Jazzer.js is a coverage-guided, in-process fuzzer for the Node.js platform developed by Code Intelligence. It is based on libFuzzer and brings many of its instrumentation-powered mutation features to the JavaScript ecosystem.

Jazzer.js currently supports the following platforms:

• Linux x86_64
• macOS x86_64 and arm64
• Windows x86_64

To use Jazzer.js in your own project follow these few simple steps:

1. Add the @jazzer.js/core dev-dependency

   npm install --save-dev @jazzer.js/core

2. Create a fuzz target invoking your code

   // file "FuzzTarget.js"
   module.exports.fuzz = function (data /*: Buffer */) {
   	const fuzzerData = data.toString();
   	myAwesomeCode(fuzzerData);
   };

3. Start the fuzzer using the fuzz target

   npx jazzer FuzzTarget

4. Enjoy fuzzing!

Jazzer.js requires an entry point for the fuzzer, this is commonly referred to as fuzz target. A simple example is shown below.

module.exports.fuzz = function (data) {
	myAwesomeCode(data.toString());
};

A fuzz target module needs to export a function called fuzz, which takes a Buffer parameter and executes the actual code under test.

The Buffer, a subclass of Uint8Array, can be used to create needed parameters for the actual code under test, so that the fuzzer can detect the usage of parts of the input and mutate them in the next iterations to reach new code paths. In this use-case Buffer is not the nicest abstraction to work with and will be replaced with a more suitable one in the future. An example on how to use the data parameter is shown below, documentation on Buffer can be found in the Node.js documentation.

module.exports.fuzz = function (data) {
	const intParam = data.readInt32BE(0);
	const stringParam = data.toString("utf-8", 4);
	myAwesomeCode(intParam, stringParam);
};

Jazzer.js supports asynchronous fuzz targets out of the box, no special handling or configuration is needed.

The resolution of a Promise returned by a fuzz target is awaited before the next fuzzing input is provided. This enables the fuzzing of async/await, Promise and callback based code.

Asynchronous code needs careful synchronization between the Node.js Event Loop and the fuzzing thread, hence provides a lower throughput compared to synchronous fuzzing. Even so, asynchronous fuzzing is the default mode of Jazzer.js due to its prevalence in the JavaScript ecosystem and because it works for all fuzz targets.

Solely synchronous code can participate in the enhanced performance of synchronous fuzzing by setting the --sync flag when starting the fuzzer.

An example of a Promise based fuzz target can be found at examples/promise/fuzz.js.

It is also possible to use TypeScript, or in that matter any other language transpiling to JavaScript, to write fuzz targets, as long as a modules exporting a fuzz function is generated.

An example on how to use TypeScript to fuzz a library can be found at examples/js-yaml/package.json.

After adding @jazzer.js/core as dev-dependency to a project the fuzzer can be executed using the jazzer npm command. To do so use npx:

npx jazzer <fuzzer parameters>

Or add a new script to your package.json:

"scripts": {
"fuzz": "jazzer <fuzzer parameters>"
}

The general command format is:

jazzer <fuzzTarget> <fuzzerFlags> [corpus...] [-- <fuzzingEngineFlags>]

Detailed documentation and some example calls are available using the --help flag, so that only the most important ones are discussed here.

Further documentation is available at docs/readme.md.

We recorded a live demo in which shows how to get Jazzer.js up and running for your own projects. If you are just getting started, this might be helpful.

You can watch the recording here.

Jazzer.js is inspired by its namesake Jazzer, also developed by Code Intelligence.