Made of Bugs

Property-Based Testing Is Fuzzing

“Property-based testing” refers to the idea of writing statements that should be true of your code (“properties”), and then using automated tooling to generate test inputs (typically, randomly-generated inputs of an appropriate type), and observe whether the properties hold for that input. If an input violates a property, you’ve demonstrated a bug, as well as a convenient example that demonstrates it.

A classic example of property-based testing is testing a sort function:

@given(st.lists(st.integers()))
def test_sort(s):
  out = list(sorted(s))
  assert set(out) == set(s)
  assert all(x<=y for x,y in zip(out, out[1:]))

This test asserts that, given a list of integers, sorting the list - preserves the set of elements - produces a sorted output

The test framework will then automatically execute it over some set of input lists, and report if any counter-examples are found.

“Fuzzing” is a much older practice, and generally refers to passing randomly-generated data of some variety (often a purely-random bytestream, but potentially chosen in some intelligent way) to a program in the hopes of finding an input that causes a crash (and thus, also, demonstrating a bug).

In recent years, pioneered largely by AFL, the practice of coverage-guided fuzzing has used a form of code instrumentation/coverage to explore inputs more likely to exercise interesting behavior; This technique has proven to be incredibly effective for a large variety of fuzz targets.

Historically, fuzzing and property-based testing have been regarded as fairly separate practices. Property-based testing originated primarily with Haskell’s QuickCheck, and so tends to be associated with richly-typed languages, formal specifications, and related fields. Fuzzing, on the other hand, was usually practiced against C or C++ binaries, typically with a security bent – aiming at finding exploitable memory corruption bugs.

In this post, however, I want to present an argument that fuzzing and property-based testing are essentially the same practice, at least at a certain level of abstraction. I’m hopeful that the recognition of this similarity can help practitioners of each improve their tools and workflows.

Property-Based Testing is Fuzzing

If we back up one level of abstraction or so, property-based testing and fuzzing appear very similar. In both cases, we have:

  • A system under test

The traditional granularity of a property-based test is a function, and for a fuzzer is a binary1, but both are just different implementation of “some arbitary computation”

  • A property we want to ensure

Traditionally, property-based testing has us write a property as explicit code, while fuzzers only test for the property “does not crash”. However, by the simple expedient of assert(property()) we can turn any property into an assertion about not-crashing, and people have used this technique to find surprisingly subtle behavioral bugs.

  • A strategy for finding inputs that might violate the property

Quickcheck, and many derivative property-based test suites, have used type-driven generation, while fuzzers have used random bytestreams, hand-coded generators, or random mutations of known-good inputs. However, ultimately all of these approaches are just strategies to automatically generate inputs that are hoped to trigger violations of the asserted properties of the system under test.

There are numerous differences in how they’re used in practice and in the tooling. But it seems clear to me that there’s also a deep similarity, and they are less fundamentally-different practices than they may appear.

Why should we care?

Both fuzzing and property-based testing have a rich history of development, varied ecosystems of tools, and communities of users and fans. However, in my experience, they relatively rarely overlap, and there’s not – by and large – an enormous amount of cross-pollination between the ecosystems. I think that’s a mistake, and the tools should be converging more than they have been to date. In a future post, I hope to go into a bit more detail about some of the specific techniques I’d love to see making their way into property-based testing tools.

Postscript: Hypothesis

Hypothesis is an open-source property-based testing tool, primarily implemented for Python. It is, in my opinion, worlds ahead of any other tool I’m aware of in many ways. Relevantly for this post, however, is that its author is way ahead of me on recognizing the fundamental similarily between fuzzing and property-based testing, and has written about it, as well as adopted many ideas from the fuzzing world into the tool.

If you maintain a Python codebase, you should probably be using Hypothesis. If you don’t, you should probably understand Hypothesis, so you can crib its best ideas for yourself.


  1. Although more recent tools, such as libfuzzer, have also supported writing fuzz targets with the granularity of “a function” [return]