# fuzzing

Guides agents in writing fuzz targets for C and C++, configuring libFuzzer or AFL++, and wiring coverage-guided tests into CI pipelines.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 618 installs · ★ 267
- Author: mohitmishra786
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/runtimes/fuzzing/SKILL.md
- Updated: 2026-06-27
- HTML: https://affpapa.org/en/nevibecode/skills/fuzzing

## What it does

The skill instructs coding agents on setting up coverage-guided fuzz testing for C and C++ codebases. It covers in-process testing with libFuzzer, compilation flags in Clang, and sanitizer support such as AddressSanitizer.

It includes templates for persistent mode in AFL++ using `__AFL_LOOP` and corpus minimization with `afl-cmin`. It also outlines custom mutators via `LLVMFuzzerCustomMutator` alongside Python testing with Atheris.

Flags like `-fsanitize=fuzzer` provide a built-in `main()`, so declaring a manual `main` in the harness breaks the build. Crash artifacts from `crash-*` files can be passed directly into GDB.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill fuzzing -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/fuzzing/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/fuzzing/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill fuzzing -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/fuzzing/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill fuzzing -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/fuzzing/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install mohitmishra786/low-level-dev-skills fuzzing
```

2. **Manually: put the files in place**
- File: `.github/skills/fuzzing/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill fuzzing -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/mohitmishra786/low-level-dev-skills --path skills/runtimes/fuzzing
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/fuzzing/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill fuzzing -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/runtimes/fuzzing/SKILL.md

## When to use it
- Write a `LLVMFuzzerTestOneInput` harness for C and C++ parsers
- Compile targets using Clang with AddressSanitizer and UBSan flags
- Run AFL++ persistent mode to avoid per-run `fork()` overhead
- Minimize fuzzing corpus directories before running CI regression checks

## Example prompts
- Help me write a libFuzzer target for my C parser
- How do I build this library with AFL++ persistent mode?
- Set up a short 60-second libFuzzer regression step in GitHub Actions

## Source text · SKILL.md · MIT

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BEGIN THIRD-PARTY FILE 7ce95ff7bc134afa
````text
---
name: fuzzing
description: Fuzzing skill for automated input-driven bug finding in C/C++. Use when setting up libFuzzer or AFL++ fuzz targets, defining fuzz entry points around parsing or I/O boundaries, integrating fuzzing into CI, managing corpora, or combining fuzzing with sanitizers. Activates on queries about libFuzzer, AFL, afl-fuzz, fuzz targets, corpus management, coverage-guided fuzzing, or OSS-Fuzz integration.
---

# Fuzzing

## Purpose

Guide agents through setting up and running coverage-guided fuzz testing: libFuzzer (in-process) and AFL++ (fork-based), with sanitizer integration and CI pipeline setup.

## Triggers

- "How do I fuzz-test my parser/deserializer?"
- "What is a fuzz target / how do I write one?"
- "How do I set up libFuzzer?"
- "How do I use AFL++ on my program?"
- "How do I run fuzzing in CI?"
- "Fuzzer found a crash — how do I reproduce it?"

## Workflow

### 1. Write a fuzz target (libFuzzer)

A fuzz target is a function that accepts arbitrary bytes and exercises the code under test.

```c
// fuzz_parser.c
#include <stdint.h>
#include <stddef.h>
#include "myparser.h"

// Entry point called by libFuzzer with random data
int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    // Must not abort/exit on invalid input (that's expected)
    // Must not read outside [data, data+size)

    MyParser *p = parser_create();
    if (p) {
        parser_feed(p, (const char *)data, size);
        parser_destroy(p);
    }
    return 0;  // Always return 0 (non-zero means discard input)
}
```

Key rules:

- Never call `abort()`, `exit()`, or use global state that persists across calls
- Handle all inputs gracefully (crash = bug found)
- Keep the target fast: the fuzzer calls it millions of times

### 2. Build with libFuzzer

```bash
# Clang (libFuzzer is built into Clang)
clang -fsanitize=fuzzer,address -g -O1 \
    fuzz_parser.c myparser.c -o fuzz_parser

# With UBSan too
clang -fsanitize=fuzzer,address,undefined -g -O1 \
    fuzz_parser.c myparser.c -o fuzz_parser
```

`-fsanitize=fuzzer` links libFuzzer and provides `main()`. Do not provide your own `main()` in the fuzz target.

### 3. Run libFuzzer

```bash
# Create corpus directory
mkdir -p corpus

# Seed with known-good inputs (greatly accelerates coverage)
cp tests/inputs/* corpus/

# Run the fuzzer
./fuzz_parser corpus/ -max_len=65536 -timeout=10

# Run for a time limit
./fuzz_parser corpus/ -max_total_time=3600

# Run with specific number of jobs (parallel)
./fuzz_parser corpus/ -jobs=4 -workers=4

# Minimise a corpus (remove redundant inputs)
./fuzz_parser -merge=1 corpus_min/ corpus/
```

Common flags:

| Flag | Default | Effect |
|------|---------|--------|
| `-max_len=N` | 4096 | Max input size in bytes |
| `-timeout=N` | 1200 | Kill if single run takes > N seconds |
| `-max_total_time=N` | 0 (forever) | Total fuzzing time |
| `-runs=N` | -1 (infinite) | Total number of runs |
| `-dict=file` | none | Dictionary of interesting tokens |
| `-jobs=N` | 1 | Parallel jobs (each writes its own log) |
| `-merge=1` | off | Merge mode: minimise corpus |

### 4. Reproduce a crash
````
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First 3119 of 8566 bytes. © mohitmishra786 · MIT · https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/runtimes/fuzzing/SKILL.md

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