This book constitutes the revised selected papers of the 11th International Symposium on Foundations and Practice of Security, FPS 2018, held in Montreal, QC, Canada, in March 2018. The 16 full papers, 1 short paper, 1 position paper and 2 invited papers presented in this book, were carefully reviewed and selected from 51 submissions. They cover a range of topics including mobile security; cloud security and big data; IoT security; software security, malware analysis, and vulnerability detection; cryptography; cyber physical security and hardware security; and access control.