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Fuzz test Python modules with libFuzzer.

Project description



Use libFuzzer to fuzz test Python 3.6+ C extension modules.


clang 8 or later is required.

$ apt install clang
$ pip install pyfuzzer

Example Usage

Hello world

Use the default mutator pyfuzzer.mutators.random when testing the module hello_world.

$ cd examples/hello_world
$ pyfuzzer run -l max_total_time=1 hello_world hello_world.c
<lots of libFuzzer output>

Print the function calls that found new code paths. This information is usually useful when writing unit tests.

$ pyfuzzer print_corpus
tell(b'') = 5
tell(b'@') = True
tell(None) raises:
Traceback (most recent call last):
  File "/home/erik/workspace/pyfuzzer/pyfuzzer/mutators/", line 18, in print_function
    res = func(*args)
TypeError: expected bytes, NoneType found
tell(b'@\x01\x00') = 0
tell(b'#@') = b'Hello!'

See the hello_world for all files and full output.

Hello world fatal error

Similar to the previous example, but triggers a fatal error when tell() is called with a bytes object longer than 2 bytes as its first argument.

$ cd examples/hello_world_fatal_error
$ pyfuzzer run hello_world hello_world.c
Fatal Python error: deallocating None

Current thread 0x00007f7ca99c2780 (most recent call first):

Print the function call that caused the crash. Just as expected, the first argument is clearly longer than 2 bytes.

$ pyfuzzer print_crashes
tell(b'..............................................') = None

See the hello_world_fatal_error for all files and full output.

Custom mutator

Use the custom mutator hello_world_mutator when testing the module hello_world.

Testing with a custom mutator is often more efficient than using a generic one.

$ cd examples/hello_world_custom_mutator
$ pyfuzzer run -l max_total_time=1 -m hello_world hello_world.c

See the hello_world_custom_mutator for all files and full output.


A mutator uses data from libFuzzer to test a module. A mutator must implement the functions test_one_input(module, data) and test_one_input_print(module, data), where module is the module under test and data is the data generated by libFuzzer (as a bytes object).

test_one_input(module, data) performs the actual fuzz testing, while test_one_input_print(module, data) prints corpus and crashes.

A minimal mutator fuzz testing a CRC-32 algorithm could look like below. It simply calls crc_32() with data as its only argument.

import traceback
from .pyfuzzer.mutators.utils import print_function

def test_one_input(module, data):

def test_one_input_print(module, data):
     print_function(module.crc_32, [data])


  • Use type annotations for less type errors in generic mutator.
  • Add support to fuzz test pure Python modules by generating C code using Cython.
  • Call random methods on classes.

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