This two volume constitutes the refereed proceedings of the 18th International Symposium on Foundations and Practice of Security, FPS 2025, held in Brest, France during November 25–27, 2025. The 38 full and 8 short papers presented in this book were carefully reviewed and selected from 91 submissions. These papers were organized in the following topical sections:Part I: Security, Privacy, and Trust in Emerging Distributed Systems; Cyber Resilience and Risk Management in Enterprise Architectures; Formal Methods and Automated Analysis for Secure Software Systems; Machine Learning and Intelligent Systems for Attack Detection and Trust Evaluation; Applications to Industry and Critical Infrastructure.  Part II: Advances in Privacy-Preserving Cryptography and Secure Computation; Secure and Intelligent Network Architectures for Next-Generation Communications; Secure, Explainable, and Efficient Machine Learning in Cybersecurity; Short Papers.