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Technical Perspective: What’s All the Fuss about Fuzzing?

"Boosting Fuzzer Efficiency: An Information Theoretic Perspective," by Marcel Böhme, Valentin J.M. Manès, and Sang Kil Cha, presents a novel twist to fuzzing that is shown to increase the central metric of the number of bugs found.

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Testing programs automatically is usually done using one of three possible approaches: In the simplest case, we throw random inputs at the program and see what happens. Search-based approaches tend to observe what happens inside the program and use this information to influence the choice of successive inputs. Symbolic approaches try to reason which specific inputs are needed to exercise certain program paths.

After decades of research on each of these approaches, fuzzing has emerged as an effective and successful alternative. Fuzzing consists of feeding random, often invalid, test data to programs in the hope of revealing program crashes, and is usually conducted at scale, with fuzzing campaigns exercising individual programs often for hours. A common classification of fuzzing approaches is between black-box fuzzers that assume no information about the system under test; grey-box fuzzers that inform the generation of new inputs by considering information about past executions such as code coverage; and white-box fuzzers that use symbolic reasoning to generate inputs for specific program paths. At face value, these three approaches to fuzzing appear to be identical to the three established approaches to test generation listed above. So, what’s all the fuss about fuzzing?

The fuss about fuzzing becomes clear when picking up any recent fuzzing paper; inevitably, the paper will boast numbers showing how many bugs were found in how many different software systems. This is quite different to more classical test-generation papers, and it demonstrates a different mindset. Competing on such numbers can only be achieved by building robust and flexible tools that work on real systems, and indeed, fuzzers must work well not only on individual systems, but on as many different systems as possible. This requires replacing problematic and inhibiting assumptions, for example, that a test oracle will magically appear, with more pragmatic solutions, such as considering only program crashes as bugs. Clearly, fuzzing has emerged from a practical need and is driven by applications and practitioners.

Traditional test-generation papers appear to put much less focus on building great tools and collecting bugs. That is not to say there are not great test-generation prototypes and tools, but there is a stronger focus on theory and progress quantified using proxy measurements such as code coverage. For example, search-based test-generation approaches build upon evolutionary algorithms, for which we have decades of research, studying and proving various properties of these algorithms on representative and understandable search problems, thus providing a clearer understanding of the complex processes involved while generating tests using search algorithms. Less of this appears to be available for fuzzing, where success is measured in terms of the number of bugs found, and the resulting competition around this. What has counted in fuzzing papers to date are results more than theory explaining how these are achieved.

The following paper presents a novel twist to fuzzing that is shown to increase the central metric of the number of bugs found. A grey-box fuzzer tends to have a set of seed inputs, which are mutated to generate new inputs. Whenever a mutated input covers new aspects of the code, it is added to the seeds. Usually, fuzzers are implemented to prefer seeds added later during the fuzzing process, because intuitively there may be more previously undiscovered program behavior reachable by mutating seeds that have been explored less. This paper introduces an alternative, where the probability of individual seeds being selected for mutation does not depend on the time they were discovered, but rather on how much new program behavior has been discovered so far by mutating them. The more likely a seed is to lead to new program behavior, the more likely that seed is selected for mutation. The more likely new test inputs discover new program behavior, the more likely the fuzzer reveals new bugs.

How to calculate the probabilities, that is, the “power schedule” for different seeds, is a tricky question, which this paper answers using information theory—the authors define entropy as the average information a generated test input reveals about the resulting coverage of program code. A low entropy for a seed input means mutations of that seed tend to produce similar coverage behavior, while a high entropy signals that mutating the seed input leads to new coverage behavior. Thus, entropy provides not only a means to calculate power schedules, but a more general efficiency measurement for fuzzers. In the tradition of fuzzing research, this is all implemented into a robust fuzzer tool, providing the expected impressive numbers on systems tested and bugs found.

The paper is exciting in that it not only provides an effective algorithmic update to power schedules in grey-box fuzzers, but also a deeper intuitive understanding as to why this works, how it influences behavior during fuzzer campaigns, and a general framework on which to build for future work on fuzzing. Together, these contributions enhance our understanding of fuzzing and will help build more effective test generators, and to answer related questions such as how effective fuzzers are, or when to stop fuzzing campaigns. What makes this paper stand out is that it contributes to linking the viewpoints of traditional test generation and the trendy and seemingly more pragmatic fuzzing approach. Progress on automated testing requires both theory as well as robust tools.

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