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,...