cargo-fuzz #

The cargo-fuzz tool is the de facto choice for fuzzing your Rust project when using Cargo. It uses libFuzzer as the back end. Note that if you are not using Cargo, you cannot use the cargo-fuzz tool.

By installing the cargo-fuzz crate, a Cargo subcommand is installed. Therefore, cargo-fuzz depends on using Cargo. The subcommand also automatically enables relevant compilation flags for your Rust project and even supports enabling sanitizers like AddressSanitizer.

Installation #

The cargo-fuzz tool uses features that are available only in the nightly Rust toolchain. You can install it using rustup.

rustup install nightly

Verify that the installation was successful by running the following command.

cargo +nightly --version

The recommended way of installing the tool itself is through cargo install.

cargo install cargo-fuzz

Write a fuzz test #

Let’s recall the example introduced in the introduction of this chapter, consisting of a main and a check_buf function. We want to fuzz test the check_buf function. For this purpose, we want to restructure the project so that the code we want to test is part of a library crate.

Initially, your project probably consists of a Cargo project file (i.e., a Cargo.toml file) and a The Cargo.toml specifies the name of the project, e.g. your_project.


We want to split the file into the entrypoint of the program, the main function, and the code we want to fuzz test. This results in the following files.

use std::process;

pub fn check_buf(buf: &[u8]) {
    if buf.len() > 0 && buf[0] == b'a' {
        if buf.len() > 1 && buf[1] == b'b' {
            if buf.len() > 2 && buf[2] == b'c' {
fn main() {
    let buffer: &[u8] = b"123";

The project is now structured in the following way.


The next step is to initialize the project using cargo-fuzz.

cargo fuzz init

This creates a new Cargo project in a fuzz/ subdirectory, which depends on your Cargo crate. The directory fuzz/fuzz_targets contains Rust programs that use the libfuzzer-sys dependency. We can adjust the default file generated at fuzz_targets/ directory to the following:


use libfuzzer_sys::fuzz_target;

fn harness(data: &[u8]) {

fuzz_target!(|data: &[u8]| {

The setup for cargo-fuzz is now done. We created a harness that can be executed by cargo-fuzz. The actual name of the fuzz target is defined in the fuzz/Cargo.toml file. Every fuzz test is a separate cargo binary.

Usage #

Fuzz tests can be executed by invoking cargo-fuzz in the following way.

cargo +nightly fuzz run fuzz_target_1

If the project does not contain unsafe Rust or calls into C/C++ code, then we can append the option --sanitizer none to significantly improve the fuzzing performance. By default, AddressSanitizer is enabled, which checks for memory-related bugs. Because Rust is a memory-safe language, the usage of AddressSanitizer unnecessarily slows down the execution if no unsafe Rust is used (see the AddressSanitizer section if you are unsure whether to enable or disable ASan for your use case).

The corpus is persisted in the fuzz/corpus/fuzz_fuzz_target_1/ directory. Crashes are stored in fuzz/artifacts/fuzz_fuzz_target_1/.

Re-execute a test case #

A test case can be re-executed using cargo +nightly fuzz run fuzz_target_1 <test_case>. For example, the following command re-executes a crash:

cargo +nightly fuzz run fuzz_target_1 fuzz/artifacts/fuzz_target_1/crash-04629f583cb62b4c23651a9b9b1749abbad5f932

This helps triage found bugs. If you want to re-execute a directory of test cases without actually fuzzing (-runs=0), you can run:

cargo +nightly fuzz run fuzz_target_1 <directory> -- -runs=0

For example, to re-execute the corpus you can run the following command:

cargo +nightly fuzz run fuzz_target_1 fuzz/corpus/fuzz_target_1 -- -runs=0

Fuzzer options #

Several options can be adjusted by adding command-line flags when running cargo-fuzz.

  • –-sanitizer none Controls which sanitizers are enabled. ASan is enabled by default, which is helpful when fuzzing unsafe Rust code. If you are not using unsafe Rust, then sanitizers can be disabled to achieve a significant performance boost. (See the AddressSanitizer section for more information.)

  • –-jobs 1 Enables the experimental forking features by libFuzzer, as briefly mentioned in Multi-core Fuzzing. We do not recommend using this feature.

Apart from the cargo-fuzz specific options, libFuzzer options can be used by appending a -- followed by the libFuzzer option. In the following example, we print all command-line options for libFuzzer:

cargo +nightly fuzz run fuzz_target_1 -- -help=1

For example, the following command allows one to specify a dictionary file that guides the fuzzer and allows the fuzzer to discover interesting test cases more quickly. (For more details about this, see Fuzzing dictionary.)

cargo +nightly fuzz run fuzz_target_1 -- -dict=./dict.dict

AddressSanitizer #

ASan helps detect memory errors that might otherwise go unnoticed. For a general introduction to ASan, refer to AddressSanitizer.

ASan is enabled by default when fuzzing with cargo-fuzz. This may be a bad default if you are not using unsafe Rust in your code or your dependencies. ASan may be unnecessary in this case because its goal is to detect memory corruption bugs, but Rust without unsafe code is memory-safe. The cargo-geiger project can help you determine if your project uses unsafe Rust.

A speedup of 2x can be expected from disabling ASan.

ASan can be disabled with the flag ​​--sanitizer none:

cargo +nightly fuzz run ​​--sanitizer none fuzz_target_1

Most sanitizers in Rust currently require a nightly toolchain because they are an unstable feature as of writing. So if you encounter issues when compiling your project, you might want to test a different version of the installed nightly toolchain.

Real-world examples #

Cargo crate: ogg #

The ogg crate parses ogg files, which contain media data. The ogg format is a container format for media, which means such files can host different codecs. Parsers are easy to fuzz and also a high-value target, because they have to behave correctly even when they are presented with untrusted data.

Let’s go over the process of fuzzing an existing Cargo crate. First, we check out the source-code of the crate:

git clone
cd ogg/

Now, we initialize cargo-fuzz:

cargo fuzz init

We look now for examples, unit tests, or integration tests in the project that might provide us with a good starting point for writing a harness, such as the repack example in the ogg crate. We rewrite this example to read and write to memory instead of to a file. Rust errors denoted by the Result:Err(_) enum case are ignored, because returning an error for invalid files generated by the fuzzer is good behavior. In case of an error, the current input is skipped (see the if let Ok(r) = … { statements).

The following code shows the harness and the entrypoint, stored at fuzz/fuzz_targets/ for cargo-fuzz.


use ogg::{PacketReader, PacketWriter};
use ogg::writing::PacketWriteEndInfo;
use std::fs::File;
use std::io::Cursor;

use libfuzzer_sys::fuzz_target;

fn harness(data: &[u8]) {
    let mut data = data.to_vec();
    let mut pck_rdr = PacketReader::new(Cursor::new(data));


    let output = Vec::new();

    let mut pck_wtr = PacketWriter::new(Cursor::new(output));

    if let Ok(r) = pck_rdr.read_packet() {
        if let Ok(r) = pck_rdr.read_packet() {
            match r {
                Some(pck) => {
                    let inf = if pck.last_in_stream() {
                    } else if pck.last_in_page() {
                    } else {
                    let stream_serial = pck.stream_serial();
                    let absgp_page = pck.absgp_page();
                    let _ = pck_wtr.write_packet(, stream_serial, inf, absgp_page);
                // End of stream
                None => return,

fuzz_target!(|data: &[u8]| {
Harness for the ogg library

To improve fuzzing performance, we can seed the fuzzer by downloading an initial test case:

mkdir fuzz/corpus/fuzz_target_1/
curl -o fuzz/corpus/fuzz_target_1/320x240.ogg

We can now run the fuzzer:

cargo +nightly fuzz run fuzz_target_1

The corpus is stored at fuzz/corpus/fuzz_target_1/. Check out the FAQ to learn how to use a corpus over the long term.

The next step is to investigate the coverage and see if the harness or seed corpus can be improved (refer to the Coverage analysis).

Additional resources #

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