The answer is that the firewall is dropping packets because it is only allowing traffic that matches an established session, but the application’s control channel is not being tracked correctly. This happens because a stateful firewall relies on inspecting the control channel to dynamically open pinholes for data channels when applications use dynamic ports, such as in FTP’s active mode. If the firewall fails to parse the control channel negotiation—often due to missing application-layer inspection (ALI) or an outdated rule set—it cannot recognize the subsequent data channel packets as part of the same session, so it drops them even though the initial control session was allowed. On the ISC2 Certified in Cybersecurity CC exam, this concept tests your understanding of stateful inspection versus deep packet inspection, and a common trap is assuming that simply allowing the initial connection is enough for all subsequent traffic. Remember the mnemonic: “Control the channel, or the data will be canned.”
ISC2 CC Network Security Practice Question
This CC practice question tests your understanding of network security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A company uses a stateful firewall. A user reports that an application requiring multiple dynamic ports is not working. The firewall logs show that packets from the server are being dropped. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The firewall is only allowing packets that match an established session, but the application's control channel is not being tracked correctly.
A stateful firewall tracks sessions by monitoring the control channel. For applications like FTP that use dynamic ports, the firewall must inspect the control channel to dynamically open pinholes for data channels. If the control channel is not tracked correctly, the firewall will not recognize the data channel packets as part of an established session and will drop them, even if the initial control session was allowed.
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 firewall is only allowing packets that match an established session, but the application's control channel is not being tracked correctly.
Why this is correct
The firewall may not associate the data channel with the control session if it does not inspect the application protocol.
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 firewall does not have a rule allowing inbound traffic for the application.
Why it's wrong here
If user initiated outbound, stateful firewall should allow return traffic automatically.
✗
The firewall is not inspecting application-layer data.
Why it's wrong here
Lack of app-layer inspection can cause issues but stateful firewall may still track by address/port.
✗
The firewall's state table is full and cannot track new sessions.
Why it's wrong here
Would affect many sessions, not just this application.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the misconception that a stateful firewall automatically handles all dynamic port protocols, when in fact it requires proper ALG configuration to inspect control channel payloads for port negotiation.
Detailed technical explanation
How to think about this question
Stateful firewalls maintain a state table that maps session tuples (source IP, source port, destination IP, destination port, protocol). For protocols like FTP active mode, the client sends a PORT command on the control channel (TCP/21) to specify a dynamic data port; the firewall must parse this command to create a temporary state entry for the reverse connection. If the firewall's application-layer gateway (ALG) for FTP is misconfigured or not enabled, the control channel is tracked but the dynamic port negotiation is missed, causing the firewall to drop the data channel packets as they do not match any existing session.
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.
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall is only allowing packets that match an established session, but the application's control channel is not being tracked correctly. — A stateful firewall tracks sessions by monitoring the control channel. For applications like FTP that use dynamic ports, the firewall must inspect the control channel to dynamically open pinholes for data channels. If the control channel is not tracked correctly, the firewall will not recognize the data channel packets as part of an established session and will drop them, even if the initial control session was allowed.
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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