Software security is of utmost importance for most software systems. Developers must systematically select, plan, design, implement, and especially maintain and evolve security features - functionalities to mitigate attacks or protect personal data such as cryptography or access control, to ensure the security of their software. While security features are usually available in libraries, additional code needs to be written and maintained to integrate security features and not all desired features can be reused this way. While there have been studies on the use of such libraries, surprisingly little is known about how developers engineer security features, how they select what security features to implement, and the implications on maintenance.

We therefore currently rely on assumptions that are largely based on common sense or individual examples. However, researchers require hard empirical data to understand what practitioners need and how they view security, which we currently lack to provide them with effective solutions.

We contribute an exploratory study with 26 knowledgeable industrial participants. We study how security features of software systems are selected and engineered in practice, what their code-level characteristics are, and the challenges practitioners face. Based on the empirical data gathered, we validate four common assumptions and gain insights into engineering practices.