# googleprojectzero/fuzzilli

A JavaScript Engine Fuzzer

Repository: https://github.com/googleprojectzero/fuzzilli
Canonical: https://ross.abutalabs.com/products/fuzzilli
Language: Swift
License: Apache-2.0
License Family: permissive
Last push: 2026-08-20T18:05:50+00:00

## Health v2 (maintenance only)
Score: 67/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 98, release rhythm 8, longevity 100
- inputs: {"age_days": 2723, "days_push": 13, "days_rel": null, "gap_med": null, "n_releases_24m": 0}
- flags: none
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 2334, forks 368 (observed 2026-08-28T04:06:37.941571+00:00)

## What it is
Fuzzilli is a coverage-guided fuzzer for JavaScript engines, built in Swift by Google Project Zero. It generates test programs via a custom intermediate language (FuzzIL) that is mutated and translated to JavaScript to find interpreter and JIT compiler bugs.

## Use cases
- fuzz a JavaScript engine like V8 or SpiderMonkey for security bugs
- find JIT compiler vulnerabilities in interpreters
- discover zero-day bugs in JavaScript runtimes
- run coverage-guided fuzzing campaigns on custom language interpreters
- research semantic correctness of generated test programs
- scale fuzzing across cloud compute instances

## When to choose
- you are doing vulnerability research on JavaScript engines or JIT compilers
- you need a coverage-guided, mutation-based fuzzer with a custom IL for dynamic languages
- you want to find deep interpreter bugs that naive fuzzers miss

## When to avoid
- you need to fuzz web applications or APIs rather than language interpreters
- you cannot compile the target engine with coverage instrumentation
- you want a simple black-box fuzzer without building Swift toolchains and applying engine patches

## Facets
- artifact type: cli-tool
- maturity: active
- function: fuzzing, testing, security, interpreter, compiler
- domain: security, penetration-testing, programming-languages, developer-tools
- platform: cli
- tags: javascript-engine, coverage-guided-fuzzing, vulnerability-research, intermediate-language, jit-compiler, linux, macos, docker, swift

## Member repositories
- googleprojectzero/fuzzilli (main) score 67

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:06:37.941571+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-30T02:37:45.780825+00:00, confidence not recorded.
  - readme: https://github.com/googleprojectzero/fuzzilli (fetched 2026-08-28T04:06:37.941571+00:00, sha dd35256a4449)
- Data as of 2026-08-30T08:39:29.467469+00:00.