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SKILL.md
npx skills add trailofbits/skills --skill atheris
Sets up and runs Atheris, the coverage-guided Python fuzzer built on libFuzzer. Covers TestOneInput harnesses, FuzzedDataProvider, instrumenting both pure Python and native C extensions, and running under AddressSanitizer. Use when fuzzing a Python package, hunting memory corruption in a Python C extension, or choosing between Atheris and Hypothesis for a Python target.
npx skills add trailofbits/skills --skill 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.
| 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:
Run:
Atheris supports 32-bit and 64-bit Linux, and macOS. We recommend fuzzing on Linux because it's simpler to manage and often faster.
For a fully operational Linux environment with all dependencies configured:
Build and run:
A target taking several typed arguments wastes most of the fuzzer's inputs if the harness
slices data by hand, because every mutation shifts the byte offsets of everything after it.
atheris.FuzzedDataProvider splits one bytes input into typed values instead:
See structured-input.md for the full method reference, the fixed-draw- order rule, and what each method returns once the buffer runs dry.
| 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.
For fuzzing broader parts of an application or library, use instrumentation functions:
Instrumentation Options:
atheris.instrument_func - Decorator for single function instrumentationatheris.instrument_imports() - Context manager for instrumenting all imported modulesatheris.instrument_all() - Instrument all Python code system-widePython C extensions require compilation with specific flags for instrumentation and sanitizer support.
If using the provided Dockerfile, these are already configured. For local setup:
Install the extension from source:
The --no-binary-package flag ensures the C extension is compiled locally with
instrumentation rather than pulled as a prebuilt wheel. Persist that choice with
no-binary-package = ["cbor2"] under [tool.uv] in pyproject.toml, or a later
uv sync can silently swap in an uninstrumented wheel.
Create cbor2-fuzz.py:
Run:
Important: When running locally (not in Docker), you must set
LD_PRELOADmanually.
Run with corpus:
Atheris inherits corpus minimization from libFuzzer:
See Also: For corpus creation strategies, dictionaries, and seed selection, see the fuzzing-corpus technique skill.
| 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 |
AddressSanitizer is automatically integrated when using the provided Docker environment or when compiling with appropriate flags.
For local setup:
Configure ASan behavior:
For native extension fuzzing:
See Also: For detailed sanitizer configuration, common issues, and advanced flags, see the address-sanitizer and undefined-behavior-sanitizer technique skills.
| 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 |
| 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 |
Fine-tune what gets instrumented:
| 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 |
Add UBSan to catch additional bugs:
Note: Modify flags in Dockerfile if using containerized setup.
Two complete harnesses — a pure-Python parser and an HTTP response parser — are in examples.md.
| 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 |
| 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 |
| Skill | When to Consider |
|---|---|
| hypothesis | Property-based testing with type-aware generation |
| python-afl | AFL-style fuzzing for Python when Atheris isn't available |
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.
Videos and tutorials are available in the main Atheris documentation and libFuzzer resources.
uv run python fuzz.pyuv run python fuzz.pyuv init --bare # once, if the harness directory is not yet a uv project
uv add atheris# 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"]docker build -t atheris .
docker run -it atherispython -c "import atheris; print(atheris.__version__)"import sys
import atheris
@atheris.instrument_func
def TestOneInput(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, TestOneInput)
atheris.Fuzz()
if __name__ == "__main__":
main()fdp = atheris.FuzzedDataProvider(data)
name = fdp.ConsumeUnicodeNoSurrogates(fdp.ConsumeIntInRange(0, 64))
strict = fdp.ConsumeBool()import atheris
with atheris.instrument_imports():
import your_module
from another_module import target_function
def TestOneInput(data: bytes):
target_function(data)
atheris.Setup(sys.argv, TestOneInput)
atheris.Fuzz()export CC="clang"
export CFLAGS="-fsanitize=address,fuzzer-no-link"
export CXX="clang++"
export CXXFLAGS="-fsanitize=address,fuzzer-no-link"
export LDSHARED="clang -shared"CBOR2_BUILD_C_EXTENSION=1 uv add --no-binary-package cbor2 'cbor2==5.6.4'import sys
import atheris
# _cbor2 ensures the C library is imported
from _cbor2 import loads
def TestOneInput(data: bytes):
try:
loads(data)
except Exception:
# We're searching for memory corruption, not Python exceptions
pass
def main():
atheris.Setup(sys.argv, TestOneInput)
atheris.Fuzz()
if __name__ == "__main__":
main()uv run python cbor2-fuzz.pymkdir corpus
# Add seed inputs
echo "test data" > corpus/seed1
echo '{"key": "value"}' > corpus/seed2uv run python fuzz.py corpus/uv run python fuzz.py corpus/uv run python fuzz.py -merge=1 new_corpus/ old_corpus/# Run for 10 minutes
uv run python fuzz.py -max_total_time=600
# Limit input size
uv run python fuzz.py -max_len=1024
# Run with multiple workers
uv run python fuzz.py -workers=4 -jobs=4export CFLAGS="-fsanitize=address,fuzzer-no-link"
export CXXFLAGS="-fsanitize=address,fuzzer-no-link"export ASAN_OPTIONS="allocator_may_return_null=1,detect_leaks=0"export LD_PRELOAD="$(python -c 'import atheris; import os; print(os.path.join(os.path.dirname(atheris.__file__), "asan_with_fuzzer.so"))')"import atheris
# Instrument only specific modules
with atheris.instrument_imports():
import target_module
# Don't instrument test harness code
def TestOneInput(data: bytes):
target_module.parse(data)export CFLAGS="-fsanitize=address,undefined,fuzzer-no-link"
export CXXFLAGS="-fsanitize=address,undefined,fuzzer-no-link"