libfuzzer
Coverage-guided fuzzer built into LLVM for C/C++ projects. Use for fuzzing C/C++ code that can be compiled with Clang.
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Coverage-guided fuzzer built into LLVM for C/C++ projects. Use for fuzzing C/C++ code that can be compiled with Clang.
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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.
| 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:
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.
#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/
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
# Using Homebrew
brew install llvm
# Or using Nix
nix-env -i clang
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.
clang++ --version
# Should show LLVM version information
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;
}
| 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:
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.
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.
The key flag is -fsanitize=fuzzer, which:
main function)memcmpclang++ -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 functionAddressSanitizer (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.
| 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 |
For projects that produce static libraries:
export CC=clang CFLAGS="-fsanitize=fuzzer-no-link -fsanitize=address"
export CXX=clang++ CXXFLAGS="$CFLAGS"
./configure --enable-shared=no
make
clang++ -fsanitize=fuzzer -fsanitize=address harness.cc libmylib.a -o fuzz
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 .
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:
The corpus directory contains:
a9993e364706816aba3e25717850c26c9cd0d89d)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.
./fuzz corpus/
This runs until a crash is found or you stop it (Ctrl+C).
./fuzz -fork=1 -ignore_crashes=1 corpus/
The -fork and -ignore_crashes flags (experimental but widely used) allow fuzzing to continue after finding crashes.
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
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 processesOption 2: Fork mode:
./fuzz -fork=4 -ignore_crashes=1 corpus/
Note: For serious multi-core fuzzing, consider switching to AFL++, Honggfuzz, or LibAFL.
Re-run a single crash:
./fuzz ./crash-a9993e364706816aba3e25717850c26c9cd0d89d
Test all inputs in a directory without fuzzing:
./fuzz -runs=0 corpus/
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 |
On crash:
==11672== ERROR: libFuzzer: deadly signal
artifact_prefix='./'; Test unit written to ./crash-a9993e364706816aba3e25717850c26c9cd0d89d
0x61,0x62,0x63,
abc
Base64: YWJj
The crash is saved to ./crash-<hash> with the input shown in hex, UTF-8, and Base64.
Reproducibility: Use -seed=<value> to reproduce a fuzzing campaign (single-core only).
Dictionaries help the fuzzer discover interesting inputs faster by providing hints about the input format.
Create a text file with quoted strings (one per line):
# Lines starting with '#' are comments
# Magic bytes
magic="\x89PNG"
magic2="IEND"
# Keywords
"GET"
"POST"
"Content-Type"
# Hex sequences
delimiter="\xFF\xD8\xFF"
./fuzz -dict=./format.dict corpus/
From header files:
grep -o '".*"' header.h > header.dict
From man pages:
man curl | grep -oP '^\s*(--|-)\K\S+' | sed 's/[,.]$//' | sed 's/^/"&/; s/$/&"/' | sort -u > man.dict
From binary strings:
strings ./binary | sed 's/^/"&/; s/$/&"/' > strings.dict
Using LLMs: Ask ChatGPT or similar to generate a dictionary for your format (e.g., "Generate a libFuzzer dictionary for a JSON parser").
See Also: For advanced dictionary generation, format-specific dictionaries, and dictionary optimization strategies, see the fuzzing-dictionaries technique skill.
While libFuzzer shows basic coverage stats (cov: N), detailed coverage analysis requires additional tools.
1. Recompile with coverage instrumentation:
clang++ -fsanitize=fuzzer -fprofile-instr-generate -fcoverage-mapping harness.cc target.cc -o fuzz
2. Run fuzzer to collect coverage:
LLVM_PROFILE_FILE="coverage-%p.profraw" ./fuzz -runs=10000 corpus/
3. Merge coverage data:
llvm-profdata merge -sparse coverage-*.profraw -o coverage.profdata
4. Generate coverage report:
llvm-cov show ./fuzz -instr-profile=coverage.profdata
5. Generate HTML report:
llvm-cov show ./fuzz -instr-profile=coverage.profdata -format=html > coverage.html
Tips:
See Also: For detailed coverage analysis techniques, identifying coverage gaps, systematic coverage improvement, and comparing coverage across fuzzers, see the coverage-analysis technique skill.
ASan detects memory errors like buffer overflows and use-after-free bugs. Highly recommended for fuzzing.
Enable ASan:
clang++ -fsanitize=fuzzer,address -g -O2 -U_FORTIFY_SOURCE harness.cc target.cc -o fuzz
Example ASan output:
==1276163==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6020000c4ab1
WRITE of size 1 at 0x6020000c4ab1 thread T0
#0 0x55555568631a in check_buf(char*, unsigned long) main.cc:13:25
#1 0x5555556860bf in LLVMFuzzerTestOneInput harness.cc:7:3
Configure ASan with environment variables:
ASAN_OPTIONS=verbosity=1:abort_on_error=1 ./fuzz corpus/
Important flags:
verbosity=1: Show ASan is activedetect_leaks=0: Disable leak detection (leaks reported at end)abort_on_error=1: Call abort() instead of _exit() on errorsDrawbacks:
-rss_limit_mb=0)See Also: For comprehensive ASan configuration, common pitfalls, symbolization, and combining with other sanitizers, see the address-sanitizer technique skill.
UBSan detects undefined behavior like integer overflow, null pointer dereference, etc.
Enable UBSan:
clang++ -fsanitize=fuzzer,undefined -g -O2 harness.cc target.cc -o fuzz
Combine with ASan:
clang++ -fsanitize=fuzzer,address,undefined -g -O2 harness.cc target.cc -o fuzz
MSan detects uninitialized memory reads. More complex to use (requires rebuilding all dependencies).
clang++ -fsanitize=fuzzer,memory -g -O2 harness.cc target.cc -o fuzz
| Issue | Solution |
|---|---|
| ASan slows fuzzing too much | Use -fsanitize-recover=address for non-fatal errors |
| Out of memory | Set ASAN_OPTIONS=rss_limit_mb=0 or -rss_limit_mb=0 |
| Stack exhaustion | Increase stack size: ASAN_OPTIONS=stack_size=8388608 |
False positives with _FORTIFY_SOURCE | Use -U_FORTIFY_SOURCE flag |
| MSan reports in dependencies | Rebuild all dependencies with -fsanitize=memory |
libpng is a widely-used library for reading/writing PNG images. Bugs can lead to security issues.
1. Get source code:
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/
2. Install dependencies:
apt install zlib1g-dev
3. Compile with fuzzing instrumentation:
export CC=clang CFLAGS="-fsanitize=fuzzer-no-link -fsanitize=address"
export CXX=clang++ CXXFLAGS="$CFLAGS"
./configure --enable-shared=no
make
4. Get a harness (or write your own):
curl -O https://raw.githubusercontent.com/glennrp/libpng/f8e5fa92b0e37ab597616f554bee254157998227/contrib/oss-fuzz/libpng_read_fuzzer.cc
5. Prepare corpus and dictionary:
mkdir corpus/
curl -o corpus/input.png https://raw.githubusercontent.com/glennrp/libpng/acfd50ae0ba3198ad734e5d4dec2b05341e50924/contrib/pngsuite/iftp1n3p08.png
curl -O https://raw.githubusercontent.com/glennrp/libpng/2fff013a6935967960a5ae626fc21432807933dd/contrib/oss-fuzz/png.dict
6. Link and compile fuzzer:
clang++ -fsanitize=fuzzer -fsanitize=address libpng_read_fuzzer.cc .libs/libpng16.a -lz -o fuzz
7. Run fuzzing campaign:
./fuzz -close_fd_mask=3 -dict=./png.dict corpus/
Harness that finds a division-by-zero bug:
#include <stdint.h>
#include <stddef.h>
double divide(uint32_t numerator, uint32_t denominator) {
// Bug: No check if denominator is zero
return numerator / denominator;
}
extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
if(size != 2 * sizeof(uint32_t)) {
return 0;
}
uint32_t numerator = *(uint32_t*)(data);
uint32_t denominator = *(uint32_t*)(data + sizeof(uint32_t));
divide(numerator, denominator);
return 0;
}
Compile and fuzz:
clang++ -fsanitize=fuzzer harness.cc -o fuzz
./fuzz
The fuzzer will quickly find inputs causing a crash.
| Tip | Why It Helps |
|---|---|
| Start with single-core, switch to AFL++ for multi-core | libFuzzer harnesses work with AFL++ |
| Use dictionaries for structured formats | 10-100x faster bug discovery |
Close file descriptors with -close_fd_mask=3 | Speed boost if SUT writes output |
Set reasonable -max_len | Prevents wasted time on huge inputs |
| Run for days/weeks, not minutes | Coverage plateaus take time to break |
| Use seed corpus from test suites | Starts fuzzing from valid inputs |
For highly structured inputs (e.g., complex protocols, file formats), use libprotobuf-mutator:
See structure-aware fuzzing documentation for details.
libFuzzer allows custom mutators for specialized fuzzing:
extern "C" size_t LLVMFuzzerCustomMutator(uint8_t *Data, size_t Size,
size_t MaxSize, unsigned int Seed) {
// Custom mutation logic
return new_size;
}
extern "C" size_t LLVMFuzzerCustomCrossOver(const uint8_t *Data1, size_t Size1,
const uint8_t *Data2, size_t Size2,
uint8_t *Out, size_t MaxOutSize,
unsigned int Seed) {
// Custom crossover logic
return new_size;
}
| Setting | Impact |
|---|---|
-close_fd_mask=3 | Closes stdout/stderr, speeds up fuzzing |
-max_len=<reasonable_size> | Avoids wasting time on huge inputs |
-timeout=<seconds> | Detects hangs, prevents stuck executions |
| Disable ASan for baseline | 2-4x speed boost (but misses memory bugs) |
Use -jobs and -workers | Limited multi-core support |
| Run on Linux | Best platform support and performance |
| Problem | Cause | Solution |
|---|---|---|
| No crashes found after hours | Poor corpus, low coverage | Add seed inputs, use dictionary, check harness |
| Very slow executions/sec (<100) | Target too complex, excessive logging | Optimize target, use -close_fd_mask=3, reduce logging |
| Out of memory | ASan's 20TB virtual memory | Set -rss_limit_mb=0 to disable RSS limit |
| Fuzzer stops after first crash | Default behavior | Use -fork=1 -ignore_crashes=1 to continue |
| Can't reproduce crash | Non-determinism in harness/target | Remove random number generation, global state |
Linking errors with -fsanitize=fuzzer |
| Skill | Use Case |
|---|---|
| fuzz-harness-writing | Detailed guidance on writing effective harnesses, structure-aware fuzzing, and FuzzedDataProvider usage |
| address-sanitizer | Memory error detection configuration, ASAN_OPTIONS, and troubleshooting |
| undefined-behavior-sanitizer | Detecting undefined behavior during fuzzing |
| coverage-analysis | Measuring fuzzing effectiveness and identifying untested code paths |
| fuzzing-corpus | Building and managing seed corpora, corpus minimization strategies |
| fuzzing-dictionaries | Creating format-specific dictionaries for faster bug discovery |
| Skill | When to Consider |
|---|---|
| aflpp | When you need serious multi-core fuzzing, or when libFuzzer coverage plateaus |
| honggfuzz | When you want hardware-based coverage feedback on Linux |
| libafl | When building custom fuzzers or conducting fuzzing research |
-U_FORTIFY_SOURCE| Disable fortification (can interfere with ASan) |
| Resident memory usage |
| Missing libFuzzer runtime |
| Ensure using Clang, check LLVM installation |
| GCC project won't compile with Clang | GCC-specific code | Switch to AFL++ with gcc_plugin instead |
| Coverage not improving | Corpus plateau | Run longer, add dictionary, improve seeds, check coverage report |
| Crashes but ASan doesn't trigger | Memory error not detected without ASan | Recompile with -fsanitize=address |
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