The answer is that no firewall rule explicitly permits traffic from 10.0.0.25 to 203.0.113.50 on port 443. This is correct because firewalls operate on a default-deny model, meaning any packet that does not match an explicit permit rule in the access control list is automatically dropped; the log entry confirms a denied HTTPS session, and without a rule allowing that specific source, destination, and port combination, the firewall has no choice but to block it. On the CISSP exam, this tests your understanding of implicit deny and stateful inspection fundamentals—a common trap is assuming a deny log always indicates a malicious attack, when it often simply reflects missing policy. To remember this, think of the firewall as a bouncer with a guest list: if the source IP isn’t on the list for that specific door and port, entry is denied, no questions asked.
CISSP Security and Risk Management Practice Question
This CISSP practice question tests your understanding of security and risk management. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Based on the firewall log entry, what is the most likely cause of the denied traffic?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
No firewall rule explicitly permits traffic from 10.0.0.25 to 203.0.113.50 on port 443.
The firewall log entry shows a packet from source IP 10.0.0.25 to destination IP 203.0.113.50 on destination port 443 (HTTPS) being denied. The most likely cause is that no firewall rule explicitly permits this traffic. Firewalls operate on a default-deny or explicit-permit model; if no rule matches the source, destination, and port, the packet is dropped. This is a fundamental principle of access control lists (ACLs) and stateful inspection.
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.
✓
No firewall rule explicitly permits traffic from 10.0.0.25 to 203.0.113.50 on port 443.
Why this is correct
The reason 'No matching rule' indicates no permit rule exists for this flow.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The source IP is attempting a port scan on the destination.
Why it's wrong here
The log shows a single denied packet, not a scan pattern.
✗
The destination port should be 80 instead of 443.
Why it's wrong here
The log shows port 443, which is valid for HTTPS; no rule exists for this traffic.
✗
The destination server's SSL certificate has expired.
Why it's wrong here
Certificate issues are not logged by a stateful firewall; this is a network-layer deny.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse network-layer denial with application-layer issues (like SSL certificates) or misinterpret a single denied packet as evidence of a port scan, when the core concept is that firewalls enforce explicit permit rules and deny all other traffic by default.
Trap categories for this question
Command / output trap
The log shows a single denied packet, not a scan pattern.
Detailed technical explanation
How to think about this question
Firewalls evaluate packets against a sequential set of rules, typically using a first-match or best-match algorithm. If no rule explicitly permits the traffic (e.g., 'permit tcp host 10.0.0.25 host 203.0.113.50 eq 443'), the implicit deny rule at the end of the ACL drops the packet. In real-world scenarios, misconfigured ACLs or missing rules for new services are common causes of denied traffic, and administrators must verify rule order and content using tools like 'show access-lists' on Cisco devices or equivalent on other platforms.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Security and Risk Management — This question tests Security and Risk Management — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: No firewall rule explicitly permits traffic from 10.0.0.25 to 203.0.113.50 on port 443. — The firewall log entry shows a packet from source IP 10.0.0.25 to destination IP 203.0.113.50 on destination port 443 (HTTPS) being denied. The most likely cause is that no firewall rule explicitly permits this traffic. Firewalls operate on a default-deny or explicit-permit model; if no rule matches the source, destination, and port, the packet is dropped. This is a fundamental principle of access control lists (ACLs) and stateful inspection.
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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Question Discussion
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