This book constitutes the refereed proceedings of the 18th International Conference on Network and System Security, NSS 2024, held in Abu Dhabi, United Arab Emirates, during November 20-22, 2024. The 21 full papers presented in this book were carefully reviewed and selected from 62 submissions. They are grouped into these topical sections: authentication and security; privacy and encryption; malware detection and prevention; system security and prevention; network and infrastructure security; blockchain and smart contracts; and data security.