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

import sys
import atheris

@atheris.instrument_func
def TestOneInput(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, TestOneInput)
    atheris.Fuzz()

if __name__ == "__main__":
    main()

Run:

uv run 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)
• For Docker users: Docker Desktop

Linux/macOS

uv init --bare   # once, if the harness directory is not yet a uv project
uv add atheris

Docker Environment (Recommended)

For a fully operational Linux environment with all dependencies configured:

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

FROM python:$PYTHON_VERSION-slim-bookworm

RUN python --version