cargo-fuzz by trailofbits

---
name: cargo-fuzz
type: fuzzer
description: "Sets up and runs cargo-fuzz, the standard fuzzing tool for Cargo-based Rust projects. Covers cargo fuzz init, the nightly toolchain requirement, fuzz_target! harnesses, Arbitrary-derived structured inputs, sanitizer options, cargo fuzz coverage, and reproducing a crash artifact. Use when fuzzing a Rust crate, writing a fuzz_target!, exercising unsafe blocks or FFI in Rust, or triaging a cargo fuzz crash."
---

# 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

```rust
#![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:
```bash
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](https://rustup.rs/)
- Nightly toolchain

### Linux/macOS

```bash
# Install nightly toolchain
rustup install nightly

# Install cargo-fuzz
cargo install cargo-fuzz
```

### Verification

```bash
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:

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

Initialize fuzzing:
```bash
cargo fuzz init
```

This creates:
```text
fuzz/
├── Cargo.toml
└── fuzz_targets/
    └── fuzz_target_1.rs
```

### Harness Structure

```rust
#![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](https://github.com/rust-fuzz/arbitrary) crate for structure-aware fuzzing:

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

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

```rust
// 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`:
```toml
[dependencies]
arbitrary = { version = "1", features = ["derive"] }
```

## Running Campaigns

### Basic Run

```bash
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:

```bash
cargo +nightly fuzz run --sanitizer none fuzz_target_1
```

Check if your project uses unsafe code:
```bash
cargo install cargo-geiger
cargo geiger
```

### Re-executing Test Cases

```bash
# 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

```bash
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:

```bash
cargo +nightly fuzz run fuzz_target_1
```

### Disabling Sanitizers

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

```bash
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

```bash
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

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

### Generating Coverage Reports

```bash
# Generate coverage data from corpus
cargo +nightly fuzz coverage fuzz_target_1
```

Create coverage generation script:

```bash
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:
```bash
./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 `--`:

```bash
# 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

```bash
# 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](https://github.com/RustAudio/ogg) parses Ogg media container files. Parsers are excellent fuzzing targets because they handle untrusted data.

```bash
# 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`:

```rust
#![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:
```bash
mkdir fuzz/corpus/fuzz_target_1/
curl -o fuzz/corpus/fuzz_target_1/320x240.ogg \
  https://commons.wikimedia.org/wiki/File:320x240.ogg
```

Run:
```bash
cargo +nightly fuzz run fuzz_target_1
```

Analyze coverage:
```bash
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](https://rust-fuzz.github.io/book/cargo-fuzz.html)**
Official documentation for cargo-fuzz covering installation, usage, and advanced features.

**[arbitrary crate documentation](https://docs.rs/arbitrary/latest/arbitrary/)**
Guide to structure-aware fuzzing with automatic derivation for Rust types.

**[cargo-fuzz GitHub Repository](https://github.com/rust-fuzz/cargo-fuzz)**
Source code, issue tracker, and examples for cargo-fuzz.