(ISC)2 Certified in Cybersecurity Practice Exam

What firewall rule receives traffic not explicitly allowed by a firewall rule?

1. Implicit Deny

2. Explicit Deny

3. Strict Deny

4. Closed Deny

The correct answer is: Implicit Deny

The concept of an "Implicit Deny" is fundamental in firewall configurations and network security as a whole. This rule dictates that any traffic that is not explicitly permitted by other rules is automatically denied. It serves as a safety net, ensuring that only specified, trusted data can enter or leave a network, thereby minimizing potential vulnerabilities. The reasoning behind the implicit deny principle is rooted in the idea of maintaining a secure environment by default. When a firewall is set up, unless there are specific rules that allow particular traffic types, all other traffic will be blocked. This approach effectively defends against unauthorized access and helps prevent various types of attacks by ensuring that unapproved traffic does not have the opportunity to reach sensitive systems. In contrast, the other options suggest different types of denial rules, such as explicit or strict denials; however, these do not represent the default behavior of a firewall. Explicit deny rules, for instance, would require an administrator to specifically define what traffic should be denied, which does not account for the default behavior of blocking all unapproved traffic. Thus, the concept of "Implicit Deny" is crucial for establishing a proactive security posture in firewall management.