atheris

SKILL.md

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 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:

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 pip install 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

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:

docker build -t atheris .
docker run -it atheris

Verification

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

Writing a Harness

Harness Structure for Pure 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:

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:

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:

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:

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:

python cbor2-fuzz.py

Important: When running locally (not in Docker), you must set LD_PRELOAD manually.

Corpus Management

Creating Initial Corpus

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

Run with corpus:

python fuzz.py corpus/

Corpus Minimization

Atheris inherits corpus minimization from libFuzzer:

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

python fuzz.py

With Corpus Directory

python fuzz.py corpus/

Common Options

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

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

# Run with multiple workers
python fuzz.py -workers=4 -jobs=4

Interpreting Output

Output
Meaning

NEW    cov: X
Found new coverage, corpus expanded

pulse  cov: X
Periodic status update

exec/s: X
Executions per second (throughput)

corp: X/Yb
Corpus size: X inputs, Y bytes total

ERROR: libFuzzer
Crash detected

Sanitizer Integration

AddressSanitizer (ASan)

AddressSanitizer is automatically integrated when using the provided Docker environment or when compiling with appropriate flags.

For local setup:

export CFLAGS="-fsanitize=address,fuzzer-no-link"
export CXXFLAGS="-fsanitize=address,fuzzer-no-link"

Configure ASan behavior:

export ASAN_OPTIONS="allocator_may_return_null=1,detect_leaks=0"

LD_PRELOAD Configuration

For native extension fuzzing:

export LD_PRELOAD="$(python -c 'import atheris; import os; print(os.path.join(os.path.dirname(atheris.__file__), "asan_with_fuzzer.so"))')"

See Also: For detailed sanitizer configuration, common issues, and advanced flags,
see the address-sanitizer and undefined-behavior-sanitizer technique skills.

Common Sanitizer Issues

Issue
Solution

LD_PRELOAD not set
Export LD_PRELOAD to point to asan_with_fuzzer.so

Memory allocation failures
Set ASAN_OPTIONS=allocator_may_return_null=1

Leak detection noise
Set ASAN_OPTIONS=detect_leaks=0

Missing symbolizer
Set ASAN_SYMBOLIZER_PATH to llvm-symbolizer

Advanced Usage

Tips and Tricks

Tip
Why It Helps

Use atheris.instrument_imports() early
Ensures all imports are instrumented for coverage

Start with small max_len
Faster initial fuzzing, gradually increase

Use dictionaries for structured formats
Helps fuzzer understand format tokens

Run multiple parallel instances
Better coverage exploration

Custom Instrumentation

Fine-tune what gets instrumented:

import atheris

# Instrument only specific modules
with atheris.instrument_imports():
    import target_module
# Don't instrument test harness code

def test_one_input(data: bytes):
    target_module.parse(data)

Performance Tuning

Setting
Impact

-max_len=N
Smaller values = faster execution

-workers=N -jobs=N
Parallel fuzzing for faster coverage

ASAN_OPTIONS=fast_unwind_on_malloc=0
Better stack traces, slower execution

UndefinedBehaviorSanitizer (UBSan)

Add UBSan to catch additional bugs:

export CFLAGS="-fsanitize=address,undefined,fuzzer-no-link"
export CXXFLAGS="-fsanitize=address,undefined,fuzzer-no-link"

Note: Modify flags in Dockerfile if using containerized setup.

Real-World Examples

Example: Pure Python Parser

import sys
import atheris
import json

@atheris.instrument_func
def test_one_input(data: bytes):
    try:
        # Fuzz Python's JSON parser
        json.loads(data.decode('utf-8', errors='ignore'))
    except (ValueError, UnicodeDecodeError):
        pass

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

if __name__ == "__main__":
    main()

Example: HTTP Request Parsing

import sys
import atheris

with atheris.instrument_imports():
    from urllib3 import HTTPResponse
    from io import BytesIO

def test_one_input(data: bytes):
    try:
        # Fuzz HTTP response parsing
        fake_response = HTTPResponse(
            body=BytesIO(data),
            headers={},
            preload_content=False
        )
        fake_response.read()
    except Exception:
        pass

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

if __name__ == "__main__":
    main()

Troubleshooting

Problem
Cause
Solution

No coverage increase
Poor seed corpus or target not instrumented
Add better seeds, verify instrument_imports()

Slow execution
ASan overhead or large inputs
Reduce max_len, use ASAN_OPTIONS=fast_unwind_on_malloc=1

Import errors
Modules imported before instrumentation
Move imports inside instrument_imports() context

Segfault without ASan output
Missing LD_PRELOAD
Set LD_PRELOAD to asan_with_fuzzer.so path

Build failures
Wrong compiler or missing flags
Verify CC, CFLAGS, and clang version

Related Skills

Technique Skills

Skill
Use Case

fuzz-harness-writing
Detailed guidance on writing effective harnesses

address-sanitizer
Memory error detection during fuzzing

undefined-behavior-sanitizer
Catching undefined behavior in C extensions

coverage-analysis
Measuring and improving code coverage

fuzzing-corpus
Building and managing seed corpora

Related Fuzzers

Skill
When to Consider

hypothesis
Property-based testing with type-aware generation

python-afl
AFL-style fuzzing for Python when Atheris isn't available

Resources

Key External Resources

Atheris GitHub Repository
Official repository with installation instructions, examples, and documentation for fuzzing both pure Python and native extensions.

Native Extension Fuzzing Guide
Comprehensive guide covering compilation flags, LD_PRELOAD setup, sanitizer configuration, and troubleshooting for Python C extensions.

Continuously Fuzzing Python C Extensions
Trail of Bits blog post covering CI/CD integration, ClusterFuzzLite setup, and real-world examples of fuzzing Python C extensions in continuous integration pipelines.

ClusterFuzzLite Python Integration
Guide for integrating Atheris fuzzing into CI/CD pipelines using ClusterFuzzLite for automated continuous fuzzing.

Video Resources

Videos and tutorials are available in the main Atheris documentation and libFuzzer resources.