This volume constitutes the refereed proceedings of the 6th International Conference on Frontiers in Cyber Security, FCS 2023, held in Chengdu, China, in August 2023. The 44 full papers included in this book were carefully reviewed and selected from 89 submissions. They were organized in topical sections as follows: Blockchain and Distributed Systems; Network Security and Privacy Protection; Cryptography and Encryption Techniques; Machine Learning and Security; and Internet of Things and System Security.