cargo-fuzz

cargo-fuzz is the de facto choice for fuzzing Rust projects when using Cargo. It uses libFuzzer as the backend and provides a convenient Cargo subcommand that automatically enables relevant compilation flags for your Rust project, including support for sanitizers like AddressSanitizer.

When to Use

cargo-fuzz is currently the primary and most mature fuzzing solution for Rust projects using Cargo.

Fuzzer Best For Complexity

cargo-fuzz Cargo-based Rust projects, quick setup Low

AFL++ Multi-core fuzzing, non-Cargo projects Medium

LibAFL Custom fuzzers, research, advanced use cases High

Choose cargo-fuzz when:

• Your project uses Cargo (required)

• You want simple, quick setup with minimal configuration

• You need integrated sanitizer support

• You're fuzzing Rust code with or without unsafe blocks

Quick Start

#![no_main]

use libfuzzer_sys::fuzz_target;

fn harness(data: &[u8]) { your_project::check_buf(data); }

fuzz_target!(|data: &[u8]| { harness(data); });

Initialize and run:

cargo fuzz init

Edit fuzz/fuzz_targets/fuzz_target_1.rs with your harness

cargo +nightly fuzz run fuzz_target_1

Installation

cargo-fuzz requires the nightly Rust toolchain because it uses features only available in nightly.

Prerequisites

• Rust and Cargo installed via rustup

• Nightly toolchain

Linux/macOS

Install nightly toolchain

rustup install nightly

Install cargo-fuzz

cargo install cargo-fuzz

Verification

cargo +nightly --version cargo fuzz --version

Writing a Harness

Project Structure

cargo-fuzz works best when your code is structured as a library crate. If you have a binary project, split your main.rs into:

src/main.rs # Entry point (main function) src/lib.rs # Code to fuzz (public functions) Cargo.toml

Initialize fuzzing:

cargo fuzz init

This creates:

fuzz/ ├── Cargo.toml └── fuzz_targets/ └── fuzz_target_1.rs

Harness Structure

#![no_main]

use libfuzzer_sys::fuzz_target;

fn harness(data: &[u8]) { // 1. Validate input size if needed if data.is_empty() { return; }

// 2. Call target function with fuzz data
your_project::target_function(data);

}

fuzz_target!(|data: &[u8]| { harness(data); });

Harness Rules

Do Don't

Structure code as library crate Keep everything in main.rs

Use fuzz_target! macro Write custom main function

Handle Result::Err gracefully Panic on expected errors

Keep harness deterministic Use random number generators

See Also: For detailed harness writing techniques and structure-aware fuzzing with the arbitrary crate, see the fuzz-harness-writing technique skill.

Structure-Aware Fuzzing

cargo-fuzz integrates with the arbitrary crate for structure-aware fuzzing:

// In your library crate use arbitrary::Arbitrary;

#[derive(Debug, Arbitrary)] pub struct Name { data: String }

// In your fuzz target #![no_main] use libfuzzer_sys::fuzz_target;

fuzz_target!(|data: your_project::Name| { data.check_buf(); });

Add to your library's Cargo.toml :

[dependencies] arbitrary = { version = "1", features = ["derive"] }

Running Campaigns

Basic Run

cargo +nightly fuzz run fuzz_target_1

Without Sanitizers (Safe Rust)

If your project doesn't use unsafe Rust, disable sanitizers for 2x performance boost:

cargo +nightly fuzz run --sanitizer none fuzz_target_1

Check if your project uses unsafe code:

cargo install cargo-geiger cargo geiger

Re-executing Test Cases

Run a specific test case (e.g., a crash)

cargo +nightly fuzz run fuzz_target_1 fuzz/artifacts/fuzz_target_1/crash-<hash>

Run all corpus entries without fuzzing

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

Using Dictionaries

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

Interpreting Output

Output Meaning

NEW

New coverage-increasing input discovered

pulse

Periodic status update

INITED

Fuzzer initialized successfully

Crash with stack trace Bug found, saved to fuzz/artifacts/

Corpus location: fuzz/corpus/fuzz_target_1/

Crashes location: fuzz/artifacts/fuzz_target_1/

Sanitizer Integration

AddressSanitizer (ASan)

ASan is enabled by default and detects memory errors:

cargo +nightly fuzz run fuzz_target_1

Disabling Sanitizers

For pure safe Rust (no unsafe blocks in your code or dependencies):

cargo +nightly fuzz run --sanitizer none fuzz_target_1

Performance impact: ASan adds ~2x overhead. Disable for safe Rust to improve fuzzing speed.

Checking for Unsafe Code

cargo install cargo-geiger cargo geiger

See Also: For detailed sanitizer configuration, flags, and troubleshooting, see the address-sanitizer technique skill.

Coverage Analysis

cargo-fuzz integrates with Rust's coverage tools to analyze fuzzing effectiveness.

Prerequisites

rustup toolchain install nightly --component llvm-tools-preview cargo install cargo-binutils cargo install rustfilt

Generating Coverage Reports

Generate coverage data from corpus

cargo +nightly fuzz coverage fuzz_target_1

Create coverage generation script:

cat <<'EOF' > ./generate_html #!/bin/sh if [ $# -lt 1 ]; then echo "Error: Name of fuzz target is required." echo "Usage: $0 fuzz_target [sources...]" exit 1 fi FUZZ_TARGET="$1" shift SRC_FILTER="$@" TARGET=$(rustc -vV | sed -n 's|host: ||p') cargo +nightly cov -- show -Xdemangler=rustfilt
"target/$TARGET/coverage/$TARGET/release/$FUZZ_TARGET"
-instr-profile="fuzz/coverage/$FUZZ_TARGET/coverage.profdata"
-show-line-counts-or-regions -show-instantiations
-format=html -o fuzz_html/ $SRC_FILTER EOF chmod +x ./generate_html

Generate HTML report:

./generate_html fuzz_target_1 src/lib.rs

HTML report saved to: fuzz_html/

See Also: For detailed coverage analysis techniques and systematic coverage improvement, see the coverage-analysis technique skill.

Advanced Usage

Tips and Tricks

Tip Why It Helps

Start with a seed corpus Dramatically speeds up initial coverage discovery

Use --sanitizer none for safe Rust 2x performance improvement

Check coverage regularly Identifies gaps in harness or seed corpus

Use dictionaries for parsers Helps overcome magic value checks

Structure code as library Required for cargo-fuzz integration

libFuzzer Options

Pass options to libFuzzer after -- :

See all options

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

Set timeout per run

cargo +nightly fuzz run fuzz_target_1 -- -timeout=10

Use dictionary

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

Limit maximum input size

cargo +nightly fuzz run fuzz_target_1 -- -max_len=1024

Multi-Core Fuzzing

Experimental forking support (not recommended)

cargo +nightly fuzz run --jobs 1 fuzz_target_1

Note: The multi-core fuzzing feature is experimental and not recommended. For parallel fuzzing, consider running multiple instances manually or using AFL++.

Real-World Examples

Example: ogg Crate

The ogg crate parses Ogg media container files. Parsers are excellent fuzzing targets because they handle untrusted data.

Clone and initialize

git clone https://github.com/RustAudio/ogg.git cd ogg/ cargo fuzz init

Harness at fuzz/fuzz_targets/fuzz_target_1.rs :

#![no_main]

use ogg::{PacketReader, PacketWriter}; use ogg::writing::PacketWriteEndInfo; use std::io::Cursor; use libfuzzer_sys::fuzz_target;

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

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

if let Ok(_) = pck_rdr.read_packet() {
    if let Ok(r) = pck_rdr.read_packet() {
        match r {
            Some(pck) => {
                let inf = if pck.last_in_stream() {
                    PacketWriteEndInfo::EndStream
                } else if pck.last_in_page() {
                    PacketWriteEndInfo::EndPage
                } else {
                    PacketWriteEndInfo::NormalPacket
                };
                let stream_serial = pck.stream_serial();
                let absgp_page = pck.absgp_page();
                let _ = pck_wtr.write_packet(
                    pck.data, stream_serial, inf, absgp_page
                );
            }
            None => return,
        }
    }
}

}

fuzz_target!(|data: &[u8]| { harness(data); });

Seed the corpus:

mkdir fuzz/corpus/fuzz_target_1/ curl -o fuzz/corpus/fuzz_target_1/320x240.ogg
https://commons.wikimedia.org/wiki/File:320x240.ogg

Run:

cargo +nightly fuzz run fuzz_target_1

Analyze coverage:

cargo +nightly fuzz coverage fuzz_target_1 ./generate_html fuzz_target_1 src/lib.rs

Troubleshooting

Problem Cause Solution

"requires nightly" error Using stable toolchain Use cargo +nightly fuzz

Slow fuzzing performance ASan enabled for safe Rust Add --sanitizer none flag

"cannot find binary" No library crate Move code from main.rs to lib.rs

Sanitizer compilation issues Wrong nightly version Try different nightly: rustup install nightly-2024-01-01

Low coverage Missing seed corpus Add sample inputs to fuzz/corpus/fuzz_target_1/

Magic value not found No dictionary Create dictionary file with magic values

Related Skills

Technique Skills

Skill Use Case

fuzz-harness-writing Structure-aware fuzzing with arbitrary crate

address-sanitizer Understanding ASan output and configuration

coverage-analysis Measuring and improving fuzzing effectiveness

fuzzing-corpus Building and managing seed corpora

fuzzing-dictionaries Creating dictionaries for format-aware fuzzing

Related Fuzzers

Skill When to Consider

libfuzzer Fuzzing C/C++ code with similar workflow

aflpp Multi-core fuzzing or non-Cargo Rust projects

libafl Advanced fuzzing research or custom fuzzer development

Resources

Rust Fuzz Book - cargo-fuzz Official documentation for cargo-fuzz covering installation, usage, and advanced features.

arbitrary crate documentation Guide to structure-aware fuzzing with automatic derivation for Rust types.

cargo-fuzz GitHub Repository Source code, issue tracker, and examples for cargo-fuzz.