This book constitutes the revised selected papers of the 12th International Symposium on Foundations and Practice of Security, FPS 2019, held in Toulouse, France, in November 2019. The 19 full papers and 9 short papers presented in this book were carefully reviewed and selected from 50 submissions. They cover a range of topics such as machine learning approaches; attack prevention and trustworthiness; and access control models and cryptography.