This book constitutes the proceedings of the 12th International Conference on Network and System Security, NSS 2018, held in Hong Kong, China, in August 2018. The 26 revised full papers and 9 short papers presented in this book were carefully reviewed and selected from 88 initial submissions. The papers cover a wide range of topics in the field, including blockchain, mobile security, applied cryptography, authentication, biometrics, IoT, privacy, and education.