In blockchains, mempool controls transaction flow before consensus, denial of whose service hurts the health and security of blockchain networks. This paper presents MPFUZZ, the first mempool fuzzer to find asymmetric DoS bugs by symbolically exploring mempool state space and optimistically estimating the promisingness an intermediate state is in reaching bug oracles. Compared to the baseline blockchain fuzzers, MPFUZZ achieves a > 100x speedup in finding known DETER exploits. Running MPFUZZ on six major Ethereum clients leads to the discovering of new mempool vulnerabilities, which exhibit a wide variety of sophisticated patterns including stealthy mempool eviction and mempool locking. Rule-based mitigation schemes are proposed against newly discovered vulnerabilities.
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