atheris

What this skill does

# Atheris

Atheris is a coverage-guided Python fuzzer built on libFuzzer. It enables fuzzing of both pure Python code and Python C extensions with integrated AddressSanitizer support for detecting memory corruption issues.

## When to Use

| Fuzzer | Best For | Complexity |
|--------|----------|------------|
| Atheris | Python code and C extensions | Low-Medium |
| Hypothesis | Property-based testing | Low |
| python-afl | AFL-style fuzzing | Medium |

**Choose Atheris when:**
- Fuzzing pure Python code with coverage guidance
- Testing Python C extensions for memory corruption
- Integration with libFuzzer ecosystem is desired
- AddressSanitizer support is needed

## Quick Start

```python
import sys
import atheris

@atheris.instrument_func
def test_one_input(data: bytes):
    if len(data) == 4:
        if data[0] == 0x46:  # "F"
            if data[1] == 0x55:  # "U"
                if data[2] == 0x5A:  # "Z"
                    if data[3] == 0x5A:  # "Z"
                        raise RuntimeError("You caught me")

def main():
    atheris.Setup(sys.argv, test_one_input)
    atheris.Fuzz()

if __name__ == "__main__":
    main()
```

Run:
```bash
python fuzz.py
```

## Installation

Atheris supports 32-bit and 64-bit Linux, and macOS. We recommend fuzzing on Linux because it's simpler to manage and often faster.

### Prerequisites

- Python 3.7 or later
- Recent version of clang (preferably [latest release](https://github.com/llvm/llvm-project/releases))
- For Docker users: [Docker Desktop](https://www.docker.com/products/docker-desktop/)

### Linux/macOS

```bash
uv pip install atheris
```

### Docker Environment (Recommended)

For a fully operational Linux environment with all dependencies configured:

```dockerfile
# https://hub.docker.com/_/python
ARG PYTHON_VERSION=3.11

FROM python:$PYTHON_VERSION-slim-bookworm

RUN python --version

RUN apt update && apt install -y \
    ca-certificates \
    wget \
    && rm -rf /var/lib/apt/lists/*

# LLVM builds version 15-19 for Debian 12 (Bookworm)
# https://apt.llvm.org/bookworm/dists/
ARG LLVM_VERSION=19

RUN echo "deb http://apt.llvm.org/bookworm/ llvm-toolchain-bookworm-$LLVM_VERSION main" > /etc/apt/sources.list.d/llvm.list
RUN echo "deb-src http://apt.llvm.org/bookworm/ llvm-toolchain-bookworm-$LLVM_VERSION main" >> /etc/apt/sources.list.d/llvm.list
RUN wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key > /etc/apt/trusted.gpg.d/apt.llvm.org.asc

RUN apt update && apt install -y \
    build-essential \
    clang-$LLVM_VERSION \
    && rm -rf /var/lib/apt/lists/*

ENV APP_DIR "/app"
RUN mkdir $APP_DIR
WORKDIR $APP_DIR

ENV VIRTUAL_ENV "/opt/venv"
RUN python -m venv $VIRTUAL_ENV
ENV PATH "$VIRTUAL_ENV/bin:$PATH"

# https://github.com/google/atheris/blob/master/native_extension_fuzzing.md#step-1-compiling-your-extension
ENV CC="clang-$LLVM_VERSION"
ENV CFLAGS "-fsanitize=address,fuzzer-no-link"
ENV CXX="clang++-$LLVM_VERSION"
ENV CXXFLAGS "-fsanitize=address,fuzzer-no-link"
ENV LDSHARED="clang-$LLVM_VERSION -shared"
ENV LDSHAREDXX="clang++-$LLVM_VERSION -shared"
ENV ASAN_SYMBOLIZER_PATH="/usr/bin/llvm-symbolizer-$LLVM_VERSION"

# Allow Atheris to find fuzzer sanitizer shared libs
# https://github.com/google/atheris#building-from-source
RUN LIBFUZZER_LIB=$($CC -print-file-name=libclang_rt.fuzzer_no_main-$(uname -m).a) \
    python -m pip install --no-binary atheris atheris

# https://github.com/google/atheris/blob/master/native_extension_fuzzing.md#option-a-sanitizerlibfuzzer-preloads
ENV LD_PRELOAD "$VIRTUAL_ENV/lib/python3.11/site-packages/asan_with_fuzzer.so"

# 1. Skip memory allocation failures for now, they are common, and low impact (DoS)
# 2. https://github.com/google/atheris/blob/master/native_extension_fuzzing.md#leak-detection
ENV ASAN_OPTIONS "allocator_may_return_null=1,detect_leaks=0"

CMD ["/bin/bash"]
```

Build and run:
```bash
docker build -t atheris .
docker run -it atheris
```

### Verification

```bash
python -c "import atheris; print(atheris.__version__)"
```

## Writing a Harness

### Harness Structure for Pure Python

```python
import sys
import atheris

@atheris.instrument_func
def test_one_input(data: bytes):
    """
    Fuzzing entry point. Called with random byte sequences.

    Args:
        data: Random bytes generated by the fuzzer
    """
    # Add input validation if needed
    if len(data) < 1:
        return

    # Call your target function
    try:
        your_target_function(data)
    except ValueError:
        # Expected exceptions should be caught
        pass
    # Let unexpected exceptions crash (that's what we're looking for!)

def main():
    atheris.Setup(sys.argv, test_one_input)
    atheris.Fuzz()

if __name__ == "__main__":
    main()
```

### Harness Rules

| Do | Don't |
|----|-------|
| Use `@atheris.instrument_func` for coverage | Forget to instrument target code |
| Catch expected exceptions | Catch all exceptions indiscriminately |
| Use `atheris.instrument_imports()` for libraries | Import modules after `atheris.Setup()` |
| Keep harness deterministic | Use randomness or time-based behavior |

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

## Fuzzing Pure Python Code

For fuzzing broader parts of an application or library, use instrumentation functions:

```python
import atheris
with atheris.instrument_imports():
    import your_module
    from another_module import target_function

def test_one_input(data: bytes):
    target_function(data)

atheris.Setup(sys.argv, test_one_input)
atheris.Fuzz()
```

**Instrumentation Options:**
- `atheris.instrument_func` - Decorator for single function instrumentation
- `atheris.instrument_imports()` - Context manager for instrumenting all imported modules
- `atheris.instrument_all()` - Instrument all Python code system-wide

## Fuzzing Python C Extensions

Python C extensions require compilation with specific flags for instrumentation and sanitizer support.

### Environment Configuration

If using the provided Dockerfile, these are already configured. For local setup:

```bash
export CC="clang"
export CFLAGS="-fsanitize=address,fuzzer-no-link"
export CXX="clang++"
export CXXFLAGS="-fsanitize=address,fuzzer-no-link"
export LDSHARED="clang -shared"
```

### Example: Fuzzing cbor2

Install the extension from source:
```bash
CBOR2_BUILD_C_EXTENSION=1 python -m pip install --no-binary cbor2 cbor2==5.6.4
```

The `--no-binary` flag ensures the C extension is compiled locally with instrumentation.

Create `cbor2-fuzz.py`:
```python
import sys
import atheris

# _cbor2 ensures the C library is imported
from _cbor2 import loads

def test_one_input(data: bytes):
    try:
        loads(data)
    except Exception:
        # We're searching for memory corruption, not Python exceptions
        pass

def main():
    atheris.Setup(sys.argv, test_one_input)
    atheris.Fuzz()

if __name__ == "__main__":
    main()
```

Run:
```bash
python cbor2-fuzz.py
```

> **Important:** When running locally (not in Docker), you must [set `LD_PRELOAD` manually](https://github.com/google/atheris/blob/master/native_extension_fuzzing.md#option-a-sanitizerlibfuzzer-preloads).

## Corpus Management

### Creating Initial Corpus

```bash
mkdir corpus
# Add seed inputs
echo "test data" > corpus/seed1
echo '{"key": "value"}' > corpus/seed2
```

Run with corpus:
```bash
python fuzz.py corpus/
```

### Corpus Minimization

Atheris inherits corpus minimization from libFuzzer:
```bash
python fuzz.py -merge=1 new_corpus/ old_corpus/
```

> **See Also:** For corpus creation strategies, dictionaries, and seed selection,
> see the **fuzzing-corpus** technique skill.

## Running Campaigns

### Basic Run

```bash
python fuzz.py
```

### With Corpus Directory

```bash
python fuzz.py corpus/
```

### Common Options

```bash
# Run for 10 minutes
python fuzz.py -max_total_time=600

# Limit input size
python fuzz.py -max_len=1024

# Run with