Atheris

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Introduction

Atheris is a coverage-guided Python fuzzing engine. It fuzzes Python code and native extensions written for CPython, and is based on libFuzzer. For native code it can be combined with Address Sanitizer or Undefined Behavior Sanitizer. It supports Linux and macOS.

The source code is at google/atheris on GitHub under the Apache-2.0 licence, with about 1,700 stars; the most recent commit was on 17 June 2026.

Features

1. Python fuzzing: Coverage-guided for pure Python.
2. Native extensions: Fuzz CPython C extensions.
3. Sanitizers: ASan and UBSan for native code.
4. FuzzedDataProvider: Helpers to build typed inputs.
5. OSS-Fuzz support: Used for Python projects.

Pros

1. Free to use: Open source under Apache-2.0.
2. Maintained: Commits as recent as 17 June 2026.

Cons

1. Platform limits: Linux and macOS only.
2. Last commit June 2026: Check activity before adopting.

Usage and Links (Example)

import atheris, sys
with atheris.instrument_imports():
    import mylib

def TestOneInput(data):
    mylib.parse(data)

atheris.Setup(sys.argv, TestOneInput)
atheris.Fuzz()

Resources:

• Atheris website

Pricing

Atheris is free and open source under the Apache-2.0 licence.

Recommended for?

1. Python library maintainers.
2. Teams with C extensions.
3. Not a fit on Windows.

Conclusion

Atheris is the standard coverage-guided fuzzer for Python.