, if possible, requires an expensive exploit - in terms of cost to develop. With the advent of Arbitrary Code Guard, and Code Integrity Guard, executing unsigned code within a popular user-mode exploitation “target”, such as a browser, is essentially impossible when these mitigations are enforced properly (and without an existing vulnerability).

Another popular target for exploit writers is the Windows kernel. Just like with user-mode targets, such as Microsoft Edge (pre-Chromium), Microsoft has invested extensively into preventing execution of unsigned, attacker-supplied code in the kernel. This is why Hypervisor-Protected Code Integrity (HVCI) is sometimes called “the ACG of kernel mode”. HVCI is a mitigation, as the name insinuates, that is provided by the Windows hypervisor - Hyper-V.

HVCI is a part of a suite of hypervisor-provided security features known as Virtualization-Based Security (VBS). HVCI uses some of the same technologies employed for virtualization in order to mitigate the ability to execute shellcode/unsigned-code within the Windows kernel. It is worth noting that VBS isn’t HVCI. HVCI is a feature under the umbrella of all that VBS offers (Credential Guard, etc.).

How can exploit writers deal with this “shellcode-less” era? Let’s start by taking a look into how a typical kernel-mode exploit may work and then examine how HVCI affects that mission statement.

“We guarantee an elevated process, or your money back!” - The Kernel Exploit Committee’s Mission Statement

Kernel exploits are (usually) locally-executed for local privilege escalation (LPE). Remotely-detonated kernel exploits over a protocol handled in the kernel, such as SMB, are usually more rare - so we will focus on local exploitation.

When locally-executed kernel exploits are exploited, they usually follow the below process (key word here - usually):

The exploit (which usually is a medium-integrity process if executed locally) uses a kernel vulnerability to read and write kernel memory.

The exploit uses the ability to read/write to overwrite a function pointer in kernel-mode (or finds some other way) to force the kernel to redirect execution into attacker-controlled memory.

The attacker-controlled memory contains shellcode.

The attacker-supplied shellcode executes. The shellcode could be used to arbitrarily call kernel-mode APIs, further corrupt kernel-mode memory, or perform token stealing in order to escalate to NT AUTHORITY\SYSTEM.

Since token stealing is extremely prevalent, let’s focus on it.

We can quickly perform token stealing using WinDbg. If we open up an instance of cmd.exe, we can use the whoami command to understand which user this Command Prompt is running in context of.

Using WinDbg, in a kernel-mode debugging session, we then can locate where in the EPROCESS structure the Token member is, using the dt command. Then, using the WinDbg Debugger Object Model, we then can leverage the following commands to locate the cmd.exe EPROCESS object, the System process EPROCESS object, and their Token objects.

dx -g @$cursession.Processes.Where(p => p.Name == "System").Select(p => new { Name = p.Name, EPROCESS = &p.KernelObject, Token = p.KernelObject.Token.Object})

dx -g @$cursession.Processes.Where(p => p.Name == "cmd.exe").Select(p => new { Name = p.Name, EPROCESS = &p.KernelObject, Token = p.KernelObject.Token.Object})

The above commands will:

Enumerate all of the current session’s active processes and filter out processes named System (or cmd.exe in the second command)

View the name of the process, the address of the corresponding EPROCESS object, and the Token object

Then, using the ep command to overwrite a pointer, we can overwrite the cmd.exe EPROCESS.Token object with the System EPROCESS.Token object - which elevates cmd.exe to NT AUTHORITY\SYSTEM privileges.

It is truly a story old as time - and this is what most kernel-mode exploit authors attempt to do. This can usually be achieved through shellcode, which usually looks something like the image below.

However, with the advent of HVCI - many exploit authors have moved to data-only attacks, as HVCI prevents unsigned-code execution, like shellcode, from running (we will examine why shortly). These so-called “data-only attacks” may work something like the following, in order to achieve the same thing (token stealing):

NtQuerySystemInformation allows a medium-integrity process to leak any EPROCESS object. Using this function, an adversary can...