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name: libfuzzer type: fuzzer description: "Sets up and runs libFuzzer, the coverage-guided fuzzer built into LLVM, on C/C++ code that compiles with Clang. Covers harness structure, -fsanitize=fuzzer builds, corpus and dictionary management, sanitizer integration, and campaign triage. Use when writing or debugging an LLVMFuzzerTestOneInput harness, starting fuzzing on a C/C++ library, choosing between libFuzzer and AFL++, or working out why a libFuzzer run finds nothing."

libFuzzer

libFuzzer is an in-process, coverage-guided fuzzer that is part of the LLVM project. It's the recommended starting point for fuzzing C/C++ projects due to its simplicity and integration with the LLVM toolchain. While libFuzzer has been in maintenance-only mode since late 2022, it is easier to install and use than its alternatives, has wide support, and will be maintained for the foreseeable future.

When to Use

| Fuzzer | Best For | Complexity | |--------|----------|------------| | libFuzzer | Quick setup, single-project fuzzing | Low | | AFL++ | Multi-core fuzzing, diverse mutations | Medium | | LibAFL | Custom fuzzers, research projects | High | | Honggfuzz | Hardware-based coverage | Medium |

Choose libFuzzer when:

• You need a simple, quick setup for C/C++ code
• Project uses Clang for compilation
• Single-core fuzzing is sufficient initially
• Transitioning to AFL++ later is an option (harnesses are compatible)

Note: Fuzzing harnesses written for libFuzzer are compatible with AFL++, making it easy to transition if you need more advanced features like better multi-core support.

Quick Start

#include <stdint.h>
#include <stddef.h>

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    // Validate input if needed
    if (size < 1) return 0;

    // Call your target function with fuzzer-provided data
    my_target_function(data, size);

    return 0;
}

Compile and run:

clang++ -fsanitize=fuzzer,address -g -O2 harness.cc target.cc -o fuzz
mkdir corpus/
./fuzz corpus/

Installation

Prerequisites

• LLVM/Clang compiler (includes libFuzzer)
• LLVM tools for coverage analysis (optional)

Linux (Ubuntu/Debian)

apt install clang llvm

For the latest LLVM version:

# Add LLVM repository from apt.llvm.org
# Then install specific version, e.g.:
apt install clang-18 llvm-18

macOS

# Using Homebrew
brew install llvm

# Or using Nix
nix-env -i clang

Windows

Install Clang through Visual Studio. Refer to Microsoft's documentation for setup instructions.

Recommendation: If possible, fuzz on a local x86_64 VM or rent one on DigitalOcean, AWS, or Hetzner. Linux provides the best support for libFuzzer.

Verification

clang++ --version
# Should show LLVM version information

Writing a Harness

Harness Structure

The harness is the entry point for the fuzzer. libFuzzer calls the LLVMFuzzerTestOneInput function repeatedly with different inputs.

#include <stdint.h>
#include <stddef.h>

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    // 1. Optional: Validate input size
    if (size < MIN_REQUIRED_SIZE) {
        return 0;  // Reject inputs that are too small
    }

    // 2. Optional: Convert raw bytes to structured data
    // Example: Parse two integers from byte array
    if (size >= 2 * sizeof(uint32_t)) {
        uint32_t a = *(uint32_t*)(data);
        uint32_t b = *(uint32_t*)(data + sizeof(uint32_t));
        my_function(a, b);
    }

    // 3. Call target function
    target_function(data, size);

    // 4. Always return 0 (non-zero reserved for future use)
    return 0;
}

Harness Rules

| Do | Don't | |----|-------| | Handle all input types (empty, huge, malformed) | Call exit() - stops fuzzing process | | Join all threads before returning | Leave threads running | | Keep harness fast and simple | Add excessive logging or complexity | | Maintain determinism | Use random number generators or read /dev/random | | Reset global state between runs | Rely on state from previous executions | | Use narrow, focused targets | Mix unrelated data formats (PNG + TCP) in one harness |

Rationale:

• Speed matters: Aim for 100s-1000s executions per second per core
• Reproducibility: Crashes must be reproducible after fuzzing completes
• Isolation: Each execution should be independent

Using FuzzedDataProvider for Complex Inputs

For complex inputs (strings, multiple parameters), use the FuzzedDataProvider helper:

#include <stdint.h>
#include <stddef.h>
#include "FuzzedDataProvider.h"  // From LLVM project

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    FuzzedDataProvider fuzzed_data(data, size);

    // Extract structured data
    size_t allocation_size = fuzzed_data.ConsumeIntegral<size_t>();
    std::vector<char> str1 = fuzzed_data.ConsumeBytesWithTerminator<char>(32, 0xFF);
    std::vector<char> str2 = fuzzed_data.ConsumeBytesWithTerminator<char>(32, 0xFF);

    // Call target with extracted data
    char* result = concat(&str1[0], str1.size(), &str2[0], str2.size(), allocation_size);
    if (result != NULL) {
        free(result);
    }

    return 0;
}

Download FuzzedDataProvider.h from the LLVM repository.

Interleaved Fuzzing

Use a single harness to test multiple related functions:

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    if (size < 1 + 2 * sizeof(int32_t)) {
        return 0;
    }

    uint8_t mode = data[0];
    int32_t numbers[2];
    memcpy(numbers, data + 1, 2 * sizeof(int32_t));

    // Select function based on first byte
    switch (mode % 4) {
        case 0: add(numbers[0], numbers[1]); break;
        case 1: subtract(numbers[0], numbers[1]); break;
        case 2: multiply(numbers[0], numbers[1]); break;
        case 3: divide(numbers[0], numbers[1]); break;
    }

    return 0;
}

See Also: For detailed harness writing techniques, patterns for handling complex inputs, structure-aware fuzzing, and protobuf-based fuzzing, see the fuzz-harness-writing technique skill.

Compilation

Basic Compilation

The key flag is -fsanitize=fuzzer, which:

• Links the libFuzzer runtime (provides main function)
• Enables SanitizerCoverage instrumentation for coverage tracking
• Disables built-in functions like memcmp

clang++ -fsanitize=fuzzer -g -O2 harness.cc target.cc -o fuzz

Flags explained:

• -fsanitize=fuzzer: Enable libFuzzer
• -g: Add debug symbols (helpful for crash analysis)
• -O2: Production-level optimizations (recommended for fuzzing)
• -DNO_MAIN: Define macro if your code has a main function

With Sanitizers

AddressSanitizer (recommended):

clang++ -fsanitize=fuzzer,address -g -O2 -U_FORTIFY_SOURCE harness.cc target.cc -o fuzz

Multiple sanitizers:

clang++ -fsanitize=fuzzer,address,undefined -g -O2 harness.cc target.cc -o fuzz

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

Build Flags

| Flag | Purpose | |------|---------| | -fsanitize=fuzzer | Enable libFuzzer runtime and instrumentation | | -fsanitize=address | Enable AddressSanitizer (memory error detection) | | -fsanitize=undefined | Enable UndefinedBehaviorSanitizer | | -fsanitize=fuzzer-no-link | Instrument without linking fuzzer (for libraries) | | -g | Include debug symbols | | -O2 | Production optimization level | | -U_FORTIFY_SOURCE | Disable fortification (can interfere with ASan) |

Building Static Libraries

For projects that produce static libraries:

1. Build the library with fuzzing instrumentation:

export CC=clang CFLAGS="-fsanitize=fuzzer-no-link -fsanitize=address"
export CXX=clang++ CXXFLAGS="$CFLAGS"
./configure --enable-shared=no
make

2. Link the static library with your harness:

clang++ -fsanitize=fuzzer -fsanitize=address harness.cc libmylib.a -o fuzz

CMake Integration

project(FuzzTarget)
cmake_minimum_required(VERSION 3.0)

add_executable(fuzz main.cc harness.cc)
target_compile_definitions(fuzz PRIVATE NO_MAIN=1)
target_compile_options(fuzz PRIVATE -g -O2 -fsanitize=fuzzer -fsanitize=address)
target_link_libraries(fuzz -fsanitize=fuzzer -fsanitize=address)

Build with:

cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ .
cmake --build .

Corpus Management

Creating Initial Corpus

Create a directory for the corpus (can start empty):

mkdir corpus/

Optional but recommended: Provide seed inputs (valid example files):

# For a PNG parser:
cp examples/*.png corpus/

# For a protocol parser:
cp test_packets/*.bin corpus/

Benefits of seed inputs:

• Fuzzer doesn't start from scratch
• Reaches valid code paths faster
• Significantly improves effectiveness

Corpus Structure

The corpus directory contains:

• Input files that trigger unique code paths
• Minimized versions (libFuzzer automatically minimizes)
• Named by content hash (e.g., a9993e364706816aba3e25717850c26c9cd0d89d)

Corpus Minimization

libFuzzer automatically minimizes corpus entries during fuzzing. To explicitly minimize:

mkdir minimized_corpus/
./fuzz -merge=1 minimized_corpus/ corpus/

This creates a deduplicated, minimized corpus in minimized_corpus/.

See Also: For corpus creation strategies, seed selection, format-specific corpus building, and corpus maintenance, see the fuzzing-corpus technique skill.

Running Campaigns

Basic Run

./fuzz corpus/

This runs until a crash is found or you stop it (Ctrl+C).

Recommended: Continue After Crashes

./fuzz -fork=1 -ignore_crashes=1 corpus/

The -fork and -ignore_crashes flags (experimental but widely used) allow fuzzing to continue after finding crashes.

Common Options

Control input size:

./fuzz -max_len=4000 corpus/

Rule of thumb: 2x the size of minimal realistic input.

Set timeout:

./fuzz -timeout=2 corpus/

Abort test cases that run longer than 2 seconds.

Use a dictionary:

./fuzz -dict=./format.dict corpus/

Close stdout/stderr (speed up fuzzing):

./fuzz -close_fd_mask=3 corpus/

See all options:

./fuzz -help=1

Multi-Core Fuzzing

Option 1: Jobs and workers (recommended):

./fuzz -jobs=4 -workers=4 -fork=1 -ignore_crashes=1 corpus/

• -jobs=4: Run 4 sequential campaigns
• -workers=4: Process jobs in parallel with 4 processes
• Test cases are shared between jobs

Option 2: Fork mode:

./fuzz -fork=4 -ignore_crashes=1 corpus/

Note: For serious multi-core fuzzing, consider switching to AFL++, Honggfuzz, or LibAFL.

Re-executing Test Cases

Re-run a single crash:

./fuzz ./crash-a9993e364706816aba3e25717850c26c9cd0d89d

Test all inputs in a directory without fuzzing:

./fuzz -runs=0 corpus/

Interpreting Output

When fuzzing runs, you'll see statistics like:

INFO: Seed: 3517090860
INFO: Loaded 1 modules (9 inline 8-bit counters)
#2      INITED cov: 3 ft: 4 corp: 1/1b exec/s: 0 rss: 26Mb
#57     NEW    cov: 4 ft: 5 corp: 2/4b lim: 4 exec/s: 0 rss: 26Mb

| Output | Meaning | |--------|---------| | INITED | Fuzzing initialized | | NEW | New coverage found, added to corpus | | REDUCE | Input minimized while keeping coverage | | cov: N | Number of coverage edges hit | | corp: X/Yb | Corpus size: X entries, Y total bytes | | exec/s: N | Executions per second | | rss: NMb | Resident memory usag