The answer is that an external host is performing a port scan against internal systems. This is the most likely reason because the firewall logs show repeated denied TCP packets from a single external source, 192.0.2.10, to multiple internal hosts, all blocked by the access-group "OUTSIDE_IN". In port scan detection, a firewall denies unsolicited inbound packets that match a deny ACE, and the pattern of repeated denied packets from one external IP to various internal destinations is the classic signature of a reconnaissance scan, not a denial-of-service attack or data exfiltration. On the ISC2 Certified in Cybersecurity CC exam, this scenario tests your ability to interpret firewall logs and distinguish between malicious activities; a common trap is confusing a port scan with a DoS attack, but remember that a scan targets multiple ports or hosts with single probes, while a DoS floods a single target. Memory tip: think "one source, many destinations" equals a scan, not an attack.
ISC2 CC Security Operations Practice Question
This CC 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.
=== syslog output ===
Jan 15 09:23:45 firewall01 %ASA-4-106023: Deny tcp src outside:192.0.2.10/3456 dst inside:10.0.0.5/22 by access-group "OUTSIDE_IN" [0x0, 0x0]
Jan 15 09:23:46 firewall01 %ASA-4-106023: Deny tcp src outside:192.0.2.10/3457 dst inside:10.0.0.5/23 by access-group "OUTSIDE_IN" [0x0, 0x0]
Jan 15 09:23:47 firewall01 %ASA-4-106023: Deny tcp src outside:192.0.2.10/3458 dst inside:10.0.0.5/80 by access-group "OUTSIDE_IN" [0x0, 0x0]
Jan 15 09:23:48 firewall01 %ASA-4-106023: Deny tcp src outside:192.0.2.10/3459 dst inside:10.0.0.6/22 by access-group "OUTSIDE_IN" [0x0, 0x0]
Jan 15 09:23:49 firewall01 %ASA-4-106023: Deny tcp src outside:192.0.2.10/3460 dst inside:10.0.0.6/23 by access-group "OUTSIDE_IN" [0x0, 0x0]
Refer to the exhibit. A security analyst is reviewing firewall logs and notices repeated denied TCP packets from 192.0.2.10 to internal hosts. The packets are being denied by the access-group "OUTSIDE_IN". What is the most likely reason for these denials?
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
✓
An external host is performing a port scan against internal systems.
The repeated denied TCP packets from 192.0.2.10 (an external IP) to multiple internal hosts indicate a port scan. The access-group 'OUTSIDE_IN' is applied to the outside interface, and the firewall is denying these packets because they match a deny ACE (access control entry) that blocks unsolicited inbound traffic. This pattern of multiple denied connections from a single external source to different internal destinations is characteristic of a reconnaissance scan, not a DoS attack or data exfiltration.
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.
✓
An external host is performing a port scan against internal systems.
Why this is correct
The sequential source ports and different destination ports across multiple hosts indicate a scan.
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 is misconfigured and blocking legitimate traffic.
Why it's wrong here
There is no indication of legitimate traffic; the pattern suggests scanning.
✗
An internal host has been compromised and is exfiltrating data.
Why it's wrong here
Exfiltration would show outbound traffic, not inbound denied packets.
✗
An external host is launching a denial-of-service (DoS) attack.
Why it's wrong here
DoS attacks typically flood a single port, not scan multiple ports and hosts.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the distinction between a port scan and a DoS attack, where candidates mistakenly choose DoS because they see 'repeated denied packets' without recognizing the pattern of multiple destinations versus a single target flood.
Trap categories for this question
Command / output trap
Exfiltration would show outbound traffic, not inbound denied packets.
Detailed technical explanation
How to think about this question
Port scans often use TCP SYN packets to probe for open ports; when a firewall denies these with a 'deny ip any any' or specific deny ACE, the firewall sends a TCP RST or drops the packet silently, depending on the configuration. In Cisco ASA firewalls, the 'access-group OUTSIDE_IN' is applied to the outside interface, and the implicit deny at the end of the ACL will log these denials if 'log' is configured on the deny entry. This behavior is distinct from a DoS attack, which would generate a high rate of traffic that might trigger threat detection features like TCP Intercept or connection limits.
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 team runs a vulnerability scan on a web application and discovers an unpatched SQL injection flaw. The team prioritises remediation by CVSS score — critical flaws are patched within 24 hours, high within 7 days. Questions like this test whether you understand vulnerability management processes, scanning tools, and remediation prioritisation.
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: An external host is performing a port scan against internal systems. — The repeated denied TCP packets from 192.0.2.10 (an external IP) to multiple internal hosts indicate a port scan. The access-group 'OUTSIDE_IN' is applied to the outside interface, and the firewall is denying these packets because they match a deny ACE (access control entry) that blocks unsolicited inbound traffic. This pattern of multiple denied connections from a single external source to different internal destinations is characteristic of a reconnaissance scan, not a DoS attack or data exfiltration.
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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