# Fuzz testing in Chromium

[go/chrome-fuzzing](https://goto.google.com/chrome-fuzzing)

[Fuzzing] is a testing technique that feeds auto-generated inputs to a piece
of target code in an attempt to crash the code. It's one of the most effective
methods we have for finding security and stability issues (see
[go/fuzzing-success](http://go/fuzzing-success)). You can learn more about the
benefits of fuzzing at [go/why-fuzz](http://go/why-fuzz).

This documentation covers the in-process guided fuzzing approach employed by
different fuzzing engines, such as [libFuzzer] or [AFL]. To learn more about
out-of-process fuzzers, please refer to the [Blackbox fuzzing] page in the
ClusterFuzz documentation.

[TOC]

## Getting Started

In Chromium, you can easily create and submit fuzz targets. The targets are
automatically discovered by buildbots, built with different fuzzing engines,
then uploaded to the distributed [ClusterFuzz] fuzzing system to run at scale.

Create your first fuzz target and submit it by stepping through our [Getting
Started Guide].

## Advanced Topics

* [Improving fuzz target efficiency].
* [Creating a fuzz target that expects a protobuf] instead of a byte stream as
  input.

  *** note
  **Note:** You can also fuzz code that needs multiple mutated
  inputs, or to generate inputs defined by a grammar.
  ***

* [Reproducing bugs] found by libFuzzer/AFL and reported by ClusterFuzz.
* [Fuzzing mojo interfaces] using automatically generated libprotobuf-mutator fuzzers.

## Further Reading

* [LibFuzzer integration] with Chromium and ClusterFuzz.
* [AFL integration] with Chromium and ClusterFuzz.
* [Detailed references] for other integration parts.
* Writing fuzzers for the [non-browser parts of Chrome OS].
* [Fuzzing browsertests] if you need to fuzz multiple Chrome subsystems.

## Trophies
* [Issues automatically filed] by ClusterFuzz.
* [Issues filed manually] after running fuzz targets.
* [Bugs found in PDFium] by manual fuzzing.
* [Bugs found in open-source projects] with libFuzzer.

## Other Links
* [Guided in-process fuzzing of Chrome components] blog post.
* [ClusterFuzz Stats] for fuzz targets built with AddressSanitizer and
  libFuzzer.

[AFL]: http://lcamtuf.coredump.cx/afl/
[AFL integration]: AFL_integration.md
[Blackbox fuzzing]: https://google.github.io/clusterfuzz/setting-up-fuzzing/blackbox-fuzzing/
[Bugs found in open-source projects]: http://llvm.org/docs/LibFuzzer.html#trophies
[Bugs found in PDFium]: https://bugs.chromium.org/p/pdfium/issues/list?can=1&q=libfuzzer&colspec=ID+Type+Status+Priority+Milestone+Owner+Summary&cells=tiles
[ClusterFuzz]: https://clusterfuzz.com/
[ClusterFuzz Stats]: https://clusterfuzz.com/fuzzer-stats/by-fuzzer/fuzzer/libFuzzer/job/libfuzzer_chrome_asan
[Creating a fuzz target that expects a protobuf]: libprotobuf-mutator.md
[Detailed references]: reference.md
[Fuzzing]: https://en.wikipedia.org/wiki/Fuzzing
[Fuzzing browsertests]: fuzzing_browsertests.md
[Fuzzing mojo interfaces]: ../../mojo/docs/mojolpm.md
[Getting Started Guide]: getting_started.md
[Guided in-process fuzzing of Chrome components]: https://security.googleblog.com/2016/08/guided-in-process-fuzzing-of-chrome.html
[Improving fuzz target efficiency]: efficient_fuzzing.md
[Issues automatically filed]: https://bugs.chromium.org/p/chromium/issues/list?sort=-modified&colspec=ID%20Pri%20M%20Stars%20ReleaseBlock%20Component%20Status%20Owner%20Summary%20OS%20Modified&q=label%3AStability-LibFuzzer%2CStability-AFL%20label%3AClusterFuzz%20-status%3AWontFix%2CDuplicate&can=1
[Issues filed manually]: https://bugs.chromium.org/p/chromium/issues/list?can=1&q=label%3AStability-LibFuzzer+-label%3AClusterFuzz&sort=-modified&colspec=ID+Pri+M+Stars+ReleaseBlock+Component+Status+Owner+Summary+OS+Modified&x=m&y=releaseblock&cells=ids
[non-browser parts of Chrome OS]: https://chromium.googlesource.com/chromiumos/docs/+/main/testing/fuzzing.md
[Reproducing bugs]: reproducing.md
[crbug.com/539572]: https://bugs.chromium.org/p/chromium/issues/detail?id=539572
[go/fuzzing-success]: https://goto.google.com/fuzzing-success
[libFuzzer]: http://llvm.org/docs/LibFuzzer.html
[libFuzzer integration]: libFuzzer_integration.md