SAA-C03 Design Secure Architectures Practice Question
A company runs an EC2 Auto Scaling group behind an internet-facing Application Load Balancer. The security team must ensure that the instances accept HTTP traffic only from the ALB and never directly from the internet, while the ALB itself must accept traffic only from a specific corporate CIDR range. Which combination of security group configurations should a solutions architect implement?
⚠ Common exam trap
The trap here is assuming the instances can match on the original client CIDR, when the ALB rewrites the source address so only a security group reference reliably identifies the load balancer.
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
✓
Create an inbound rule on the instance security group allowing TCP 80 from the ALB's security group, and an inbound rule on the ALB security group allowing TCP 80 from the corporate CIDR range.
The secure pattern is to make the instances trust only the load balancer by referencing the ALB's security group as the source, and to make the ALB trust only the intended clients by scoping its inbound rule to the corporate CIDR range. This creates a chained trust model where the instances never accept direct internet connections and the ALB is not exposed beyond the approved network range.
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 an inbound rule on the instance security group allowing TCP 80 from 0.0.0.0/0, and an inbound rule on the ALB security group allowing TCP 80 from the corporate CIDR range.
Why it's wrong here
Opening the instances to 0.0.0.0/0 means anyone who reaches their addresses can bypass the load balancer, directly violating the first requirement. Although the ALB rule is correct, the instance rule defeats the defense-in-depth goal and exposes the fleet to direct internet traffic on port 80.
- ✓
Create an inbound rule on the instance security group allowing TCP 80 from the ALB's security group, and an inbound rule on the ALB security group allowing TCP 80 from the corporate CIDR range.
Why this is correct
Referencing the ALB's security group as the source means only traffic that the ALB forwards can reach the instances, regardless of the instances' public IPs. Restricting the ALB security group to the corporate CIDR range enforces the second requirement. This is the standard two-tier security group pattern and satisfies both conditions without extra infrastructure.
- ✗
Create an inbound rule on the instance security group allowing TCP 80 from the corporate CIDR range, and an inbound rule on the ALB security group allowing TCP 80 from the ALB's security group.
Why it's wrong here
A security group cannot use itself as the source for inbound rules in a way that permits external clients, and the corporate CIDR on the instances would not match the ALB's source address. The ALB would also receive no permitted inbound traffic from the corporate network, so client requests would be dropped before reaching the target group.
- ✗
Create an inbound rule on the instance security group allowing TCP 80 from the corporate CIDR range, and an inbound rule on the ALB security group allowing TCP 80 from 0.0.0.0/0.
Why it's wrong here
Allowing the corporate CIDR on the instances does not restrict traffic to the load balancer, because the source IP the instances see is the load balancer's node address, not the original client. Opening the ALB to the world also violates the requirement that the ALB accept traffic only from the corporate range, so both rules are wrong here.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
This SAA-C03 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 SAA-C03 exam.