AppSpear: Bytecode Decrypting and DEX Reassembling for Packed Android Malware

摘要

As the techniques for Android malware detection are progressing, malware also fights back through deploying advanced code encryption with the help of Android packers. An effective Android malware detection therefore must take the unpacking issue into consideration to prove the accuracy. Unfortunately, this issue is not easily addressed. Android packers often adopt multiple complex anti-analysis defenses and are evolving frequently. Current unpacking approaches are either based on manual efforts, which are slow and tedious, or based on coarse-grained memory dumping, which are susceptible to a variety of anti-monitoring defenses. This paper conducts a systematic study on existing Android malware which is packed. A thorough investigation on 37,688 Android malware samples is conducted to take statistics of how widespread are those samples protected by Android packers. The anti-analysis techniques of related commercial Android packers are also summarized. Then, we propose AppSpear, a generic and fine-grained system for automatically malware unpacking. Its core technique is a bytecode decrypting and Dalvik executable (DEX) reassembling method, which is able to recover any protected bytecode effectively without the knowledge of the packer. AppSpear directly instruments the Dalvik VM to collect the decrypted bytecode information from the Dalvik Data Struct (DDS), and performs the unpacking by conducting a refined reassembling process to create a new DEX file. The unpacked app is then available for being analyzed by common program analysis tools or malware detection systems. Our experimental evaluation shows that AppSpear could sanitize mainstream Android packers and help detect more malicious behaviors. To the best of our knowledge, AppSpear is the first automatic and generic unpacking system for current commercial Android packers.