The answer is to configure an inbound rule on the RDS security group that allows MySQL traffic on port 3306 sourced from the security group ID of the application server. This is the most secure and precise method because it uses a security group as the source, which dynamically adapts to any IP changes in the application server—whether from scaling, replacement, or auto-recovery—without requiring manual CIDR updates. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of security group chaining versus subnet-level access; a common trap is selecting “allow from the VPC CIDR” or “allow from the subnet CIDR,” which would expose the database to any resource in that IP range. The core concept here is that security group IDs are logical references, not IP-based, making them ideal for controlling traffic between tiers in the same VPC. Memory tip: “Group to group, not IP to IP” — always think of linking security groups for inter-service communication to maintain least privilege.
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 an Amazon RDS for MySQL database in a private subnet. The security team wants to ensure that only an application server in the same VPC can connect to the database. Which security group configuration should be used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Inbound rule on the RDS security group: allow MySQL on port 3306 from the security group ID of the application server.
Option C is correct because referencing the application server's security group ID as the source in the inbound rule for MySQL (port 3306) on the RDS security group allows traffic only from instances that are members of that security group, regardless of their IP addresses. This is the most secure and precise method, as it automatically adapts to changes in the application server's IP (e.g., after scaling or replacement) and avoids opening the database to the entire subnet or VPC CIDR.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Inbound rule on the RDS security group: allow MySQL on port 3306 from the VPC's CIDR.
Why it's wrong here
This allows any resource in the VPC, not just the application server.
✗
Inbound rule on the RDS security group: allow MySQL on port 3306 from the subnet CIDR of the application server.
Why it's wrong here
This allows any instance in that subnet, not just the application server.
✓
Inbound rule on the RDS security group: allow MySQL on port 3306 from the security group ID of the application server.
Why this is correct
This restricts access to instances that are in the specified security group.
Related concept
Read the scenario before looking for a memorised answer.
✗
Inbound rule on the application server's security group: allow outbound MySQL to the RDS security group.
Why it's wrong here
Security groups are stateful; inbound rules on the RDS side control access.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse inbound vs. outbound rules or mistakenly think that allowing a subnet CIDR is equivalent to allowing a specific instance, when in fact security group ID-based rules provide instance-level granularity and are the recommended approach for this use case.
Detailed technical explanation
How to think about this question
Security group rules are stateful — if you allow inbound traffic from a security group ID, the corresponding outbound return traffic is automatically permitted, so no explicit outbound rule is needed. Under the hood, AWS evaluates security group rules by checking the source security group's network interfaces; this allows dynamic IP changes (e.g., after an Auto Scaling event) without updating the rule. In a real-world scenario, if the application server is part of an Auto Scaling group, using its security group ID ensures new instances automatically gain access without manual CIDR updates.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Infrastructure Security — This question tests Infrastructure Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Inbound rule on the RDS security group: allow MySQL on port 3306 from the security group ID of the application server. — Option C is correct because referencing the application server's security group ID as the source in the inbound rule for MySQL (port 3306) on the RDS security group allows traffic only from instances that are members of that security group, regardless of their IP addresses. This is the most secure and precise method, as it automatically adapts to changes in the application server's IP (e.g., after scaling or replacement) and avoids opening the database to the entire subnet or VPC CIDR.
What should I do if I get this SCS-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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