SysWhispers2 – AV/EDR Evasion Via Direct System Calls

SysWhispers helps with evasion by generating header/ASM files implants can use to make direct system calls.

All core syscalls are supported and example generated files available in the example-output/ folder.

Difference Between SysWhispers 1 and 2

The usage is almost identical to SysWhispers1 but you don’t have to specify which versions of Windows to support. Most of the changes are under the hood. It no longer relies on @j00ru‘s syscall tables, and instead uses the “sorting by system call address” technique popularized by @modexpblog. This significantly reduces the size of the syscall stubs.

The specific implementation in SysWhispers2 is a variation of @modexpblog’s code. One difference is that the function name hashes are randomized on each generation. @ElephantSe4l, who had published this technique earlier, has another implementation based in C++17 which is also worth checking out.

The original SysWhispers repository is still up but may be deprecated in the future.

Introduction

Various security products place hooks in user-mode API functions which allow them to redirect execution flow to their engines and detect for suspicious behaviour. The functions in ntdll.dll that make the syscalls consist of just a few assembly instructions, so re-implementing them in your own implant can bypass the triggering of those security product hooks. This technique was popularized by @Cn33liz and his blog post has more technical details worth reading.

SysWhispers provides red teamers the ability to generate header/ASM pairs for any system call in the core kernel image (ntoskrnl.exe). The headers will also include the necessary type definitions.

Installation

> git clone https://github.com/jthuraisamy/SysWhispers2.git
> cd SysWhispers2
> py .\syswhispers.py --help

Usage and Examples

Command Lines

# Export all functions with compatibility for all supported Windows versions (see example-output/).
py .\syswhispers.py --preset all -o syscalls_all

# Export just the common functions (see below for list).
py .\syswhispers.py --preset common -o syscalls_common

# Export NtProtectVirtualMemory and NtWriteVirtualMemory with compatibility for all versions.
py .\syswhispers.py --functions NtProtectVirtualMemory,NtWriteVirtualMemory -o syscalls_mem

Script Output

PS C:\Projects\SysWhispers2> py .\syswhispers.py --preset common --out-file syscalls_common

. ,--.
,-. . . ,-. . , , |-. o ,-. ,-. ,-. ,-. ,-. /
`-. | | `-. |/|/ | | | `-. | | |-' | `-. ,-'
`-' `-| `-' ' ' ' ' ' `-' |-' `-' ' `-' `---
/| | @Jackson_T
`-' ' @modexpblog, 2021

SysWhispers2: Why call the kernel when you can whisper?

Common functions selected.

Complete! Files written to:
syscalls_common.h
syscalls_common.c
syscalls_common_stubs.asm

Before-and-After Example of Classic CreateRemoteThread DLL Injection

py .\syswhispers.py -f NtAllocateVirtualMemory,NtWriteVirtualMemory,NtCreateThreadEx -o syscalls
#include <Windows.h>

void InjectDll(const HANDLE hProcess, const char* dllPath)
{
LPVOID lpBaseAddress = VirtualAllocEx(hProcess, NULL, strlen(dllPath), MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
LPVOID lpStartAddress = GetProcAddress(GetModuleHandle(L"kernel32.dll"), "LoadLibraryA");

WriteProcessMemory(hProcess, lpBaseAddress, dllPath, strlen(dllPath), nullptr);
CreateRemoteThread(hProcess, nullptr, 0, (LPTHREAD_START_ROUTINE)lpStartAddress, lpBaseAddress, 0, nullptr);
}
#include <Windows.h>
#include "syscalls.h" // Import the generated header.

void InjectDll(const HANDLE hProcess, const char* dllPath)
{
HANDLE hThread = NULL;
LPVOID lpAllocationStart = nullptr;
SIZE_T szAllocationSize = strlen(dllPath);
LPVOID lpStartAddress = GetProcAddress(GetModuleHandle(L"kernel32.dll"), "LoadLibraryA");

NtAllocateVirtualMemory(hProcess, &lpAllocationStart, 0, (PULONG)&szAllocationSize, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
NtWriteVirtualMemory(hProcess, lpAllocationStart, (PVOID)dllPath, strlen(dllPath), nullptr);
NtCreateThreadEx(&hThread, GENERIC_EXECUTE, NULL, hProcess, lpStartAddress, lpAllocationStart, FALSE, 0, 0, 0, nullptr);
}

Common Functions

Using the --preset common switch will create a header/ASM pair with the following functions:

  • NtCreateProcess (CreateProcess)
  • NtCreateThreadEx (CreateRemoteThread)
  • NtOpenProcess (OpenProcess)
  • NtOpenThread (OpenThread)
  • NtSuspendProcess
  • NtSuspendThread (SuspendThread)
  • NtResumeProcess
  • NtResumeThread (ResumeThread)
  • NtGetContextThread (GetThreadContext)
  • NtSetContextThread (SetThreadContext)
  • NtClose (CloseHandle)
  • NtReadVirtualMemory (ReadProcessMemory)
  • NtWriteVirtualMemory (WriteProcessMemory)
  • NtAllocateVirtualMemory (VirtualAllocEx)
  • NtProtectVirtualMemory (VirtualProtectEx)
  • NtFreeVirtualMemory (VirtualFreeEx)
  • NtQuerySystemInformation (GetSystemInfo)
  • NtQueryDirectoryFile
  • NtQueryInformationFile
  • NtQueryInformationProcess
  • NtQueryInformationThread
  • NtCreateSection (CreateFileMapping)
  • NtOpenSection
  • NtMapViewOfSection
  • NtUnmapViewOfSection
  • NtAdjustPrivilegesToken (AdjustTokenPrivileges)
  • NtDeviceIoControlFile (DeviceIoControl)
  • NtQueueApcThread (QueueUserAPC)
  • NtWaitForMultipleObjects (WaitForMultipleObjectsEx)

Importing into Visual Studio

  1. Copy the generated H/C/ASM files into the project folder.
  2. In Visual Studio, go to ProjectBuild Customizations… and enable MASM.
  3. In the Solution Explorer, add the .h and .c/.asm files to the project as header and source files, respectively.
  4. Go to the properties of the ASM file, and set the Item Type to Microsoft Macro Assembler.
  5. Ensure that the project platform is set to x64. 32-bit projects are not supported at this time.

Caveats and Limitations

  • Only 64-bit Windows is supported at this time.
  • System calls from the graphical subsystem (win32k.sys) are not supported.
  • Tested on Visual Studio 2019 (v142) with Windows 10 SDK.

Troubleshooting

  • Type redefinitions errors: a project may not compile if typedefs in syscalls.h have already been defined.
    • Ensure that only required functions are included (i.e. --preset all is rarely necessary).
    • If a typedef is already defined in another used header, then it could be removed from syscalls.h.

Credits

Developed by @Jackson_T and @modexpblog, but builds upon the work of many others:

Related Articles and Projects

References to SysWhispers

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