Robust app clone detection based on similarity of UI structure

App clone is a serious threat to the mobile app ecosystem, which not only damages the benefits of original developers, but also contributes to spreading malware. App clone detection has received extensive attentions from our research community, and a number of approaches were proposed, which mainly rely on code or visual similarity of the apps. However, the tricky plagiarists in the wild may specifically modify the code or the content of User Interface (UI), which will lead to the ineffectiveness of current methods. In this paper, we propose a robust app clone detection method based on the similarity of UI structure. The key idea behind our approach is based on the finding that content features (e.g., background color) are more likely to be modified by plagiarists, while structure features (e.g., overall hierarchy structure, widget hierarchy structure) are relative stable, which could be used to detect different levels of clone attacks. Experiment results on a labeled benchmark of 4,720 similar app pairs show that our approach could achieve an accuracy of 99.6%. Compare with existing approaches, our approach works in practice with high effectiveness. We have implemented a prototype system and applied it to more than 404,650 app pairs, and we found 1,037 app clone pairs, most of them are piggybacking apps that introduced malicious payloads.