Many assisting exploration strategies have been proposed to assist grey-box fuzzers in exploring program states guarded by tight and complex branch conditions such as equality constraints. Although they have shown promising results in their original papers, their evaluations seldom follow equivalent protocols, e.g., they are rarely evaluated on identical benchmarks. Moreover, there is a lack of sufficient investigations on the specifics of the program states explored by these strategies which can obfuscate the future application and development of such strategies. Consequently, there is a pressing need for a comprehensive study of assisting exploration strategies on their effectiveness, versatility, and limitations to enlighten their future development. To this end, we perform the first comprehensive study about the assisting exploration strategies for grey-box fuzzers. Specifically, we first collect nine recent fuzzers representing the mainstream assisting exploration strategies as our studied subjects and 21 real-world projects to form our benchmark suite. After evaluating the subjects on the benchmark suite, we then surprisingly find that the dictionary strategy is most promising since it not only achieves similar or even slightly better performance over the other studied assisting exploration strategies in terms of exploring program states but also is more practical to be enhanced. Accordingly, we propose CDFUZZ, which generates a customized dictionary for each seed upon the baseline fuzzer AFL to improve over the original dictionary strategy. The evaluation results demonstrate that CDFUZZ increases the edge coverage by 16.1% on average for all benchmark projects over the best performer in our study (i.e., AFL++ with the dictionary strategy). CDFUZZ also successfully exposed 37 previously unknown bugs, with nine confirmed and seven fixed by the corresponding developers.

For coverage-guided fuzzers, many of their adopted seeds are usually underused by exploring limited program states since essentially all their executions have to abide by rigorous program dependencies while only limited seeds are capable of accessing dependencies. Moreover, even when iteratively executing such limited seeds, the fuzzers have to repeatedly access the covered program states before uncovering new states. Such facts indicate that exploration power on program states of seeds has not been sufficiently leveraged by the existing coverage-guided fuzzing strategies. To tackle these issues, we propose a coverage-guided fuzzer, namely MirageFuzz, to mitigate the program dependencies when executing seeds for enhancing their exploration power on program states. Specifically, MirageFuzz first creates a “phantom” program of the target program by reducing its program dependencies corresponding to conditional statements while retaining their original semantics. Accordingly, MirageFuzz performs dual fuzzing, i.e., the source fuzzing to fuzz the original program and the phantom fuzzing to fuzz the phantom program simultaneously. Then, MirageFuzz applies the taint-based mutation mechanism to generate a new seed by updating the target conditional statement of a given seed from the source fuzzing with the corresponding condition value derived by the phantom fuzzing. To evaluate the effectiveness of MirageFuzz, we build a benchmark suite with 18 projects commonly adopted by recent fuzzing papers, and select seven open-source fuzzers as baselines for performance comparison with MirageFuzz. The experiment results suggest that MirageFuzz outperforms our baseline fuzzers from 13.42% to 77.96% averagely. Furthermore, MirageFuzz exposes 29 previously unknown bugs where 4 of them have been confirmed and 3 have been fixed by the corresponding developers.

Coverage-guided fuzzing has become mainstream in fuzzing to automatically expose program vulnerabilities. Recently, a group of fuzzers are proposed to adopt a random search mechanism namely Havoc, explicitly or implicitly, to augment their edge exploration. However, they only tend to adopt the default setup of Havoc as an implementation option while none of them attempts to explore its power under diverse setups or inspect its rationale for potential improvement. In this paper, to address such issues, we conduct the first empirical study on Havoc to enhance the understanding of its characteristics. Specifically, we first find that applying the default setup of Havoc to fuzzers can significantly improve their edge coverage performance. Interestingly, we further observe that even simply executing Havoc itself without appending it to any fuzzer can lead to strong edge coverage performance and outperform most of our studied fuzzers. Moreover, we also extend the execution time of Havoc and find that most fuzzers can not only achieve significantly higher edge coverage, but also tend to perform similarly (i.e., their performance gaps get largely bridged). Inspired by the findings, we further propose HavocMAB, which models the Havoc mutation strategy as a multi-armed bandit problem to be solved by dynamically adjusting the mutation strategy. The evaluation result presents that HavocMAB can significantly increase the edge coverage by 11.1% on average for all the benchmark projects compared with Havoc and even slightly outperform state-of-the-art QSYM which augments its computing resource by adopting three parallel threads. We further execute HavocMAB with three parallel threads and result in 9% higher average edge coverage over QSYM upon all the benchmark projects.

Fuzzing nowadays has been commonly modeled as an optimization problem, e.g., maximizing code coverage under a given time budget via typical search-based solutions such as evolutionary algorithms. However, such solutions are widely argued to cause inefficient computing resource usage, i.e., inefficient mutations. To address this issue, two neural program-smoothing-based fuzzers, Neuzz and MTFuzz, have been recently proposed to approximate program branching behaviors via neural network models, which input byte sequences of a seed and output vectors representing program branching behaviors. Moreover, assuming that mutating the bytes with larger gradients can better explore branching behaviors, they develop strategies to mutate such bytes for generating new seeds as test cases. Meanwhile, although they have been shown to be effective in the original papers, they were only evaluated upon a limited dataset. In addition, it is still unclear how their key technical components and whether other factors can impact fuzzing performance. To further investigate neural program-smoothing-based fuzzing, we first construct a large-scale benchmark suite with a total of 28 popular open-source projects. Then, we extensively evaluate Neuzz and MTFuzz on such benchmarks. The evaluation results suggest that their edge coverage performance can be unstable. Moreover, neither neural network models nor mutation strategies can be consistently effective, and the power of their gradient-guidance mechanisms have been compromised. Inspired by such findings, we propose a simplistic technique, PreFuzz, which improves neural program-smoothing-based fuzzers with a resource-efficient edge selection mechanism to enhance their gradient guidance and a probabilistic byte selection mechanism to further boost mutation effectiveness. Our evaluation results indicate that PreFuzz can significantly increase the edge coverage of Neuzz/MTFuzz, and also reveal multiple practical guidelines to advance future research on neural program-smoothing-based fuzzing.