This book constitutes the proceedings of the 11th International Conference on Network and System Security, NSS 2017, held in Helsinki, Finland, in August 2017. The 24 revised full papers presented in this book were carefully reviewed and selected from 83 initial submissions. The papers are organized in topical sections on Cloud and IoT Security; Network Security; Platform and Hardware Security; Crypto and Others; and Authentication and Key Management. This volume also contains 35 contributions of the following workshops: Security Measurements of Cyber Networks (SMCN-2017); Security in Big Data (SECBD-2017); 5G Security and Machine Learning (IW5GS-2017); of the Internet of Everything (SECIOE-2017).