This book constitutes the thoroughly refereed post-conference proceedings of the IFIP WG 11.4 International Workshop on Open Problems in Network Security, iNetSec 2015, held in Zurich, Switzerland, in October 2015. iNetSec is the main workshop of the IFIP working group WG 11.4, its objective is to present and discuss open problems and new research directions on all aspects related to network security. The 9 revised full papers presented in this volume were carefully reviewed and selected from 13 submissions. They were organized in topical sections named: network security, intrusion detection, anonymous communication, and cryptography.