- A
Increased DNS query responses
Why wrong: DNS amplification attacks increase DNS traffic.
- B
High number of ICMP echo replies
Why wrong: ICMP echo replies are part of ping flood.
- C
Unusual outbound traffic on port 80
Why wrong: Outbound traffic is not typical for SYN flood.
- D
Large number of half-open connections
The server keeps connections in SYN-RECEIVED state.
- E
High number of SYN packets with no ACK
Attackers send SYN packets without completing the handshake.
Question 153 of 1,000
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 is investigating a potential DDoS attack on the company's web server. Which two symptoms are indicative of a SYN flood attack? (Select TWO.)
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Large number of half-open connections
Option D is correct because a SYN flood attack exploits the TCP three-way handshake by sending a high volume of SYN packets to a target server without completing the handshake, resulting in a large number of half-open connections that exhaust server resources. These connections remain in a SYN_RECEIVED state, consuming memory and preventing legitimate connections from being established.
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.
- ✗
Increased DNS query responses
Why it's wrong here
DNS amplification attacks increase DNS traffic.
- ✗
High number of ICMP echo replies
Why it's wrong here
ICMP echo replies are part of ping flood.
- ✗
Unusual outbound traffic on port 80
Why it's wrong here
Outbound traffic is not typical for SYN flood.
- ✓
Large number of half-open connections
Why this is correct
The server keeps connections in SYN-RECEIVED state.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
High number of SYN packets with no ACK
Why this is correct
Attackers send SYN packets without completing the handshake.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between the symptom of 'half-open connections' (the server-side resource exhaustion) and the traffic pattern of 'SYN packets with no ACK' (the attacker's behavior), expecting candidates to recognize both as correct indicators of a SYN flood.
Detailed technical explanation
How to think about this question
In a SYN flood, the attacker sends SYN packets with spoofed source IP addresses, causing the server to allocate Transmission Control Blocks (TCBs) and respond with SYN-ACKs to unreachable hosts, leaving connections in the SYN_RECEIVED state until a timeout (typically 30-60 seconds per RFC 793). Modern mitigations include SYN cookies (RFC 4987), which encode connection state in the SYN-ACK sequence number to avoid resource allocation until the handshake completes, and rate-limiting with tools like iptables or hardware-based DDoS protection appliances.
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
Network Security — study guide chapter
Learn the concepts, then practise the questions
- →
Network Security practice questions
Targeted practice on this topic area only
- →
All CC questions
1,000 questions across all exam domains
- →
ISC2 Certified in Cybersecurity CC study guide
Full concept coverage aligned to exam objectives
- →
CC practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related CC practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Security Principles practice questions
Practise CC questions linked to Security Principles.
Business Continuity, Disaster Recovery, and Incident Response practice questions
Practise CC questions linked to Business Continuity, Disaster Recovery, and Incident Response.
Access Controls Concepts practice questions
Practise CC questions linked to Access Controls Concepts.
Security Operations practice questions
Practise CC questions linked to Security Operations.
Network Security practice questions
Practise CC questions linked to Network Security.
Business Continuity, DR & Incident Response practice questions
Practise CC questions linked to Business Continuity, DR & Incident Response.
CC fundamentals practice questions
Practise CC questions linked to CC fundamentals.
CC scenario practice questions
Practise CC questions linked to CC scenario.
CC troubleshooting practice questions
Practise CC questions linked to CC troubleshooting.
Practice this exam
Start a free CC practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this CC question test?
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: Large number of half-open connections — Option D is correct because a SYN flood attack exploits the TCP three-way handshake by sending a high volume of SYN packets to a target server without completing the handshake, resulting in a large number of half-open connections that exhaust server resources. These connections remain in a SYN_RECEIVED state, consuming memory and preventing legitimate connections from being established.
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.
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 →
Last reviewed: Jul 4, 2026
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.