Evaluating and Improving Hybrid Fuzzing.

To date, various hybrid fuzzers have been proposed for maximal program vulnerability exposure by integrating the power of fuzzing strategies and concolic executors. While the existing hybrid fuzzers have shown their superiority over conventional coverage-guided fuzzers, they seldom follow equivalent evaluation setups, e.g., benchmarks and seed corpora. Thus, there is a pressing need for a comprehensive study on the existing hybrid fuzzers to provide implications and guidance for future research in this area. To this end, in this paper, we conduct the first extensive study on state-of-the-art hybrid fuzzers. Surprisingly, our study shows that the performance of existing hybrid fuzzers may not well generalize to other experimental settings. Meanwhile, their performance advantages over conventional coverage-guided fuzzers are overall limited. In addition, instead of simply updating the fuzzing strategies or concolic executors, updating their coordination modes potentially poses crucial performance impact of hybrid fuzzers. Accordingly, we propose CoFuzz to improve the effectiveness of hybrid fuzzers by upgrading their coordination modes. Specifically, based on the baseline hybrid fuzzer QSYM, CoFuzz adopts edge-oriented scheduling to schedule edges for applying concolic execution via an online linear regression model with Stochastic Gradient Descent. It also adopts sampling-augmenting synchronization to derive seeds for applying fuzzing strategies via the interval path abstraction and John walk as well as incrementally updating the model. Our evaluation results indicate that CoFuzz can significantly increase the edge coverage (e.g., 16.31% higher than the best existing hybrid fuzzer in our study) and expose around 2X more unique crashes than all studied hybrid fuzzers. Moreover, CoFuzz successfully detects 37 previously unknown bugs where 30 are confirmed with 8 new CVEs and 20 are fixed.