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🐍 Experimentations in trying to find 0-days in numpy
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Fuzzing Numpy

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🐍 Experimentations in trying to find 0-days in numpy

Inspired by Murmus CTF's amazing live streams on fuzzing numpy, I decided to write my own custom fuzzer as well, and try to fuzz numpy.

You can find my notes on the basics of fuzzing and genetic fuzzing on my security-notes repo. These notes were inspired by live streams by Gynvael Coldwind (links #1 #2 #3).

This repository contains my explorations and experimentations in trying to fuzz numpy. Over time, I might expand into fuzzing other parts of CPython itself (or other libraries for Python).


Run ./ to download and compile CPython and Numpy with ASAN (address sanitizer).

Then, use ./ to start the fuzzer, and watch the crashing inputs get dropped into the crashes directory.

Quick Start

It is possible to start off quickly by spinning up a new virtual machine, and running the fuzzer inside it (it also would prevent any unintended side-effects that might occur due to fuzzing).

This has been implemented as a Vagrantfile in this directory itself, which runs an Ubuntu-14.04 virtual machine with all required configuration etc, and auto-starts the fuzzing process into the background whenever it boots. It can be booted simply by running vagrant up inside this repository. Following this, running vagrant ssh will let you access the box, where the running process can be seen with screen -r (and disconnected without killing by pressing Ctrl+a, d). To stop the fuzzer and the virtual machine, merely run vagrant halt.

Note: Starting up the vagrant box also creates a crashes/ directory in the repository directory, which is symlinked inside the virtual machine, so that the crashes can be obtained outside the VM wiht ease.

Structure of the Fuzzer

The fuzzer consists of multiple parts working in unison to lead to effective finding of bugs.

  1. We have the ASAN compilation for both CPython and numpy (see ./ This allows us to catch even subtle errors (that might take longer to manifest as a crash), such as heap corruption etc.

  2. We have a harness that handles crashing applications (see harness/harness.c). Specifically, it is compiled as a shared object (.so file) which is loaded into python using ctypes and it sets up signal handlers for SIGABRT (signifying an error caught by ASAN) and SIGSEGV (more serious error; missed by ASAN for some reason). It also uses an mmap'd region where testcases can be registered before running, thereby allowing the crashed process to be able to cleanly store the crashing input.

  3. We have the the main fuzzer itself that handles repeatedly trying newly generated testcases and puts all the different parts of the fuzzer together (see fuzzer/

  4. We have the generator, whose job is to keep track of the corpus, generate cases to test out, keep track of valid cases to increase corpus, etc. (see fuzzer/

  5. We have the wrapper utility (see ./ which repeatedly calls the fuzzer with the right configuration for ASAN etc, in order to get a whole host of bugs rather than stopping after just one bug that otherwise be done by the main fuzzer.


Copyright 2017 Jay Bosamiya

All files and works in this repository are licensed under the Apache License, Version 2.0

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