The prevalent usage of runtime packers has compli- cated Android malware analysis, as both legitimate and malicious apps are leveraging packing mechanisms to protect themselves against reverse engineer. Although recent efforts have been made to analyze particular packing techniques, little has been done to study the unique characteristics of Android packers. In this paper, we report the first systematic study on mainstream Android packers, in an attempt to understand their security implications. For this purpose, we developed DROIDUNPACK, a whole-system emulation based Android unpacking analysis framework, which compared with existing tools, relies on intrinsic characteristics of Android runtime (rather than heuristics), and further enables virtual machine inspection to precisely recover hidden code and reveal packing behaviors. Running our tool on 6 major commercial packers, 93,910 packed Android malware samples and 3 existing state-of-the-art unpackers, we found that not only are commercial packing services abused to encrypt malicious or plagiarized contents, they themselves also introduce security- critical vulnerabilities to the apps being packed. Our study further reveals the prevalence and rapid evolution of custom packers used by malware authors, which cannot be defended against using existing techniques, due to their design weaknesses.