Question 1hardmultiple choice
Review the full subnetting walkthrough →Question 314 of 1,010
Quick Answer
The correct answer is that the EC2 instance’s security group blocks direct internet traffic, allowing HTTP/HTTPS only from the load balancer’s security group and SSH solely from the internal IP range, which explains why an Nmap SYN scan against the instance’s public IP fails and why port 22 appears filtered. This occurs because security groups are stateful and evaluate inbound rules before any response; the initial SYN packet to port 22 from an external source is silently dropped, making the host appear down during a SYN scan, while a TCP connect scan reveals ports 80 and 443 as open only because the load balancer forwards traffic to those ports on the instance. On the Certified Ethical Hacker CEH exam, this scenario tests your understanding of AWS security group vs network ACL Nmap scan behavior—specifically how security groups filter at the instance level versus network ACLs at the subnet level, and how load balancer traffic masking can mislead reconnaissance. A common trap is assuming a public IP means direct internet access, but security groups enforce strict source-based rules. Memory tip: “Security groups are instance firewalls—SYN scans hit the wall, not the host.”
CEH Scanning Networks and Enumeration Practice Question
This CEH practice question tests your understanding of scanning networks and enumeration. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
You are conducting a security assessment for a company that hosts a web application on AWS. The application consists of a public-facing load balancer, an EC2 instance running a Linux web server, and an RDS MySQL database in a private subnet. The web server is configured to allow SSH access only from the company's internal IP range (203.0.113.0/24). During initial reconnaissance, you discover that the load balancer's security group allows inbound HTTP/HTTPS from anywhere. You attempt an Nmap SYN scan against the EC2 instance's public IP but receive no response (host appears down). Using a TCP connect scan, you find that ports 80 and 443 are open on the EC2 instance's public IP, but port 22 is filtered. You then launch an EC2 instance in the same region and run a scan from that internal AWS IP, and you find that port 22 is open on the target EC2 instance's private IP. Which of the following is the most likely reason for the initial scan failure and the filtered SSH port?
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 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
✓
The EC2 instance's security group only allows HTTP/HTTPS traffic from the load balancer's security group and SSH from the internal IP range. The load balancer's security group allows internet traffic to the load balancer, but the EC2 instance's security group does not allow direct internet traffic to the instance.
Option C is correct because the EC2 instance's security group is configured to allow HTTP/HTTPS only from the load balancer's security group (not from the internet directly) and SSH only from the internal IP range (203.0.113.0/24). The initial Nmap SYN scan against the EC2 instance's public IP failed because the instance's security group drops all inbound traffic not explicitly allowed, including SYN packets to port 22 from external IPs, making the host appear down. The TCP connect scan revealed ports 80 and 443 open because the load balancer forwards traffic to those ports on the instance, but the security group still blocks direct internet access to the instance's public IP for SSH, resulting in a filtered state. The internal scan from another EC2 instance in the same VPC succeeded because the security group allows SSH from the internal IP range, and the private IP is reachable within the VPC.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse the behavior of security groups (stateful, instance-level) with network ACLs (stateless, subnet-level) and incorrectly assume that a filtered port or unresponsive host must be due to a NACL blocking SYN packets, rather than recognizing that the security group's explicit deny of direct internet traffic to the instance's public IP causes the scan to fail.
Detailed technical explanation
How to think about this question
In AWS, security groups act as stateful virtual firewalls at the instance level, while network ACLs are stateless at the subnet level. When a security group denies inbound traffic, the instance does not respond to SYN packets, causing Nmap's SYN scan to mark the host as 'down' or 'filtered' because no SYN-ACK or RST is returned. The TCP connect scan (using the three-way handshake) succeeded on ports 80 and 443 because the load balancer's health checks or forwarded traffic may have established connections that keep the state table open, or because the security group allows HTTP/HTTPS from the load balancer's security group, which indirectly permits responses to the internet via the load balancer's public IP. A real-world scenario is a multi-tier web app where the web server in a private subnet only accepts traffic from the load balancer, preventing direct internet access to the instance for security.
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 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
Questions learners often ask
What does this CEH question test?
Scanning Networks and Enumeration — This question tests Scanning Networks and Enumeration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The EC2 instance's security group only allows HTTP/HTTPS traffic from the load balancer's security group and SSH from the internal IP range. The load balancer's security group allows internet traffic to the load balancer, but the EC2 instance's security group does not allow direct internet traffic to the instance. — Option C is correct because the EC2 instance's security group is configured to allow HTTP/HTTPS only from the load balancer's security group (not from the internet directly) and SSH only from the internal IP range (203.0.113.0/24). The initial Nmap SYN scan against the EC2 instance's public IP failed because the instance's security group drops all inbound traffic not explicitly allowed, including SYN packets to port 22 from external IPs, making the host appear down. The TCP connect scan revealed ports 80 and 443 open because the load balancer forwards traffic to those ports on the instance, but the security group still blocks direct internet access to the instance's public IP for SSH, resulting in a filtered state. The internal scan from another EC2 instance in the same VPC succeeded because the security group allows SSH from the internal IP range, and the private IP is reachable within the VPC.
What should I do if I get this CEH 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.
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Last reviewed: Jun 11, 2026
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