The correct answer is that the server is under a SYN flood attack, filling the connection queue. This occurs because a SYN flood exploits the TCP three-way handshake by sending a high volume of SYN packets without completing the handshake, which saturates the server’s half-open connection backlog. When the backlog is full, the server stops sending SYN-ACK replies to new requests, even though CPU and memory remain normal—a key distinction from resource exhaustion attacks. On the ISC2 Certified in Cybersecurity CC exam, this scenario tests your understanding of network-based denial-of-service attacks, often appearing as a trap where normal resource usage might mislead you toward a hardware or software fault. A common memory tip: think of the backlog as a parking lot—once it’s full, no new cars (connections) can enter, even if the engine (CPU) is fine.
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 security analyst notices that an internal web server is receiving a high volume of TCP SYN packets from a single external IP address, but the server is not sending SYN-ACK replies. The server's CPU and memory usage are normal. 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 server is under a SYN flood attack, filling the connection queue
The correct answer is B because a SYN flood attack works by sending a high volume of TCP SYN packets to a server, filling its half-open connection queue (the backlog) and preventing it from completing the three-way handshake. Since the server's CPU and memory are normal, the issue is not resource exhaustion but rather the queue being saturated, causing the server to stop sending SYN-ACK replies to new connection requests.
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.
✗
A firewall rule is blocking inbound SYN-ACK packets
Why it's wrong here
Firewalls typically block inbound SYNs, not SYN-ACKs; the issue is the server not sending SYN-ACKs.
✓
The server is under a SYN flood attack, filling the connection queue
Why this is correct
SYN flood attacks fill the server's half-open connection queue, preventing it from sending SYN-ACKs.
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 server's TCP/IP stack has crashed
Why it's wrong here
A crash would show abnormal CPU/memory usage, which is normal here.
✗
The server is experiencing a distributed denial-of-service (DDoS) attack
Why it's wrong here
A DDoS involves multiple sources, but here only one IP is mentioned.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the distinction between a SYN flood (which fills the connection queue) and a DDoS (which involves multiple sources), so candidates mistakenly choose D when the key detail is the single external IP address.
Trap categories for this question
Command / output trap
A crash would show abnormal CPU/memory usage, which is normal here.
Detailed technical explanation
How to think about this question
The TCP SYN flood exploits the backlog queue (somaxconn or tcp_max_syn_backlog in Linux), which stores half-open connections awaiting the final ACK. When the queue is full, the server drops new SYN packets, and tools like netstat -s can show 'SYN to SYN/ACK' retransmission failures. In real-world scenarios, attackers often spoof source IPs to avoid detection, but here the single IP suggests a simpler attack or a misconfigured client.
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 server is under a SYN flood attack, filling the connection queue — The correct answer is B because a SYN flood attack works by sending a high volume of TCP SYN packets to a server, filling its half-open connection queue (the backlog) and preventing it from completing the three-way handshake. Since the server's CPU and memory are normal, the issue is not resource exhaustion but rather the queue being saturated, causing the server to stop sending SYN-ACK replies to new connection requests.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This CC practice question is part of Courseiva's free ISC2 certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CC exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.