This book constitutes the refereed proceedings of the 5th International Symposium on Mobile Internet Security, MobiSec 2021, held in Jeju Island, Republic of Korea, in October 2021. The 28 revised full papers presented were carefully reviewed and selected from 66 submissions. The papers are organized in the topical sections: ​IoT and cyber security; blockchain security; digital forensic and malware analysis; 5G virtual Infrastructure, cryptography and network security.