The correct answer is to configure security groups for the EC2 instances that only allow outbound traffic to the RDS security group, and the RDS security group that allows inbound traffic from the EC2 security group. This works because security groups are stateful and support instance-level granularity, allowing you to reference another security group as a source or destination—creating a tightly scoped, two-way communication channel between the private subnets without opening access to any other resources in the VPC. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of the difference between security groups (stateful, instance-level) and NACLs (stateless, subnet-level); a common trap is choosing NACLs alone, which lack the granularity to restrict traffic between specific instances. Remember the memory tip: “Group-to-group for granular, NACL for broad”—when you need to lock down private subnet to private subnet communication to only specific instances, always reach for security group referencing first.
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 security engineer is designing a network segmentation strategy for a VPC that hosts sensitive data. The engineer needs to ensure that EC2 instances in a private subnet can communicate with an RDS database in a different private subnet, but cannot communicate with any other resources in the same VPC. Which configuration should be used?
Create a VPC peering connection between the subnets.
Why wrong: VPC peering connects entire VPCs, not subnets.
B
Configure security groups for the EC2 instances that only allow outbound traffic to the RDS security group, and RDS security group allows inbound from the EC2 security group.
Security groups provide instance-level granularity and stateful filtering.
C
Assign the same security group to both the EC2 instances and the RDS database.
Why wrong: Same security group allows all traffic between them, but also to others.
D
Use network ACLs with deny rules for all traffic except between the two subnets.
Why wrong: NACLs are stateless and apply to entire subnets, not specific instances.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure security groups for the EC2 instances that only allow outbound traffic to the RDS security group, and RDS security group allows inbound from the EC2 security group.
Option D is correct because a combination of security groups and network ACLs can enforce least-privilege rules. Option A is wrong because VPC peering is for cross-VPC. Option B is wrong because a single security group shared would allow broader communication. Option C is wrong because NACLs alone are not enough for instance-level granularity.
Key principle: Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Create a VPC peering connection between the subnets.
Why it's wrong here
VPC peering connects entire VPCs, not subnets.
✓
Configure security groups for the EC2 instances that only allow outbound traffic to the RDS security group, and RDS security group allows inbound from the EC2 security group.
Why this is correct
Security groups provide instance-level granularity and stateful filtering.
Related concept
Authentication checks who the user is.
✗
Assign the same security group to both the EC2 instances and the RDS database.
Why it's wrong here
Same security group allows all traffic between them, but also to others.
✗
Use network ACLs with deny rules for all traffic except between the two subnets.
Why it's wrong here
NACLs are stateless and apply to entire subnets, not specific instances.
Common exam traps
Common exam trap: authentication is not authorization
Logging in proves the user can authenticate. It does not automatically mean the user is allowed to enter privileged or configuration mode. Watch for AAA authorization, privilege level and command authorization details.
Detailed technical explanation
How to think about this question
This kind of question is testing the difference between identity and permission. A user may successfully log in to a router because authentication is working, but still fail to enter configuration mode because authorization is missing, misconfigured or mapped to a lower privilege level.
KKey Concepts to Remember
Authentication checks who the user is.
Authorization controls what the user is allowed to do after login.
Privilege levels affect access to EXEC and configuration commands.
AAA, TACACS+ and RADIUS can separate login success from command access.
TExam Day Tips
→Do not assume successful login means full administrative access.
→Look for words such as cannot enter configuration mode, privilege level, authorization or command access.
→Separate login problems from permission problems before choosing the answer.
Key takeaway
Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
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 Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related SCS-C02 questions on access control and AAA configuration.
Infrastructure Security — This question tests Infrastructure Security — Authentication checks who the user is..
What is the correct answer to this question?
The correct answer is: Configure security groups for the EC2 instances that only allow outbound traffic to the RDS security group, and RDS security group allows inbound from the EC2 security group. — Option D is correct because a combination of security groups and network ACLs can enforce least-privilege rules. Option A is wrong because VPC peering is for cross-VPC. Option B is wrong because a single security group shared would allow broader communication. Option C is wrong because NACLs alone are not enough for instance-level granularity.
What should I do if I get this SCS-C02 question wrong?
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related SCS-C02 questions on access control and AAA configuration.
What is the key concept behind this question?
Authentication checks who the user is.
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