Which of the following methods denies all services unless explicitly permitted?

The method that denies all services unless explicitly permitted is known as the Deny by Default approach. This security model is fundamentally designed to establish a secure baseline by ensuring that, initially, all access is blocked. In this way, only explicitly authorized services or communications are allowed, reducing the surface area for potential attacks or unauthorized access.

By denying everything by default, organizations can closely control what is permitted, ensuring that only trusted users and systems have access. This proactive stance serves to minimize vulnerabilities that could be exploited by malicious actors, as it requires an explicit allowance for any service or traffic to be granted access.

In contrast, other methods such as Allow by Default or Open Policy can lead to openings within the network where unintended access might occur, as they permit access unless specifically denied. Similarly, Restricted Access refers to limiting access to certain users or systems but does not inherently deny all by default. Thus, while these approaches have their applications, they do not provide the same level of baseline security as the Deny by Default strategy does.