The correct choice is to restrict the security group source to the API Gateway’s VPC endpoint or the ALB security group. This is because a Lambda function inside a VPC should never have its ENI security group open to the internet (0.0.0.0/0) on port 443; doing so allows direct inbound traffic, bypassing API Gateway entirely. The Lambda ENI security group must only allow inbound from the API Gateway’s VPC endpoint (or an ALB security group) to ensure the function is invoked exclusively through the API Gateway, preserving the intended security boundary. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding of VPC-based Lambda security and the principle of least privilege—a common trap is assuming outbound rules or VPC attachment are the issue, when the real vulnerability is the overly permissive inbound rule. Memory tip: “Lambda in VPC? Lock the ENI to API Gateway only—never 0.0.0.0/0.”
SAP-C02 Continuous Improvement for Existing Solutions Practice Question
This SAP-C02 practice question tests your understanding of continuous improvement for existing solutions. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
Network Topology
A security engineer runs the command above and finds an ENI attached to a Lambda function. The security group sg-12345678 allows inbound traffic on port 443 from 0.0.0.0/0. The Lambda function is used to process API requests. The engineer is concerned about security. What should the engineer do?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Restrict the security group source to the API Gateway's VPC endpoint or the ALB security group.
Option B is correct because the Lambda function should only be invoked via the API Gateway, not directly from the internet. The security group should restrict inbound traffic from the API Gateway's source, not from all IPs. Option A is incorrect because outbound traffic is needed for the function to send responses. Option C is incorrect because VPC attachment is necessary for accessing internal resources. Option D is incorrect because the Lambda function is already in a VPC.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Modify the Lambda function to not use a VPC.
Why it's wrong here
This may break functionality if the function needs VPC resources.
✗
Add a deny rule for inbound 0.0.0.0/0 and allow only from the Lambda function's own IP.
Why it's wrong here
Lambda functions don't have static IPs without a NAT.
✗
Remove the VPC attachment from the Lambda function and use a VPC endpoint.
Why it's wrong here
VPC attachment might be needed for accessing other VPC resources.
✓
Restrict the security group source to the API Gateway's VPC endpoint or the ALB security group.
Why this is correct
Limits inbound traffic to only the expected source.
Related concept
Standard ACLs match source addresses.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related SAP-C02 ACL questions on filtering logic and placement.
Continuous Improvement for Existing Solutions — This question tests Continuous Improvement for Existing Solutions — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Restrict the security group source to the API Gateway's VPC endpoint or the ALB security group. — Option B is correct because the Lambda function should only be invoked via the API Gateway, not directly from the internet. The security group should restrict inbound traffic from the API Gateway's source, not from all IPs. Option A is incorrect because outbound traffic is needed for the function to send responses. Option C is incorrect because VPC attachment is necessary for accessing internal resources. Option D is incorrect because the Lambda function is already in a VPC.
What should I do if I get this SAP-C02 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related SAP-C02 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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