ANS-C01 Security group referencing Practice Question
A company has a VPC with multiple subnets. They have an Application Load Balancer (ALB) in a public subnet and a web server fleet in private subnets. The security team wants to ensure that only the ALB can communicate with the web servers. Which security group configuration should be used?
⚠ Common exam trap
A common pitfall is using source CIDR blocks (e.g., subnet or VPC CIDR) instead of security group IDs, which grants broader access than intended.
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
✓
Set the web server security group inbound rule to allow traffic from the ALB's security group.
Referencing the ALB's security group as the source allows only traffic from instances or resources associated with that security group, strictly limiting access to the web servers. This follows the principle of least privilege. Option B is wrong because allowing traffic from the public subnet CIDR is too broad and would permit any instance in that subnet, not just the ALB. Option C is wrong because allowing traffic from the VPC CIDR permits any resource in the VPC, which is overly permissive. Option D is wrong because allowing traffic from 0.0.0.0/0 exposes the web servers to the internet.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Set the web server security group inbound rule to allow traffic from the ALB's security group.
Why this is correct
This restricts inbound traffic to only the ALB.
- ✗
Set the web server security group inbound rule to allow traffic from the public subnet CIDR.
Why it's wrong here
This allows traffic from any instance in the public subnet, not just the ALB.
- ✗
Set the web server security group inbound rule to allow traffic from the VPC CIDR.
Why it's wrong here
This allows traffic from any resource in the VPC, not just the ALB.
- ✗
Set the web server security group inbound rule to allow traffic from 0.0.0.0/0.
Why it's wrong here
This allows all inbound traffic, bypassing the ALB.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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