Quick Answer
The answer is a SYN flood attack. This is the correct choice because a SYN flood exploits the TCP three-way handshake by sending a massive volume of SYN packets to target hosts but never completing the handshake with the final ACK, leaving connections half-open and exhausting the target’s connection table. In the Certified Ethical Hacker CEH exam, this scenario tests your ability to recognize denial-of-service signatures; a common trap is confusing it with a ping flood or Smurf attack, which use ICMP rather than incomplete TCP handshakes. The key memory tip is to think of the handshake as a three-step dance: if the first step (SYN) is repeated endlessly without the third step (ACK), the server is left waiting—hence, a SYN flood.
This CEH practice question tests your understanding of network and web application attacks. 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.
As a network defender, you notice an unusually high number of incomplete TCP three-way handshakes from a single external IP to multiple internal hosts. What is the most likely attack taking place?
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.
UDP flood
Why wrong: UDP flood uses UDP packets, not TCP SYN packets.
SYN flood
SYN flood sends many SYN packets without completing handshake.
ARP spoofing
Why wrong: ARP spoofing operates at layer 2, not TCP handshake.
ICMP flood
Why wrong: ICMP flood uses ping requests, not TCP handshake packets.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
SYN flood
A SYN flood attack exploits the TCP three-way handshake by sending a high volume of SYN packets to target hosts without completing the handshake (i.e., not sending the final ACK). This leaves the target with half-open connections, exhausting its connection table and denying service to legitimate traffic. The observation of incomplete handshakes from a single external IP to multiple internal hosts is a classic signature of a SYN flood.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
UDP flood
SYN flood
Why this is correct
SYN flood sends many SYN packets without completing handshake.
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.
ARP spoofing
ICMP flood
Common exam traps
EC-Council often tests the distinction between a SYN flood and a UDP flood, where candidates mistakenly choose UDP flood because they associate 'flood' with any high-volume attack, but the key clue is the incomplete TCP three-way handshake, which is specific to SYN floods.
Detailed technical explanation
Under the hood, a SYN flood sends a barrage of SYN packets with spoofed source IP addresses, causing the target to allocate Transmission Control Blocks (TCBs) for each half-open connection. The target's backlog queue (defined by the `tcp_max_syn_backlog` kernel parameter) fills up, preventing new legitimate connections. In real-world scenarios, attackers often randomize source IPs to evade simple IP-based blocking, making mitigation techniques like SYN cookies (RFC 4987) or rate-limiting essential.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Network and Web Application Attacks — This question tests Network and Web Application Attacks — Read the scenario before looking for a memorised answer..
The correct answer is: SYN flood — A SYN flood attack exploits the TCP three-way handshake by sending a high volume of SYN packets to target hosts without completing the handshake (i.e., not sending the final ACK). This leaves the target with half-open connections, exhausting its connection table and denying service to legitimate traffic. The observation of incomplete handshakes from a single external IP to multiple internal hosts is a classic signature of a SYN flood.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
This CEH practice question is part of Courseiva's free EC-Council 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 CEH exam.
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