- A
Update the network ACL inbound rule to allow traffic only from the ALB security group. The security group is already correctly configured.
Why wrong: The security group currently allows 0.0.0.0/0, which is incorrect.
- B
Update the database security group to allow inbound traffic on port 3306 only from the ALB security group. Update the network ACL inbound rule to allow traffic from the ALB security group.
Restricting both layers to the ALB security group ensures only the ALB can reach the database.
- C
Update the database security group to allow inbound traffic on port 3306 from the VPC CIDR range. No changes to network ACL are needed.
Why wrong: Allowing the entire VPC is unnecessary and may expose the database to other instances.
- D
Move the database to a public subnet and restrict the security group to the ALB security group.
Why wrong: Moving to a public subnet increases exposure; private subnet is correct.
Quick Answer
The correct answer is to update the database security group to allow inbound traffic on port 3306 only from the ALB security group, and update the network ACL inbound rule to allow traffic from the ALB security group. This is the proper application of security group referencing for database access, where you use the logical security group ID as the source rather than an IP range, ensuring that only traffic originating from the ALB can reach the RDS instance. On the AWS Certified Security Specialty SCS-C02 exam, this concept tests your understanding of the layered defense model: security groups are stateful and support logical references, while network ACLs are stateless and must explicitly allow both inbound and outbound traffic. A common trap is to think that placing the database in a private subnet alone is sufficient, but the exam emphasizes that security group rules must also be tightened. Remember the memory tip: “SG for stateful, NACL for stateless—reference the group, not the IP.”
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 multi-tier web application hosted on AWS. The application consists of an Application Load Balancer (ALB), a fleet of EC2 instances in an Auto Scaling group, and an Amazon RDS MySQL database. The security team has implemented security groups and network ACLs. Recently, a vulnerability scan revealed that the RDS database is accessible from the internet. The security engineer investigates and finds that the database security group allows inbound traffic on port 3306 from 0.0.0.0/0. The engineer also checks the network ACLs and finds that inbound rules allow traffic on port 3306 from 0.0.0.0/0, and outbound rules allow all traffic. The database is in a private subnet. Which combination of steps should the engineer take to remediate the issue while maintaining application functionality?
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
Update the database security group to allow inbound traffic on port 3306 only from the ALB security group. Update the network ACL inbound rule to allow traffic from the ALB security group.
Option A is correct because it restricts both security group and network ACL to only the ALB security group, which is the proper source for database traffic. Option B is wrong because it only changes network ACL, not security group. Option C is wrong because allowing the VPC CIDR is too broad. Option D is wrong because it suggests public subnet for database, which is insecure.
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.
- ✗
Update the network ACL inbound rule to allow traffic only from the ALB security group. The security group is already correctly configured.
Why it's wrong here
The security group currently allows 0.0.0.0/0, which is incorrect.
- ✓
Update the database security group to allow inbound traffic on port 3306 only from the ALB security group. Update the network ACL inbound rule to allow traffic from the ALB security group.
Why this is correct
Restricting both layers to the ALB security group ensures only the ALB can reach the database.
Related concept
CIDR notation defines the prefix length.
- ✗
Update the database security group to allow inbound traffic on port 3306 from the VPC CIDR range. No changes to network ACL are needed.
Why it's wrong here
Allowing the entire VPC is unnecessary and may expose the database to other instances.
- ✗
Move the database to a public subnet and restrict the security group to the ALB security group.
Why it's wrong here
Moving to a public subnet increases exposure; private subnet is correct.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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.
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FAQ
Questions learners often ask
What does this SCS-C02 question test?
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: Update the database security group to allow inbound traffic on port 3306 only from the ALB security group. Update the network ACL inbound rule to allow traffic from the ALB security group. — Option A is correct because it restricts both security group and network ACL to only the ALB security group, which is the proper source for database traffic. Option B is wrong because it only changes network ACL, not security group. Option C is wrong because allowing the VPC CIDR is too broad. Option D is wrong because it suggests public subnet for database, which is insecure.
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.
About these practice questions
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Same concept, more angles
1 more ways this is tested on SCS-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a VPC with a public subnet and a private subnet. An Amazon RDS instance is in the private subnet, and an application server is in the public subnet. The security team needs to allow the application server to connect to the RDS instance on port 3306 (MySQL). Which configuration will meet this requirement securely?
hard- A.Add an inbound rule to the RDS security group that allows traffic from the VPC CIDR on port 3306.
- ✓ B.Add an inbound rule to the RDS security group that allows traffic from the security group of the application server on port 3306.
- C.Add an inbound rule to the RDS security group that allows traffic from the subnet CIDR of the application server on port 3306.
- D.Add an inbound rule to the RDS security group that allows traffic from 0.0.0.0/0 on port 3306.
Why B: Security groups are stateful; you can allow inbound traffic from the security group of the application server. Option A is correct. Option B is less secure because it allows all traffic from the subnet. Option C is incorrect because it allows all traffic from the VPC. Option D is incorrect because it allows traffic from the internet.
Last reviewed: Jun 20, 2026
This SCS-C02 practice question is part of Courseiva's free Amazon Web Services 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 SCS-C02 exam.
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