A configurable firewall for computing systems is disclosed. The configurable firewall provides a firewall control block that can be used as a mechanism to implement and control access privileges between various components of the computing environment. As such, the firewall control block can be used to determine whether one component (e.g., applet) can access another component in the computing environment. This allows a flexible environment where firewall boundaries can be configured in such a way that each applet can allow access to a desired set of other applets. In addition, the control block can be implemented using a variety of techniques that may be suitable for different system requirements (e.g., processing speed, memory). As such, the configurable firewall is useful for implementing security for various computing systems, especially those that operate with relatively limited processing power and/or provide highly specialized functionality (e.g., smart cards).