In the present era of ubiquitous digitalization, security is a concern for everyone. Despite enormous efforts, securing IT systems still remains an open challenge for community and industry. One of the main reasons is that the variety and complexity of IT systems keeps increasing, making it practically impossible for security experts to grasp the full system. A further problem is that security has become an interdisciplinary challenge. While interdisciplinary research does exist already, it is mostly restricted to collaborations between two individual disciplines and has been rather bottom-up by focusing on very specific problems. The idea of the Dagstuhl Seminar was to go one step back and to follow a comprehensive top-down approach instead. The goal was to identify the "biggest failures" in security and to get a comprehensive understanding on their overall impact on security. To this end, the Dagstuhl Seminar was roughly divided into two parts. First, experienced experts from different disciplines gave overview talks on the main problems of their field. Based on these, overlapping topics but also common research interests among the participants have been identified. ... mehrAfterwards, individual working groups have been formed to work on the identified questions.