The answer is that the firewall rule set is missing a default deny rule. This is a critical misconfiguration because without an explicit deny-all statement at the end of the rule base, any traffic that does not match a preceding permit rule may be implicitly allowed, depending on the firewall platform’s default behavior. Most security frameworks and best practices mandate a trailing deny rule to ensure that only explicitly authorized traffic can pass, effectively closing the door on unintended access. On the CompTIA SecurityX CAS-004 exam, this concept tests your understanding of firewall policy design and the principle of least privilege—a common trap is assuming that a missing deny rule simply means traffic is dropped, when in fact it can be implicitly permitted. A reliable memory tip is “deny at the end, or let the stranger in”—always picture a locked door with no final guard, where anything not on the list can still slip through.
CAS-004 Security Engineering Practice Question
This CAS-004 practice question tests your understanding of security engineering. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Firewall Rule Base:
1. permit tcp any any eq 80
2. permit tcp any any eq 443
3. permit udp any any eq 53
4. permit icmp any any
Refer to the exhibit. A security analyst is reviewing the firewall rule set for a corporate network. Which misconfiguration is present?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
No default deny rule present
The rule set lacks an explicit deny rule at the end. Without a default deny, traffic that does not match any rule may be implicitly permitted depending on the firewall platform. Most security best practices require a deny-all trailing rule to ensure only explicitly allowed traffic passes.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
No default deny rule present
Why this is correct
A deny rule should be the last rule to drop all other traffic; otherwise, it may be implicitly allowed.
Related concept
Standard ACLs match source addresses.
✗
Overly permissive source IP addresses
Why it's wrong here
The source is 'any' which is permissive, but for a web server that might be intentional; the missing deny is more critical.
✗
Unnecessary allowed ports
Why it's wrong here
Ports 80, 443, and 53 are common for web and DNS; they are likely necessary.
✗
Missing logging on permit rules
Why it's wrong here
Logging is important but not the primary misconfiguration; there is no indication logging is needed.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CAS-004 ACL questions on filtering logic and placement.
Security Engineering — This question tests Security Engineering — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: No default deny rule present — The rule set lacks an explicit deny rule at the end. Without a default deny, traffic that does not match any rule may be implicitly permitted depending on the firewall platform. Most security best practices require a deny-all trailing rule to ensure only explicitly allowed traffic passes.
What should I do if I get this CAS-004 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CAS-004 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Question Discussion
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