Javascript fuzzer for stand-alone shells like D8, Chakra, JSC or Spidermonkey.

Original author: Oliver Chang


This fuzzer may require versions of node that are newer than available on ClusterFuzz, so we use pkg to create a self contained binary) out of this.


You need to intall nodejs and npm. Run npm install in this directory.

Fuzzing DB

This fuzzer requires a fuzzing DB. To build one, get the latest web_tests.zip from gs://clusterfuzz-data/web_tests.zip and unzip it (note https://crbug.com/v8/10891 for making this data publicly available). Then run:

$ mkdir db
$ node build_db.js -i /path/to/web_tests -o db chakra v8 spidermonkey WebKit/JSTests

This may take a while. Optionally test the fuzzing DB with:

$ node test_db.js -i db

Building fuzzer

Then, to build the fuzzer,

$ ./node_modules/.bin/pkg -t node10-linux-x64 .

Replace “linux” with either “win” or “macos” for those platforms.

This builds a binary named ochang_js_fuzzer for Linux / macOS OR ochang_js_fuzzer.exe for Windows.


Use ./package.sh, ./package.sh win or ./package.sh macos to build and create the output.zip archive or use these raw commands:

$ mkdir output
$ cd output
$ ln -s ../db db
$ ln -s ../ochang_js_fuzzer run
$ zip -r /path/output.zip *

NOTE: Add .exe to ochang_js_fuzzer and run filename above if archiving for Windows platform.


Run the tests with:

$ npm test

When test expectations change, generate them with:

$ GENERATE=1 npm test

Generating exceptional configurations

Tests that fail to parse or show very bad performance can be automatically skipped or soft-skipped with the following script (takes >1h):

$ WEB_TESTS=/path/to/web_tests OUTPUT=/path/to/output/folder ./gen_exceptions.sh

Experimenting (limited to differential fuzzing)

To locally evaluate the fuzzer, setup a work directory as follows:

$ workdir/
$ workdir/app_dir
$ workdir/fuzzer
$ workdir/input
$ workdir/output

The app_dir folder can be a symlink or should contain the bundled version of d8 with all files required for execution. Copy the packaged ochang_js_fuzzer executable and the db folder to the fuzzer directory or use a symlink. The input directory is the root folder of the corpus, i.e. pointing to the unzipped data of gs://clusterfuzz-data/web_tests.zip. The output directory is expected to be empty. It'll contain all output of the fuzzing session. Start the experiments with:

$ # Around ~40000 corresponds to 24h of fuzzing on a workstation.
$ NUM_RUNS = 40000
$ python tools/workbench.py $NUM_RUNS

You can check current stats with:

$ cat workdir/output/stats.json | python -m json.tool

When failures are found, you can forge minimization command lines with:

$ MINIMIZER_PATH = path/to/minimizer
$ python tools/minimize.py $MINIMIZER_PATH

The path should point to a local checkout of the minimizer.