The answer is that the destination port is not allowed by the access-group. This is the most likely reason for the denial because a stateful firewall enforces security policies by checking each packet against an access control list (ACL) that specifies permitted source and destination IP addresses, protocols, and ports; when the destination port in the packet does not match an allowed entry in the ACL, the firewall drops the traffic and logs the denial as a policy violation. On the ISC2 Certified in Cybersecurity CC exam, this concept tests your understanding of how firewalls implement least-privilege access control, often appearing in scenario-based questions where a log entry shows a denied connection to a common service port like TCP 23 or 3389. A common trap is assuming the denial is due to an incorrect source IP or protocol mismatch, but the exam emphasizes that port-based restrictions are the primary filter for unauthorized services. Remember the mnemonic “Ports Protect Policy” to recall that destination port checks are the firewall’s first line of defense against unwanted traffic.
ISC2 CC Security Operations Practice Question
This CC 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.
Refer to the exhibit. A security analyst sees this log entry from a firewall. What is the most likely reason for this denial?
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
✓
The destination port is not allowed
The firewall log entry indicates a packet was denied due to a policy violation, and the most common reason for such a denial in a stateful firewall is that the destination port is not permitted by the configured access control list (ACL) or security policy. Firewalls evaluate traffic against rules that specify allowed source/destination IPs, ports, and protocols; if the destination port is not explicitly allowed, the packet is dropped. This is a standard security practice to restrict unauthorized services.
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 destination port is not allowed
Why this is correct
The access-group denies traffic to the destination port 33456.
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 blacklisted
Why it's wrong here
There is no indication the IP is blacklisted.
✗
The access-group is misconfigured
Why it's wrong here
The rule explicitly denies this traffic, so configuration is intentional.
✗
The packet is malformed
Why it's wrong here
No malformed packet indicators in the log.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the concept that a firewall's default implicit deny will drop traffic if no explicit permit exists for the destination port, leading candidates to incorrectly assume the source IP is blacklisted or the packet is malformed.
Detailed technical explanation
How to think about this question
Cisco firewalls (e.g., ASA, FTD) use a first-match approach in ACLs: traffic is compared sequentially against ACEs until a permit or deny is found, and if no match exists, an implicit deny all at the end drops the packet. The log entry's 'deny' action with a specific source/destination suggests an explicit deny ACE was hit, often due to a missing permit for the destination port (e.g., TCP/443 for HTTPS). In real-world scenarios, analysts frequently see this when a new application is deployed but the firewall rulebase hasn't been updated to allow the required port.
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 CC 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 destination port is not allowed — The firewall log entry indicates a packet was denied due to a policy violation, and the most common reason for such a denial in a stateful firewall is that the destination port is not permitted by the configured access control list (ACL) or security policy. Firewalls evaluate traffic against rules that specify allowed source/destination IPs, ports, and protocols; if the destination port is not explicitly allowed, the packet is dropped. This is a standard security practice to restrict unauthorized services.
What should I do if I get this CC 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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