This book constitutes the proceedings of the 6th International Conference on Principles of Security and Trust, POST 2017, which took place in Uppsala, Sweden in April 2017, held as Part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2017. The 14 papers presented in this volume were carefully reviewed and selected from 40 submissions. They were organized in topical sections named: information flow; security protocols; security policies; and information leakage. .