WinSharpFuzz.Instrument 1.0.0

.NET 5.0 .NET Core 2.1
dotnet tool install --global WinSharpFuzz.Instrument --version 1.0.0
This package contains a .NET tool you can call from the shell/command line.
dotnet new tool-manifest # if you are setting up this repo
dotnet tool install --local WinSharpFuzz.Instrument --version 1.0.0
This package contains a .NET tool you can call from the shell/command line.
#tool dotnet:?package=WinSharpFuzz.Instrument&version=1.0.0
nuke :add-package WinSharpFuzz.Instrument --version 1.0.0

WinSharpFuzz: Coverage-based Fuzzing for Windows .NET

  • Fully ported to Windows; can handle .NET/Core/Framework libraries
  • Supports both IL and mixed-mode assemblies
  • Uses libFuzzer heuristics to generate inputs--most libFuzzer options are supported
  • Reports any erroneous exceptions as crashes
  • Well-suited for discovering memory corruption issues in unsafe or Marshal method calls

Table of Contents

  1. Acknowledgements
  2. Requirements
  3. Installation
  4. Usage


This project is an unofficial fork of SharpFuzz for the Windows platform; as such, portions of this codebase were written by Nemanja Mijailovic (see NOTICE.txt for license information). This project wouldn't be possible without the groundwork he laid by developing SharpFuzz.

Special thanks also goes to ManTech Corporation ( for facilitating the development and open source release of this fuzzing tool.


As the name suggests, this library is capable of fuzzing C# libraries in a Windows environment. It supports both IL and mixed-mode .dll assemblies, and .NET Framework libraries can also be instrumented and fuzzed with this tool. Linux and MacOS platforms are not supported by this fuzzer. All you will need is a Windows computer with some way to compile the C# code (either .NET Core/Framework or a Visual Studio installation).

In its current state, the WinSharpFuzz library only supports fuzzing with libFuzzer. A zipped binary is available to use that sends libFuzzer inputs to code instrumented with WinSharpFuzz, so no C/C++ compiler is required; however, if you want to build the binary from source then you will need a recent version of Clang installed.


There are two tools and one library that we'll need to install.

To install the tools, use the following commands within a PowerShell instance:

  • dotnet tool install -g WinSharpFuzz.CommandLine
  • dotnet tool install -g WinSharpFuzz.Instrument

These will install everything needed to run the above commands (the -g flag installs them globally).

As for the WinSharpFuzz library, the way you will go about installing it will depend on what environment you'll be building the WinSharpFuzz harness code. If you're building the project using the dotnet command-line tool, just use dotnet add package WinSharpFuzz.


The process of fuzzing C# code using this library can be broken down into the following four steps:

  1. Writing a test harness that calls the library functions you want to fuzz
  2. Instrumenting the dll libraries using winsharpfuzz-instrument command
  3. Building an executable from the test harness + libraries
  4. Running the executable using the winsharpfuzz command

We'll go through how to perform each of these steps in order.

Writing a Test Harness

In order to run fuzzing on a library, we need to build a .NET executable that will call the appropriate functions in that library. This executable will be passed inputs from the underlying fuzzing framework (such as WinAFL or libFuzzer), and it will report the control flow taken by its code (as well as any exceptions that were thrown).

The following code shows a skeleton WinSharpFuzz test harness:

namespace TestExample1
    public class Program
        public static void Main(string[] args)
            Fuzzer.LibFuzzer.Initialize(() =>
                // Initialize the library here

            Fuzzer.LibFuzzer.Run(bytes =>
                    // pass input bytes to library functions here
                catch (ExpectedException) 
                    // Catch only exceptions that are meant to be thrown by the library

            Fuzzer.LibFuzzer.Cleanup(() =>
                // Call any cleanup functions on the library here 

As we can see, running fuzzing is as simple as calling Fuzzer.LibFuzzer.Run() from the main function, passing in a function (or in our case, a lambda) that accepts a ReadOnlySpan of bytes, and calling our library functions with the passed in bytes. The function passed into Fuzzer.LibFuzzer.Run will be called over and over again with unique data each time, and any unchecked exceptions or program crashes will be reported.

Occasionaly, you may want to fuzz a library class or method that has one-time initialization or cleanup procedures--the Fuzzer.LibFuzzer.Initialize() and Fuzzer.LibFuzzer.Cleanup() methods can be used to accomadate such a case. These methods are completely optional.

Any libraries methods or classes that have been instrumented cannot be called outside of these three functions; any attempt to do so will most likely result in a memory access violation.

Instrumenting the Target Library

In order to provide targeted fuzzing, frameworks usually require some kind of feedback on what control path was taken during a given execution. WinSharpFuzz gathers this feedback by instrumenting the target C# library, or adding hooks at each possible control path in the library. To instrument a target library, use:

winsharpfuzz-instrument \path\to\library.dll

You only need to instrument each library once. If you have multiple other libraries that the target library depends on, you can choose whether you want to instrument those or not--but remember that any classes and methods called from uninstrumented libraries won't give any control flow feedback to the fuzzer.

Building the Test Executable

This should be pretty straightforware--just make sure both WinSharpFuzz and WinSharpFuzz.Common libraries are added as references before you compile your project into an executable.

Running WinSharpFuzz with LibFuzzer

Once your project has successfully compiled, you can run libFuzzer using the following command:

winsharpfuzz --target_path="\path\to\HarnessExecutable.exe" "\path\to\corpus" [other-libfuzzer-options]

"\path\to\corpus" is optional; if specified, it is the path of a folder containing example inputs that will be mutated by the fuzzer to provide unique tests. These inputs can be a mixture of valid and invalid test cases, but they should not cause the program to crash.

Additional libFuzzer options can be used from this command, such as specifying the number of consecutive jobs to be run or maximum input size. More information on these options can be found here. It should be noted that some of the memory options (such as -malloc_limit_mb) will not be useful because of the independant way in which the libfuzzer and .NET executables operate.

Product Versions
.NET net5.0 net5.0-windows net6.0 net6.0-android net6.0-ios net6.0-maccatalyst net6.0-macos net6.0-tvos net6.0-windows
.NET Core netcoreapp2.1 netcoreapp2.2 netcoreapp3.0 netcoreapp3.1
Compatible target framework(s)
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Learn more about Target Frameworks and .NET Standard.

This package has no dependencies.

Version Downloads Last updated
1.0.0 2,584 8/21/2021