The mobile network (e.g., 4G LTE and 5G NR), the only large-scale wireless network infrastructure on par with the Internet, plays a critical role in interconnecting various mobile devices (e.g., smartphones, massive/critical IoT devices) and providing them with ubiquitous network services. In recent years, more users are accessing the Internet through mobile networks; since the first quarter of 2021, mobile devices (excluding tablets) have generated more than 54% of global website traffic. However, the security of the nowadays mobile networked systems is still far from being satisfactory. Unprecedented malicious attacks against mobile devices and the mobile network infrastructure cannot be effectively defended by the current complicated and error-prone design and pose real threats to a large number of users. In this talk, I would like to share with you my research experience in identifying various security vulnerabilities in essential mobile network services using formal and/or empirical approaches and securing billions of mobile users and the infrastructure.