Industries from very diverse domains are realising that security should not be treated in a reactive way (e.g., once the cyberattack has happened). This way, security-related requirements and risks need to be continuously managed, and the need of integrating technical measures should be continuously assessed. In some cases, some decisions led, intentionally or unintentionally, to debt related to security aspects. This security debt is thus incurred when limited approaches or solutions are applied to reach the expected security levels of the system in operation. Identifying and making explicit security debt items is a challenge for companies. In this work, we analyse the literature on security debt to provide initial insights on the topic. Concretely, we discuss its definition, identify its most salient characteristics, present approaches for integrating its management in the product life-cycle, and to present categories and examples of security debt items.