---
name: libafl
type: fuzzer
description: "Builds custom fuzzers with LibAFL, the modular Rust fuzzing library. Covers composing observers, feedbacks, mutators, schedulers, and executors into a fuzzer for targets the standard tools do not fit. Use when writing a bespoke fuzzer or mutator, fuzzing a non-standard target or architecture, implementing a fuzzing research idea, or when libFuzzer and AFL++ lack the control you need."
---

# LibAFL

LibAFL is a modular fuzzing library that implements features from AFL-based fuzzers like AFL++. Unlike traditional fuzzers, LibAFL provides all functionality in a modular and customizable way as a Rust library. It can be used as a drop-in replacement for libFuzzer or as a library to build custom fuzzers from scratch.

## When to Use

| Fuzzer | Best For | Complexity |
|--------|----------|------------|
| libFuzzer | Quick setup, single-threaded | Low |
| AFL++ | Multi-core, general purpose | Medium |
| LibAFL | Custom fuzzers, advanced features, research | High |

**Choose LibAFL when:**
- You need custom mutation strategies or feedback mechanisms
- Standard fuzzers don't support your target architecture
- You want to implement novel fuzzing techniques
- You need fine-grained control over fuzzing components
- You're conducting fuzzing research

## Quick Start

LibAFL can be used as a drop-in replacement for libFuzzer with minimal setup:

```c++
extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    // Call your code with fuzzer-provided data
    my_function(data, size);
    return 0;
}
```

Build LibAFL's libFuzzer compatibility layer:
```bash
git clone https://github.com/AFLplusplus/LibAFL
cd LibAFL/libafl_libfuzzer_runtime
./build.sh
```

Compile and run:
```bash
clang++ -DNO_MAIN -g -O2 -fsanitize=fuzzer-no-link libFuzzer.a harness.cc main.cc -o fuzz
./fuzz corpus/
```

## Installation

### Prerequisites

- Clang/LLVM 15-18
- Rust (via rustup)
- Additional system dependencies

### Linux/macOS

Install Clang:
```bash
apt install clang
```

Or install a specific version via apt.llvm.org:
```bash
wget https://apt.llvm.org/llvm.sh
chmod +x llvm.sh
sudo ./llvm.sh 15
```

Configure environment for Rust:
```bash
export RUSTFLAGS="-C linker=/usr/bin/clang-15"
export CC="clang-15"
export CXX="clang++-15"
```

Install Rust:
```bash
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
```

Install additional dependencies:
```bash
apt install libssl-dev pkg-config
```

For libFuzzer compatibility mode, install nightly Rust:
```bash
rustup toolchain install nightly --component llvm-tools
```

### Verification

Build LibAFL to verify installation:
```bash
cd LibAFL/libafl_libfuzzer_runtime
./build.sh
# Should produce libFuzzer.a
```

## Writing a Harness

LibAFL harnesses follow the same pattern as libFuzzer when using drop-in replacement mode:

```c++
extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    // Your fuzzing target code here
    return 0;
}
```

When building custom fuzzers with LibAFL as a Rust library, harness logic is integrated directly into the fuzzer. See the "Writing a Custom Fuzzer" section below for the full pattern.

> **See Also:** For detailed harness writing techniques, see the **harness-writing** technique skill.

## Usage Modes

LibAFL supports two primary usage modes:

### 1. libFuzzer Drop-in Replacement

Use LibAFL as a replacement for libFuzzer with existing harnesses.

**Compilation:**
```bash
clang++ -DNO_MAIN -g -O2 -fsanitize=fuzzer-no-link libFuzzer.a harness.cc main.cc -o fuzz
```

**Running:**
```bash
./fuzz corpus/
```

**Recommended for long campaigns:**
```bash
./fuzz -fork=1 -ignore_crashes=1 corpus/
```

### 2. Custom Fuzzer as Rust Library

Build a fully customized fuzzer using LibAFL components.

**Create project:**
```bash
cargo init --lib my_fuzzer
cd my_fuzzer
cargo add libafl@0.13 libafl_targets@0.13 libafl_bolts@0.13 libafl_cc@0.13 \
  --features "libafl_targets@0.13/libfuzzer,libafl_targets@0.13/sancov_pcguard_hitcounts"
```

**Configure Cargo.toml:**
```toml
[lib]
crate-type = ["staticlib"]
```

## Writing a Custom Fuzzer

> **See Also:** For detailed harness writing techniques, patterns for handling complex inputs,
> and advanced strategies, see the **fuzz-harness-writing** technique skill.

### Fuzzer Components

A LibAFL fuzzer consists of modular components:

1. **Observers** - Collect execution feedback (coverage, timing)
2. **Feedback** - Determine if inputs are interesting
3. **Objective** - Define fuzzing goals (crashes, timeouts)
4. **State** - Maintain corpus and metadata
5. **Mutators** - Generate new inputs
6. **Scheduler** - Select which inputs to mutate
7. **Executor** - Run the target with inputs

### Basic Fuzzer Structure

```rust
use libafl::prelude::*;
use libafl_bolts::prelude::*;
use libafl_targets::{libfuzzer_test_one_input, std_edges_map_observer};

#[no_mangle]
pub extern "C" fn libafl_main() {
    let mut run_client = |state: Option<_>, mut restarting_mgr, _core_id| {
        // 1. Setup observers
        let edges_observer = HitcountsMapObserver::new(
            unsafe { std_edges_map_observer("edges") }
        ).track_indices();
        let time_observer = TimeObserver::new("time");

        // 2. Define feedback
        let mut feedback = feedback_or!(
            MaxMapFeedback::new(&edges_observer),
            TimeFeedback::new(&time_observer)
        );

        // 3. Define objective
        let mut objective = feedback_or_fast!(
            CrashFeedback::new(),
            TimeoutFeedback::new()
        );

        // 4. Create or restore state
        let mut state = state.unwrap_or_else(|| {
            StdState::new(
                StdRand::new(),
                InMemoryCorpus::new(),
                OnDiskCorpus::new(&output_dir).unwrap(),
                &mut feedback,
                &mut objective,
            ).unwrap()
        });

        // 5. Setup mutator
        let mutator = StdScheduledMutator::new(havoc_mutations());
        let mut stages = tuple_list!(StdMutationalStage::new(mutator));

        // 6. Setup scheduler
        let scheduler = IndexesLenTimeMinimizerScheduler::new(
            &edges_observer,
            QueueScheduler::new()
        );

        // 7. Create fuzzer
        let mut fuzzer = StdFuzzer::new(scheduler, feedback, objective);

        // 8. Define harness
        let mut harness = |input: &BytesInput| {
            let buf = input.target_bytes().as_slice();
            libfuzzer_test_one_input(buf);
            ExitKind::Ok
        };

        // 9. Setup executor
        let mut executor = InProcessExecutor::with_timeout(
            &mut harness,
            tuple_list!(edges_observer, time_observer),
            &mut fuzzer,
            &mut state,
            &mut restarting_mgr,
            timeout,
        )?;

        // 10. Load initial inputs
        if state.must_load_initial_inputs() {
            state.load_initial_inputs(
                &mut fuzzer,
                &mut executor,
                &mut restarting_mgr,
                &input_dir
            )?;
        }

        // 11. Start fuzzing
        fuzzer.fuzz_loop(&mut stages, &mut executor, &mut state, &mut restarting_mgr)?;
        Ok(())
    };

    // Launch fuzzer
    Launcher::builder()
        .run_client(&mut run_client)
        .cores(&cores)
        .build()
        .launch()
        .unwrap();
}
```

## Compilation

### Verbose Mode

Manually specify all instrumentation flags:

```bash
clang++-15 -DNO_MAIN -g -O2 \
  -fsanitize-coverage=trace-pc-guard \
  -fsanitize=address \
  -Wl,--whole-archive target/release/libmy_fuzzer.a -Wl,--no-whole-archive \
  main.cc harness.cc -o fuzz
```

### Compiler Wrapper (Recommended)

Create a LibAFL compiler wrapper to handle instrumentation automatically.

**Create `src/bin/libafl_cc.rs`:**
```rust
use libafl_cc::{ClangWrapper, CompilerWrapper, Configuration, ToolWrapper};

pub fn main() {
    let args: Vec<String> = env::args().collect();
    let mut cc = ClangWrapper::new();
    cc.cpp(is_cpp)
      .parse_args(&args)
      .link_staticlib(&dir, "my_fuzzer")
      .add_args(&Configuration::GenerateCoverageMap.to_flags().unwrap())
      .add_args(&Configuration::AddressSanitizer.to_flags().unwrap())
      .run()
      .unwrap();
}
```

**Compile and use:**
```bash
cargo build --release
target/release/libafl_cxx -DNO_MAIN -g -O2 main.cc harness.cc -o fuzz
```

> **See Also:** For detailed sanitizer configuration, common issues, and advanced flags,
> see the **address-sanitizer** and **undefined-behavior-sanitizer** technique skills.

## Running Campaigns

### Basic Run

```bash
./fuzz --cores 0 --input corpus/
```

### Multi-Core Fuzzing

```bash
./fuzz --cores 0,8-15 --input corpus/
```

This runs 9 clients: one on core 0, and 8 on cores 8-15.

### With Options

```bash
./fuzz --cores 0-7 --input corpus/ --output crashes/ --timeout 1000
```

### Text User Interface (TUI)

Enable graphical statistics view:

```bash
./fuzz -tui=1 corpus/
```

### Interpreting Output

| Output | Meaning |
|--------|---------|
| `corpus: N` | Number of interesting test cases found |
| `objectives: N` | Number of crashes/timeouts found |
| `executions: N` | Total number of target invocations |
| `exec/sec: N` | Current execution throughput |
| `edges: X%` | Code coverage percentage |
| `clients: N` | Number of parallel fuzzing processes |

The fuzzer emits two main event types:
- **UserStats** - Regular heartbeat with current statistics
- **Testcase** - New interesting input discovered

## Advanced Usage

### Tips and Tricks

| Tip | Why It Helps |
|-----|--------------|
| Use `-fork=1 -ignore_crashes=1` | Continue fuzzing after first crash |
| Use `InMemoryOnDiskCorpus` | Persist corpus across restarts |
| Enable TUI with `-tui=1` | Better visualization of progress |
| Use specific LLVM version | Avoid compatibility issues |
| Set `RUSTFLAGS` correctly | Prevent linking errors |

### Crash Deduplication

Avoid storing duplicate crashes from the same bug:

**Add backtrace observer:**
```rust
let backtrace_observer = BacktraceObserver::owned(
    "BacktraceObserver",
    libafl::observers::HarnessType::InProcess
);
```

**Update executor:**
```rust
let mut executor = InProcessExecutor::with_timeout(
    &mut harness,
    tuple_list!(edges_observer, time_observer, backtrace_observer),
    &mut fuzzer,
    &mut state,
    &mut restarting_mgr,
    timeout,
)?;
```

**Update objective with hash feedback:**
```rust
let mut objective = feedback_and!(
    feedback_or_fast!(CrashFeedback::new(), TimeoutFeedback::new()),
    NewHashFeedback::new(&backtrace_observer)
);
```

This ensures only crashes with unique backtraces are saved.

### Dictionary Fuzzing

Use dictionaries to guide fuzzing toward specific tokens:

**Add tokens from file:**
```rust
let mut tokens = Tokens::new();
if let Some(tokenfile) = &tokenfile {
    tokens.add_from_file(tokenfile)?;
}
state.add_metadata(tokens);
```

**Update mutator:**
```rust
let mutator = StdScheduledMutator::new(
    havoc_mutations().merge(tokens_mutations())
);
```

**Hard-coded tokens example (PNG):**
```rust
state.add_metadata(Tokens::from([
    vec![137, 80, 78, 71, 13, 10, 26, 10], // PNG header
    "IHDR".as_bytes().to_vec(),
    "IDAT".as_bytes().to_vec(),
    "PLTE".as_bytes().to_vec(),
    "IEND".as_bytes().to_vec(),
]));
```

> **See Also:** For detailed dictionary creation strategies and format-specific dictionaries,
> see the **fuzzing-dictionaries** technique skill.

### Auto Tokens

Automatically extract magic values and checksums from the program:

**Enable in compiler wrapper:**
```rust
cc.add_pass(LLVMPasses::AutoTokens)
```

**Load auto tokens in fuzzer:**
```rust
tokens += libafl_targets::autotokens()?;
```

**Verify tokens section:**
```bash
echo "p (uint8_t *)__token_start" | gdb fuzz
```

### Performance Tuning

| Setting | Impact |
|---------|--------|
| Multi-core fuzzing | Linear speedup with cores |
| `InMemoryCorpus` | Faster but non-persistent |
| `InMemoryOnDiskCorpus` | Balanced speed and persistence |
| Sanitizers | 2-5x slowdown, essential for bugs |
| Optimization level `-O2` | Balance between speed and coverage |

### Debugging Fuzzer

Run fuzzer in single-process mode for easier debugging:

```rust
// Replace launcher with direct call
run_client(None, SimpleEventManager::new(monitor), 0).unwrap();

// Comment out:
// Launcher::builder()
//     .run_client(&mut run_client)
//     ...
//     .launch()
```

Then debug with GDB:
```bash
gdb --args ./fuzz --cores 0 --input corpus/
```

## Real-World Examples

### Example: libpng

Fuzzing libpng using LibAFL:

**1. Get source code:**
```bash
curl -L -O https://downloads.sourceforge.net/project/libpng/libpng16/1.6.37/libpng-1.6.37.tar.xz
tar xf libpng-1.6.37.tar.xz
cd libpng-1.6.37/
apt install zlib1g-dev
```

**2. Set compiler wrapper:**
```bash
export FUZZER_CARGO_DIR="/path/to/libafl/project"
export CC=$FUZZER_CARGO_DIR/target/release/libafl_cc
export CXX=$FUZZER_CARGO_DIR/target/release/libafl_cxx
```

**3. Build static library:**
```bash
./configure --enable-shared=no
make
```

**4. Get harness:**
```bash
curl -O https://raw.githubusercontent.com/glennrp/libpng/f8e5fa92b0e37ab597616f554bee254157998227/contrib/oss-fuzz/libpng_read_fuzzer.cc
```

**5. Link fuzzer:**
```bash
$CXX libpng_read_fuzzer.cc .libs/libpng16.a -lz -o fuzz
```

**6. Prepare seeds:**
```bash
mkdir seeds/
curl -o seeds/input.png https://raw.githubusercontent.com/glennrp/libpng/acfd50ae0ba3198ad734e5d4dec2b05341e50924/contrib/pngsuite/iftp1n3p08.png
```

**7. Get dictionary (optional):**
```bash
curl -O https://raw.githubusercontent.com/glennrp/libpng/2fff013a6935967960a5ae626fc21432807933dd/contrib/oss-fuzz/png.dict
```

**8. Start fuzzing:**
```bash
./fuzz --input seeds/ --cores 0 -x png.dict
```

### Example: CMake Project

Integrate LibAFL with CMake build system:

**CMakeLists.txt:**
```cmake
project(BuggyProgram)
cmake_minimum_required(VERSION 3.0)

add_executable(buggy_program main.cc)

add_executable(fuzz main.cc harness.cc)
target_compile_definitions(fuzz PRIVATE NO_MAIN=1)
target_compile_options(fuzz PRIVATE -g -O2)
```

**Build non-instrumented binary:**
```bash
cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ .
cmake --build . --target buggy_program
```

**Build fuzzer:**
```bash
export FUZZER_CARGO_DIR="/path/to/libafl/project"
cmake -DCMAKE_C_COMPILER=$FUZZER_CARGO_DIR/target/release/libafl_cc \
      -DCMAKE_CXX_COMPILER=$FUZZER_CARGO_DIR/target/release/libafl_cxx .
cmake --build . --target fuzz
```

**Run fuzzing:**
```bash
./fuzz --input seeds/ --cores 0
```

## Troubleshooting

| Problem | Cause | Solution |
|---------|-------|----------|
| No coverage increases | Instrumentation failed | Verify compiler wrapper used, check for `-fsanitize-coverage` |
| Fuzzer won't start | Empty corpus with no interesting inputs | Provide seed inputs that trigger code paths |
| Linker errors with `libafl_main` | Runtime not linked | Use `-Wl,--whole-archive` or `-u libafl_main` |
| LLVM version mismatch | LibAFL requires LLVM 15-18 | Install compatible LLVM version, set environment variables |
| Rust compilation fails | Outdated Rust or Cargo | Update Rust with `rustup update` |
| Slow fuzzing | Sanitizers enabled | Expected 2-5x slowdown, necessary for finding bugs |
| Environment variable interference | `CC`, `CXX`, `RUSTFLAGS` set | Unset after building LibAFL project |
| Cannot attach debugger | Multi-process fuzzing | Run in single-process mode (see Debugging section) |

## Related Skills

### Technique Skills

| Skill | Use Case |
|-------|----------|
| **fuzz-harness-writing** | Detailed guidance on writing effective harnesses |
| **address-sanitizer** | Memory error detection during fuzzing |
| **undefined-behavior-sanitizer** | Undefined behavior detection |
| **coverage-analysis** | Measuring and improving code coverage |
| **fuzzing-corpus** | Building and managing seed corpora |
| **fuzzing-dictionaries** | Creating dictionaries for format-aware fuzzing |

### Related Fuzzers

| Skill | When to Consider |
|-------|------------------|
| **libfuzzer** | Simpler setup, don't need LibAFL's advanced features |
| **aflpp** | Multi-core fuzzing without custom fuzzer development |
| **cargo-fuzz** | Fuzzing Rust projects with less setup |

## Resources

### Official Documentation

- [LibAFL Book](https://aflplus.plus/libafl-book/) - Official handbook with comprehensive documentation
- [LibAFL GitHub](https://github.com/AFLplusplus/LibAFL) - Source code and examples
- [LibAFL API Documentation](https://docs.rs/libafl/latest/libafl/) - Rust API reference

### Examples and Tutorials

- [LibAFL Examples](https://github.com/AFLplusplus/LibAFL/tree/main/fuzzers) - Collection of example fuzzers
- [cargo-fuzz with LibAFL](https://github.com/AFLplusplus/LibAFL/tree/main/fuzzers/fuzz_anything/cargo_fuzz) - Using LibAFL as cargo-fuzz backend
- [Testing Handbook LibAFL Examples](https://github.com/trailofbits/testing-handbook/tree/main/materials/fuzzing/libafl) - Complete working examples from this handbook