Android multitasking provides rich features to enhance user experience and offers great flexibility for app developers to promote app personalization. However, the security implication of Android multitasking remains under-investigated. With a systematic study of the complex tasks dynamics, we find design flaws of Android multitasking which make all recent versions of Android vulnerable to task hijacking attacks. We demonstrate proof-of-concept examples utilizing the task hijacking attack surface to implement UI spoofing, denial-of-service and user monitoring attacks. Attackers may steal login credentials, implement ransomware and spy on user’s activities. We have collected and analyzed over 6.8 million apps from various Android markets. Our analysis shows that the task hijacking risk is prevalent. Since many apps depend on the current multitasking design, defeating task hijacking is not easy. We have notified the Android team about these issues and we discuss possible mitigation techniques in this paper.