Book excerpt: This book constitutes the thoroughly refereed post-conference proceedings of the IFIP WG 11.4 International Workshop on Open Problems in Network Security, iNetSec 2011, held in Lucerne, Switzerland, in June 2011, co-located and under the auspices of IFIP SEC 2011, the 26th IFIP TC-11 International Information Security Conference. The 12 revised full papers were carefully reviewed and selected from 28 initial submissions; they are fully revised to incorporate reviewers' comments and discussions at the workshop. The volume is organized in topical sections on assisting users, malware detection, saving energy, policies, and problems in the cloud.