The Android system services usually play a critical role in running multiple important tasks, and delivering seamless user experiences, e.g., conveniently storing user data. In this paper, we conduct the first systematic security study on the data storing process in Android system services, and consequently discover a novel class of design flaws (named Straw), which can lead to serious DoS (Denial-of-Service) attacks, e.g., permanently crashing the whole victim Android device.Then we propose a novel directed fuzzing based approach, called StrawFuzzer, to automatically vet all system services against the straw vulnerabilities. StrawFuzzer balances the tradeoff between path exploration and vulnerability exploitation. By applying StrawFuzzer on three Android systems with the latest security updates, we identified 35 unique straw vulnerabilities affecting 474 interfaces across 77 system services and successfully generated corresponding exploits, which can be used to conduct various permanent/temporary DoS attacks. We have reported our findings with suggestions for repairing the vulnerabilities to corresponding vendors. Up to now, Google has rated our vulnerability as high severity.