Atheris (Python)

Per-engine reference for coverage-guided-fuzzing; the shared workflow and fuzzer-choice routing live in ../SKILL.md.

Overview

Atheris (per github.com/google/atheris) supports both pure-Python and native-extension targets (CPython C extensions).

For sanitiser pairing on native extensions, see sanitizer-integration.md; for corpus discipline see corpus-management.md.

When to use

• Fuzz testing a Python library (parser, serialiser, validator).
• Native CPython extensions where the C/C++ code is reachable from Python.
• Quick fuzz pass during development on Python projects already using pytest.

Authoring

Install

pip install atheris

Per the Atheris README, prebuilt wheels include libFuzzer for pure-Python fuzzing. Native-extension fuzzing may require building from source so the Clang and libFuzzer versions match.

Basic fuzz target

# fuzz_parser.py
import sys
import atheris

with atheris.instrument_imports():
    from my_library import parser

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

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

Per Atheris README:

• TestOneInput(data: bytes) is the fuzz callback - invoked with mutated input bytes each iteration.
• atheris.Setup(sys.argv, TestOneInput) initialises the fuzzer with libFuzzer flags from sys.argv.
• atheris.Fuzz() starts the fuzz loop (doesn't return until the campaign ends).

Coverage instrumentation

Atheris needs to instrument the modules under test:

with atheris.instrument_imports():
    from my_library import parser, decoder

The instrument_imports() context manager monkey-patches the import system so subsequent imports are coverage-instrumented. Module-level imports above this context manager are NOT instrumented - the fuzzer is blind to their code.

Alternative: per-function instrumentation:

import my_library
my_library.parser.parse = atheris.instrument_func(my_library.parser.parse)

Or instrument-all (heavyweight):

atheris.instrument_all()

FuzzedDataProvider

For structured input, use the Python equivalent of libFuzzer's helper:

def TestOneInput(data):
    fdp = atheris.FuzzedDataProvider(data)
    port = fdp.ConsumeInt(4)              # signed 4-byte int
    is_https = fdp.ConsumeBool()
    host = fdp.ConsumeUnicode(64)         # up to 64 chars
    body_size = fdp.ConsumeIntInRange(0, 1024)
    body = fdp.ConsumeBytes(body_size)
    parser.parse_request(host, port, is_https, body)

Per Atheris README, the provider exposes ConsumeInt, ConsumeUnicode, ConsumeFloat, ConsumeBool, PickValueInList, and related methods.

Running

Basic run

python fuzz_parser.py

Atheris by default runs indefinitely. Pass libFuzzer-style flags:

python fuzz_parser.py -max_total_time=300 corpus/

The trailing directory is the corpus (read + write). Subsequent directories are read-only seeds.

Common flags

Per Atheris README, all libFuzzer flags pass through:

Coverage report

python fuzz_parser.py -atheris_runs=100000 corpus/
# At end: prints coverage statistics

Reproducing a crash

python fuzz_parser.py crash-<sha1>
# Same crash with full traceback

Parsing results

Python tracebacks instead of sanitiser reports (unless instrumenting a CPython extension built with ASan):

[+] Loading binary contents from crash-abc123
=== Uncaught Python exception: ===
ValueError: invalid syntax
Traceback (most recent call last):
  File "fuzz_parser.py", line 12, in TestOneInput
    parser.parse(data)
  File "/path/my_library/parser.py", line 47, in parse
    return json.loads(text)
  ...

Map the traceback to a bug spec via the from-CI-failure workflow in bug-report-template (qa-bug-repro plugin).

CI integration

- uses: actions/setup-python@v6
  with: { python-version: '3.12' }
- run: pip install atheris
- name: Smoke fuzz (3 min)
  run: timeout 180 python fuzz_parser.py -max_total_time=180 corpus/ || true
- uses: actions/upload-artifact@v4
  with:
    name: atheris-crashes
    path: crash-*

Anti-patterns

Limitations

• GIL bottleneck. Python single-thread iteration; no multi-process fuzzing without -jobs (and libFuzzer's job flag works imperfectly with Python).
• Slower than libFuzzer / cargo-fuzz. Pure-Python iteration is ~1000-10000 execs/sec - orders of magnitude slower than C.
• Coverage instrumentation overhead. ~5x slowdown for instrumented modules.
• Native-extension fuzzing requires C/C++ toolchain. Build CPython + the extension with matching ASan / libFuzzer versions.
• No structured-input mutation beyond FuzzedDataProvider. For structured-aware mutation (typed records, custom grammars), pair with Hypothesis (the hypothesis-testing skill in the qa-property-based plugin) for property-based-style structured input.

References

• Atheris - github.com/google/atheris.
• LLVM libFuzzer (underlying) - llvm.org/docs/LibFuzzer.html.
• Composes: sanitizer-integration.md, corpus-management.md.
• Sibling fuzzers: libfuzzer.md, afl-plus-plus.md, cargo-fuzz.md, go-native-fuzzing.md, jazzer.md, OSS-Fuzz (google.github.io/oss-fuzz).
• Sibling-plugin overlap: hypothesis-testing - different methodology (hypothesis-driven vs coverage-guided).
• Umbrella: ../SKILL.md - fuzzer choice + shared workflow.