The answer is to modify the web server's security group to remove the inbound HTTP rule from 0.0.0.0/0. This is correct because security groups are stateful: when the web server initiates an outbound connection to download patches, the return traffic is automatically allowed, even without an explicit inbound rule. By removing the inbound HTTP rule, you block unsolicited inbound internet traffic on port 80 while preserving the server’s ability to receive patch responses. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding of stateful vs. stateless filtering and the separation of inbound and outbound rules in security groups. A common trap is to overcomplicate the solution by modifying route tables or network ACLs, but the simplest fix is to edit the security group. Remember the memory tip: “Security groups are stateful—outbound-initiated traffic gets a free pass back in, but unsolicited inbound needs an explicit rule.”
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. 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.
A company has a VPC with a public subnet and a private subnet. The public subnet contains a NAT gateway and a bastion host. The private subnet contains a web server that needs to be patched via the internet. The security engineer has configured the route tables: the public subnet route table has a default route to the Internet Gateway, and the private subnet route table has a default route to the NAT gateway. The web server can successfully initiate outbound connections to the internet to download patches. However, the security team notices that the web server is also receiving inbound connections from the internet on port 80. The web server's security group allows inbound HTTP from 0.0.0.0/0. What should the engineer do to prevent inbound internet traffic while still allowing outbound patching?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Modify the web server's security group to remove the inbound HTTP rule from 0.0.0.0/0.
Option B is correct. Removing the inbound HTTP rule from the security group will block inbound traffic from the internet, while outbound traffic is still allowed because security groups are stateful and allow return traffic for outbound-initiated connections. Option A is incorrect because changing the route table would break outbound connectivity. Option C is incorrect because the NATgateway already handles outbound traffic; changing it would not help. Option D is incorrect because a network ACL would affect all traffic and could block return traffic for outbound connections.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Change the private subnet's route table to have a default route to the Internet Gateway instead of the NAT gateway.
Why it's wrong here
This would expose the web server to the internet directly.
✓
Modify the web server's security group to remove the inbound HTTP rule from 0.0.0.0/0.
Why this is correct
This blocks inbound traffic while allowing outbound-initiated traffic.
Replace the NAT gateway with a NAT instance and configure it to block inbound traffic.
Why it's wrong here
NATgateway already blocks inbound traffic; no need to change.
✗
Add a network ACL to the private subnet that denies inbound HTTP traffic.
Why it's wrong here
Network ACLs are stateless and would also block return traffic for outbound connections.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Modify the web server's security group to remove the inbound HTTP rule from 0.0.0.0/0. — Option B is correct. Removing the inbound HTTP rule from the security group will block inbound traffic from the internet, while outbound traffic is still allowed because security groups are stateful and allow return traffic for outbound-initiated connections. Option A is incorrect because changing the route table would break outbound connectivity. Option C is incorrect because the NAT gateway already handles outbound traffic; changing it would not help. Option D is incorrect because a network ACL would affect all traffic and could block return traffic for outbound connections.
What should I do if I get this SCS-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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