SysWhispers3 : AV/EDR Evasion Via Direct System Calls

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

Why on earth didn’t I create a PR to SysWhispers2?

The reason for SysWhispers3 to be a standalone version are many, but the most important are:

  • SysWhispers3 is the de-facto “fork” used by Inceptor, and implements some utils class which are not relevant to the original version of the tool.
  • SysWhispers2 is moving towards supporting NASM compilation (for gcc/mingw), while this version is specifically designed and tested to support MSVC (because Inceptor will stay a Windows-only framework for the near future).
  • SysWhispers3 contains partially implemented features (such as egg-hunting) which would not be sensible to include in the original version of the tool.

Differences with SysWhispers2

The usage is pretty similar to SysWhispers2, with the following exceptions:

  • It also supports x86/WoW64
  • It supports syscalls instruction replacement with an EGG (to be dynamically replaced)
  • It supports direct jumps to syscalls in x86/x64 mode (in WOW64 it’s almost standard)
  • It supports direct jumps to random syscalls (borrowing @ElephantSeal’s idea)

A better explanation of these features are better outlined i the blog post SysWhispers is dead, long live SysWhispers!

Introduction

Security products, such as AVs and EDRs, usually place hooks in user-mode API functions to analyse a program execution flow, in order to detect potentially malicious activities.

SysWhispers2 is a tool designed to generate header/ASM pairs for any system call in the core kernel image (ntoskrnl.exe), which can then be integrated and called directly from C/C++ code, evading user-lands hooks.

The tool, however, generates some patters which can be included in signatures, or behaviour which can be detected at runtime.

SysWhispers3 is built on top of SysWhispers2, and integrates some helpful features to bypass these forms of detection.

Installation

C:> git clone https://github.com/klezVirus/SysWhispers3.git
C:> cd SysWhispers3
C:> python .\syswhispers.py –help

Usage and Examples

The help shows all the available commands and features of the tool:

C:>python syswhispers.py -h
usage: syswhispers.py [-h] [-p PRESET] [-a {x86,x64}] [-m {embedded,egg_hunter,jumper,jumper_randomized}] [-f FUNCTIONS] -o OUT_FILE [–int2eh] [–wow64] [-v] [-d]
SysWhispers3 – SysWhispers on steroids
optional arguments:
-h, –help show this help message and exit
-p PRESET, –preset PRESET
Preset (“all”, “common”)
-a {x86,x64}, –arch {x86,x64}
Architecture
-c {msvc,mingw,all}, –compiler {msvc,mingw,all}
Compiler
-m {embedded,egg_hunter,jumper,jumper_randomized}, –method {embedded,egg_hunter,jumper,jumper_randomized}
Syscall recovery method
-f FUNCTIONS, –functions FUNCTIONS
Comma-separated functions
-o OUT_FILE, –out-file OUT_FILE
Output basename (w/o extension)
–int2eh Use the old int 2eh instruction in place of syscall
–wow64 Use Wow64 to run x86 on x64 (only usable with x86 architecture)
-v, –verbose Enable debug output
-d, –debug Enable syscall debug (insert software breakpoint)

Command Lines

Standard SysWhispers, embedded system calls (x64)

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

SysWhispers3-only samples

Normal SysWhispers, 32-bits mode
py .\syswhispers.py –preset all -o syscalls_all -m jumper –arch x86
Normal SysWhispers, using WOW64 in 32-bits mode (only specific functions)
py .\syswhispers.py –functions NtProtectVirtualMemory,NtWriteVirtualMemory -o syscalls_mem –arch x86 –wow64
Egg-Hunting SysWhispers, to bypass the “mark of the sycall” (common function)
py .\syswhispers.py –preset common -o syscalls_common -m jumper
Jumping/Jumping Randomized SysWhispers, to bypass dynamic RIP validation (all functions) using MinGW as the compiler
py .\syswhispers.py –preset all -o syscalls_all -m jumper -c mingw

Script Output

PS C:\Projects\SysWhispers2> py .\syswhispers.py –preset common –out-file temp\syscalls_common -v . ,–.
,-. . . ,-. . , , |-. o ,-. ,-. ,-. ,-. ,-. _/ -. | |-. |/|/ | | | -. | | |-' |-. . \ -'-| -' ' ' ' ' '-‘ |-‘ -' '-‘ ”’ /| | @Jackson_T `-‘ ‘ @modexpblog, 2021 Edits by @klezVirus, 2022 SysWhispers3: Why call the kernel when you can whisper? Common functions selected. Complete! Files written to: temp\syscalls_common.h temp\syscalls_common.c temp\syscalls_common.asm
Press a key to continue…

Importing into Visual Studio

  • Copy the generated H/C/ASM files into the project folder.
  • In Visual Studio, go to Project → Build Customizations… and enable MASM.
  • In the Solution Explorer, add the .h and .c/.asm files to the project as header and source files, respectively.
  • Go to the properties of the ASM file, and set the Item Type to Microsoft Macro Assembler.

Compiling outside of Visual Studio

Windows

Makefile for 64 bits:

Makefile.msvc

OPTIONS = -Zp8 -c -nologo -Gy -Os -O1 -GR- -EHa -Oi -GS-
LIBS = libvcruntime.lib libcmt.lib ucrt.lib kernel32.lib program:
ML64 /c syscalls-asm.x64.asm /link /NODEFAULTLIB /RELEASE /MACHINE:X64
cl.exe $(OPTIONS) syscalls.c program.c
link.exe /OUT:program.x64.exe -nologo $(LIBS) /MACHINE:X64 -subsystem:console -nodefaultlib syscalls-asm.x64.obj syscalls.obj program.obj

Compile with nmake:

nmake -f Makefile.msvc

Linux

Makefile for both 64 and 32 bits:

Makefile.mingw

CC_x64 := x86_64-w64-mingw32-gcc
CC_x86 := i686-w64-mingw32-gcc
OPTIONS := -masm=intel -Wall
program:
$(CC_x64) syscalls.c program.c -o program.x64.exe $(OPTIONS)
$(CC_x86) syscalls.c program.c -o program.x86.exe $(OPTIONS)

Caveats and Limitations

  • The Egg-Hunter functionality is not implemented within this tool, it is in Inceptor.
  • System calls from the graphical subsystem (win32k.sys) are not supported.
  • Tested on Visual Studio 2019/2022 with Windows 10 SDK.
  • Support for NASM is not guaranteed.
  • Support for GCC and MinGW is not guaranteed.

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