Quick Answer
The correct answer is a Xmas scan, because a packet with the FIN, PSH, and URG flags set simultaneously is the definitive signature of this stealth scanning technique. In a Xmas scan, the lit-up combination of these three flags—like lights on a Christmas tree—exploits RFC 793’s rule that a closed port must reply with an RST packet, while an open port drops the packet silently, allowing the attacker to map live hosts without completing a full TCP handshake. On the Computer Hacking Forensic Investigator CHFI exam, this question tests your ability to recognize scan types from NIDS alerts, often appearing as a trap where you must distinguish a Xmas scan from a FIN scan (which uses only the FIN flag) or a Null scan (which uses no flags). A common memory tip is to think of the three flags—FIN, PSH, URG—as the “three lights” that make the Christmas tree glow, and remember that only a closed port will respond with an RST.
This CHFI practice question tests your understanding of network and cloud forensics. 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.
A forensic analyst is examining a network intrusion detection system (NIDS) alert that triggered on a packet with the FIN, PSH, and URG flags set. What type of scan does this indicate?
Xmas scan
Xmas scan sets FIN, PSH, and URG flags to evade detection and elicit responses.
NULL scan
Why wrong: NULL scan has no flags set.
SYN scan
Why wrong: SYN scan only sets the SYN flag.
ACK scan
Why wrong: ACK scan sets only the ACK flag.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Xmas scan
A is correct because an Xmas scan sends packets with the FIN, PSH, and URG flags set (like a Christmas tree lit up). According to RFC 793, a closed port must respond with an RST packet, while an open port should drop the packet silently (no response). The NIDS alert triggered on these three flags together, which is the signature of an Xmas scan.
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.
Xmas scan
Why this is correct
Xmas scan sets FIN, PSH, and URG flags to evade detection and elicit responses.
Related concept
Read the scenario before looking for a memorised answer.
NULL scan
Why it's wrong here
NULL scan has no flags set.
SYN scan
Why it's wrong here
SYN scan only sets the SYN flag.
ACK scan
Why it's wrong here
ACK scan sets only the ACK flag.
Common exam traps
EC-Council often tests the distinction between Xmas, NULL, and SYN scans by focusing on the exact flag combinations; the trap here is that candidates confuse the FIN, PSH, URG combination with a NULL scan (no flags) or a SYN scan (single flag).
Detailed technical explanation
Under the hood, the Xmas scan exploits TCP's behavior where an open port ignores unsolicited packets with certain flag combinations (FIN, PSH, URG) per RFC 793, while a closed port responds with RST. In real-world scenarios, modern firewalls and intrusion detection systems (like Snort or Suricata) can detect and block Xmas scans because the flag combination is abnormal for legitimate traffic, making this scan less stealthy than SYN scans.
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 Cloud Forensics — This question tests Network and Cloud Forensics — Read the scenario before looking for a memorised answer..
The correct answer is: Xmas scan — A is correct because an Xmas scan sends packets with the FIN, PSH, and URG flags set (like a Christmas tree lit up). According to RFC 793, a closed port must respond with an RST packet, while an open port should drop the packet silently (no response). The NIDS alert triggered on these three flags together, which is the signature of an Xmas scan.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
This CHFI 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 CHFI exam.
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