This CS0-003 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
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN
tcp 0 0 10.0.0.5:54321 198.51.100.20:443 ESTABLISHED
tcp 0 0 192.168.1.10:80 0.0.0.0:* LISTEN
udp 0 0 0.0.0.0:53 0.0.0.0:*
Refer to the exhibit. The output is from a Linux system running `netstat -an`. Which of the following ports is likely being used for remote command-and-control communication?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
54321
Port 54321 is a high-numbered ephemeral port that is not associated with any standard service, making it a common choice for malware or remote access tools (RATs) to establish command-and-control (C2) communication. In the netstat -an output, an established connection on a non-standard high port from the local system to a remote IP is a strong indicator of C2 activity, as legitimate services typically use well-known ports.
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.
✓
54321
Why this is correct
High port 54321 used in an established connection to an external IP is anomalous and could be C2.
Related concept
Read the scenario before looking for a memorised answer.
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the concept that high-numbered ephemeral ports (above 1024) with no associated standard service are strong indicators of C2 activity, tricking candidates into choosing common service ports like 22, 53, or 80 because they are familiar, even though those are legitimate and monitored.
Detailed technical explanation
How to think about this question
Command-and-control channels often use high-numbered ports (above 1024) to evade firewall rules that block low ports, and malware may use dynamic port selection or hardcoded ports like 54321. Tools like netstat -an show all listening and established connections; an established connection on a non-standard port to an external IP with no corresponding local service is a red flag. In real-world incidents, C2 traffic may also use protocols like HTTPS on port 443, but the question specifically highlights a non-standard port to test recognition of anomalous port usage.
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 CS0-003 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: 54321 — Port 54321 is a high-numbered ephemeral port that is not associated with any standard service, making it a common choice for malware or remote access tools (RATs) to establish command-and-control (C2) communication. In the netstat -an output, an established connection on a non-standard high port from the local system to a remote IP is a strong indicator of C2 activity, as legitimate services typically use well-known ports.
What should I do if I get this CS0-003 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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