This blog post was originally published on the GitLab Unfiltered blog. It was reviewed and republished on 2020-12-17.

What is fuzzing?

Fuzzing, also called fuzz testing, is an automated software technique that involves providing semi-random data as an input to the test program in order to uncover bugs and crashes.

In this short tutorial we will discuss using cargo-fuzz for fuzzing Rust code.

Why fuzz Rust code?

Rust is a safe language (mostly) and memory corruption issues are a thing of the past so we don’t need to fuzz our code, right? Wrong! Any code, and especially where stability, quality, and coverage are important, is worth fuzzing. Fuzzing can uncover logical bugs and denial-of-service issues in critical components that can lead to security issues as well.

As a reference to almost infinite amount of bugs found with cargo-fuzz (only the documented one) you can look at the list of bugs found by fuzz-testing Rust codebases.


cargo-fuzz is the current de-facto standard fuzzer for Rust and essentially it is a proxy layer to the well-tested libFuzzer engine. This means the algorithm and the interface is all based on libFuzzer, which is a widely-used, coverage-guided fuzzer for C/C++ and some other languages that implemented a proxy layer – just like cargo-fuzz.

libFuzzer (cargo-fuzz) and coverage-guided fuzzers in general have the following algorithm:

// pseudo code
Instrument program for code coverage
for {
  Choose random input from corpus
  Mutate input
  Execute input and collect coverage
  If new coverage/paths are hit add it to corpus (corpus - directory with test-cases)

Building and running the fuzzer

If you are already familiar with this part you can skip to Continuous Fuzzing section.

We will start with rust-fuzzing-example.

For the sake of the example, we have a simple function with an off-by-one bug:

pub fn parse_complex(data: &[u8]) -> bool{
	if data.len() == 5 {
		if data[0] == b'F' && data[1] == b'U' && data[2] == b'Z' && data[3] == b'Z' && data[4] == b'I' && data[5] == b'T' {
			return true
    return true;

Our fuzz function will look like this and will be called by libFuzzer in an infinite loop with the generated data, according to the coverage-guided algorithm.

#[macro_use] extern crate libfuzzer_sys;
extern crate example_rust;

fuzz_target!(|data: &[u8]| {
    let _ = example_rust::parse_complex(&data);

To run the fuzzer we need to build an instrumented version of the code together with the fuzz function. cargo-fuzz is doing for us the heavy lifting so it can be done using the following simple steps:

# cargo-fuzz is available in rust nightly
docker run -it rustlang/rust:nightly-stretch /bin/bash
cargo install cargo-fuzz

# Download the example repo, build, and run the fuzzer
git clone
cd example-rust
cargo fuzz run fuzz_parse_complex

## The output should look like this:
#524288 pulse  cov: 105 ft: 99 corp: 6/26b lim: 517 exec/s: 131072 rss: 93Mb
#1048576        pulse  cov: 105 ft: 99 corp: 6/26b lim: 1040 exec/s: 116508 rss: 229Mb
==2208== ERROR: libFuzzer: deadly signal
    #0 0x5588b8234961  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0x83961)
    #1 0x5588b8262dc5  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xb1dc5)
    #2 0x5588b8284734  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xd3734)
    #3 0x5588b82845e9  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xd35e9)
    #4 0x5588b826493a  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xb393a)
    #5 0x7f93737e70df  (/lib/x86_64-linux-gnu/
    #6 0x7f9373252ffe  (/lib/x86_64-linux-gnu/
    #7 0x7f9373254429  (/lib/x86_64-linux-gnu/
    #8 0x5588b82a4a06  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xf3a06)
    #9 0x5588b82a1b75  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xf0b75)
    #10 0x5588b824fa1b  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0x9ea1b)
    #11 0x5588b82a442b  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xf342b)
    #12 0x5588b82a3ee1  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xf2ee1)
    #13 0x5588b82a3dd5  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xf2dd5)
    #14 0x5588b82b6cd9  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0x105cd9)
    #15 0x5588b82b6c94  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0x105c94)
    #16 0x5588b824edda  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0x9ddda)
    #17 0x5588b81c45b7  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0x135b7)
    #18 0x5588b824f7e4  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0x9e7e4)
    #19 0x5588b827da53  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xcca53)
    #20 0x5588b82a4a18  (/example-rust/fuzz/target/x86_64-unknown-linux-gnu/debug/fuzz_parse_complex+0xf3a18)

NOTE: libFuzzer has rudimentary signal handlers.
      Combine libFuzzer with AddressSanitizer or similar for better crash reports.
SUMMARY: libFuzzer: deadly signal
MS: 2 ShuffleBytes-ChangeByte-; base unit: 89b92cdd9bcb9b861c47c0179eff7b3a9baafcde
artifact_prefix='/example-rust/fuzz/artifacts/fuzz_parse_complex/'; Test unit written to /example-rust/fuzz/artifacts/fuzz_parse_complex/crash-df779ced6b712c5fca247e465de2de474d1d23b9
Base64: RlVaWkk=

This find the bug in a few seconds, prints the “FUZZI” string that triggers the vulnerability and saves it to a file.

Running cargo-fuzz from CI

The best way to integrate go-fuzz fuzzing with Gitlab CI/CD is by adding additional stage and step to your .gitlab-ci.yml. It is straightforward and fully documented.

  - template: Coverage-Fuzzing.gitlab-ci.yml

  extends: .fuzz_base
    - apt-get update -qq && apt-get install -y -qq git make clang cmake
    - export CC=`which clang`
    - export CXX=`which clang++`
    - cargo install cargo-fuzz
    - cargo fuzz run fuzz_parse_complex -- -runs=0
    - ./gitlab-cov-fuzz run --regression=$REGRESSION -- ./fuzz/target/x86_64-unknown-linux-gnu/release/fuzz_parse_complex

For each fuzz target you will have to create a step which extends .fuzz_base that runs the following:

This will run your fuzz tests in a blocking manner inside your pipeline. There is also a possibility to run longer fuzz sessions asynchronously, as described in the docs.

Check out our full documentation and the example repo and try adding fuzz testing to your own repos!

Cover image by Zsolt Palatinus on Unsplash

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