The correct answer is that the web server is using a port other than 80 or 443. This is because the ACL, applied inbound on the perimeter router, only permits traffic destined for the standard HTTP and HTTPS ports; when the web server listens on a non-standard port like 8080 or 8443, the packets fail to match any explicit permit statement and are dropped by the implicit deny any at the end of the ACL. On the CISSP exam, this scenario tests your understanding of how ACLs filter traffic based on port numbers and the common misconfiguration of assuming all web traffic uses default ports—a trap that catches administrators who overlook server-side port customization. A useful memory tip is "Ports 80 and 443 are the only web highways; if the server takes a side road, the ACL guard blocks it."
CISSP Security Operations Practice Question
This CISSP practice question tests your understanding of security operations. 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
Refer to the exhibit.
Firewall ACL:
access-list 100 permit tcp any any eq 80
access-list 100 permit tcp any any eq 443
access-list 100 permit udp any any eq 53
access-list 100 deny ip any any log
Refer to the exhibit. The ACL is applied inbound on a perimeter router. A security analyst notices that web traffic to an internal server is being blocked. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Refer to the exhibit.
Firewall ACL:
access-list 100 permit tcp any any eq 80
access-list 100 permit tcp any any eq 443
access-list 100 permit udp any any eq 53
access-list 100 deny ip any any log
A
The connection is stateful and not returning traffic
Why wrong: This is a standard ACL, not stateful.
B
The source address is not permitted
Why wrong: ACL permits any source.
C
The ACL is applied outbound incorrectly
Why wrong: Applied inbound, that's fine.
D
The web server is using a port other than 80 or 443
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The web server is using a port other than 80 or 443
Option D is correct because the ACL is applied inbound on the perimeter router, meaning it inspects traffic as it enters the interface. If the web server is using a non-standard port (e.g., 8080 or 8443) instead of the well-known ports 80 or 443, the ACL will not match the permit statement for HTTP/HTTPS traffic, causing the packets to be dropped by the implicit deny any at the end of the ACL. This is a common misconfiguration when administrators assume all web traffic uses default ports.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The connection is stateful and not returning traffic
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the misconception that all web traffic uses ports 80 or 443, leading candidates to overlook the possibility of non-standard port configurations, while also distracting with stateful vs. stateless ACL behavior.
Detailed technical explanation
How to think about this question
Standard ACLs (numbered 1-99 or 1300-1999) filter based solely on source IP address and cannot inspect Layer 4 port numbers; however, extended ACLs (100-199 or 2000-2699) can filter on source/destination IP and protocol/port. In this scenario, if an extended ACL is used but only permits TCP/80 and TCP/443, traffic to a web server on TCP/8080 will be denied by the implicit deny any. Real-world examples include internal web servers configured on port 8080 to avoid conflicts or for application-specific requirements, and administrators forgetting to update the ACL accordingly.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A security analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Security Operations — This question tests Security Operations — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The web server is using a port other than 80 or 443 — Option D is correct because the ACL is applied inbound on the perimeter router, meaning it inspects traffic as it enters the interface. If the web server is using a non-standard port (e.g., 8080 or 8443) instead of the well-known ports 80 or 443, the ACL will not match the permit statement for HTTP/HTTPS traffic, causing the packets to be dropped by the implicit deny any at the end of the ACL. This is a common misconfiguration when administrators assume all web traffic uses default ports.
What should I do if I get this CISSP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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