  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.
