easyMultiple Choice
ISC2 CC Practice Question: A security analyst is reviewing an alert from the…
A security analyst is reviewing an alert from the IDS that shows a large number of TCP SYN packets sent to a single port on multiple internal hosts from a single external IP address. The analyst suspects a reconnaissance attack. Which type of attack is this most likely?
⚠ Common exam trap
ISC2 often tests the distinction between a SYN flood (DoS) and a SYN scan (reconnaissance), and the trap here is that candidates confuse the use of SYN packets in a volumetric attack versus a probing technique.
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
✓
SYN scan
This is most likely a SYN scan (option D), a reconnaissance technique where an attacker sends TCP SYN packets to a specific port on multiple hosts to determine if the port is open. A SYN scan is stealthier than a full TCP connect scan because it never completes the three-way handshake, leaving fewer logs. The IDS alert describes the hallmark behavior of a SYN scan: a single external IP targeting the same port across many internal hosts.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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SYN flood
Why it's wrong here
A SYN flood exhausts a single host's connection table by leaving half-open TCP handshakes; the stem describes SYNs spread across multiple internal hosts on one port, indicating scanning. It is tempting because both involve SYN packets, and would be correct if one target's backlog were being depleted.
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Ping sweep
Why it's wrong here
A ping sweep sends ICMP echo requests across many hosts to map live systems, not TCP SYN packets to one port. The described pattern is a port scan. Ping sweep would be the answer only when the alert shows ICMP traffic sweeping an address range to enumerate reachable hosts.
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Smurf attack
Why it's wrong here
A Smurf attack floods a victim with ICMP echo replies using a spoofed source address and a broadcast destination; it involves no TCP SYN packets to a single port. It is tempting as a recognised reconnaissance or DoS technique, and would be correct if the traffic were ICMP-based amplification.
- ✓
SYN scan
Why this is correct
A SYN scan sends TCP SYN packets to many hosts or ports without completing the handshake, seeking open services. The single external source hitting one port across multiple internal hosts matches horizontal reconnaissance, distinguishing it from a full connect scan.
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Related to this question
Key term
IDS
An IDS is a security system that monitors network or system traffic for suspicious activity and alerts administrators to potential threats, but does not actively block them.
Key term
Alert
An alert is a notification that something unusual or potentially harmful has happened in a computer system or network.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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