The answer is that the outbound connection was blocked by an access control list. This is the most likely reason because firewalls evaluate traffic against ACLs as the first line of defense, and the log shows a denied outbound connection from 10.0.0.5:54321 to 203.0.113.50:80 without any indication of a signature match or blacklist hit. On the CISSP exam, this tests your understanding of how firewall logs differentiate between ACL-based denials and deeper inspection failures, such as those from intrusion prevention systems. A common trap is assuming a denied connection must be due to a security threat, but in many cases, it is simply a policy rule blocking the traffic. Remember the mnemonic ACL = Always Check the Log first, as ACLs are the most frequent cause of denied outbound connections in firewall logs.
CISSP Security Operations Practice Question
This CISSP practice question tests your understanding of security operations. 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.
Exhibit
Refer to the exhibit.
Exhibit:
May 15 09:12:34 fw01 %ASA-4-106023: Deny tcp src inside:192.168.1.10/54321 dst outside:203.0.113.5/80 by access-group "outside-in" [0x0, 0x0]
Based on the firewall log entry, what is the most likely reason the connection was denied?
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 outbound connection was blocked by an access control list.
The firewall log entry shows an outbound connection attempt from source IP 10.0.0.5:54321 to destination IP 203.0.113.50:80 that was denied. Firewalls typically evaluate outbound traffic against access control lists (ACLs) before any deeper inspection. Since the log does not indicate any signature match or blacklist hit, the most straightforward reason is that an ACL rule explicitly blocked this outbound connection.
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 outbound connection was blocked by an access control list.
Why this is correct
The entry shows denial by access-group 'outside-in', which is applied to inbound on outside interface but blocks outbound traffic from inside.
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 connection was denied due to malicious signature detection.
Why it's wrong here
The log is a firewall deny, not a signature-based alert.
✗
The destination IP address is blacklisted.
Why it's wrong here
The log does not cite a blacklist, only an access-group.
✗
The source port 54321 is a prohibited port.
Why it's wrong here
No indication of port prohibition; ACLs typically use destination port.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a denied connection must be due to a security feature like blacklisting or signature detection, but the log lacks any such indicators, making a simple ACL block the most logical answer.
Detailed technical explanation
How to think about this question
Firewall ACLs are evaluated in sequential order, and the first matching rule (permit or deny) determines the action. In this case, the log entry likely corresponds to an implicit deny at the end of the ACL or an explicit deny rule for the source/destination pair. Outbound ACLs are commonly applied on the internal interface to control which internal hosts can initiate connections to external networks, and they operate at Layer 3/4 without inspecting application payloads.
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 outbound connection was blocked by an access control list. — The firewall log entry shows an outbound connection attempt from source IP 10.0.0.5:54321 to destination IP 203.0.113.50:80 that was denied. Firewalls typically evaluate outbound traffic against access control lists (ACLs) before any deeper inspection. Since the log does not indicate any signature match or blacklist hit, the most straightforward reason is that an ACL rule explicitly blocked this outbound connection.
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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