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Question 222 of 1,040

Design Secure Architectures →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is to create an interface VPC endpoint for STS and associate it with your private subnets and a security group allowing HTTPS. This resolves the connectivity error because your EC2 instances in private subnets with no NAT gateway cannot reach the public STS endpoint over the internet; a VPC endpoint for STS uses AWS PrivateLink to route traffic privately within the AWS network, bypassing the need for internet access entirely. On the SAA-C03 exam, this scenario tests your understanding of how to provide AWS service access to private resources without compromising their isolation—a common trap is to incorrectly add a NAT gateway or an internet gateway, which would break the "keep instances private" requirement. Remember that interface endpoints, not gateway endpoints, are used for STS and most API-based services like CloudWatch or KMS. A useful memory tip: "Private subnets need PrivateLink—no NAT, no problem."

SAA-C03 Design Secure Architectures Practice Question

This SAA-C03 practice question tests your understanding of design secure architectures. 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.

Your EC2 instances run in private subnets with no NAT gateway. The instances use the AWS SDK to call STS AssumeRole to obtain temporary credentials for other services. Application logs show errors like: "EndpointConnectionError: Could not connect to https://sts.<region>.amazonaws.com".

Which change most directly resolves this while keeping instances private?

Question 1mediummultiple choice

Read the full NAT/PAT explanation →

A
Create an interface VPC endpoint for STS (com.amazonaws.<region>.sts) and associate it with the instance subnets and a security group that allows HTTPS.
Interface endpoints provide private, in-VPC connectivity to AWS APIs like STS without requiring internet access or NAT.
B
Create a gateway VPC endpoint for S3 and route the STS traffic through the S3 endpoint gateway.
Why wrong: Gateway endpoints are for specific services (like S3/DynamoDB) and cannot be used to route STS traffic to the correct API.
C
Open an inbound rule in the instances’ security group to allow outbound HTTPS to the internet CIDR block directly.
Why wrong: Security groups control allowed traffic, but the subnet route table still needs a path. Without NAT/endpoint routing, traffic cannot reach STS.
D
Attach an Internet Gateway to the private subnet route table so the STS API can be reached over public internet.
Why wrong: Adding an Internet Gateway to private subnets contradicts the requirement to keep instances private and increases exposure.

Full breakdown with real-world context →

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 interface VPC endpoint for STS (com.amazonaws.<region>.sts) and associate it with the instance subnets and a security group that allows HTTPS.

The error indicates that the EC2 instances in private subnets cannot reach the STS public endpoint over the internet because there is no NAT gateway or internet gateway attached to the private subnets. Creating an interface VPC endpoint for STS (com.amazonaws.<region>.sts) allows the instances to communicate with the STS API privately using AWS PrivateLink, without requiring internet access. Associating the endpoint with the instance subnets and a security group that allows HTTPS (port 443) ensures that traffic stays within the AWS network, resolving the connectivity error while keeping the instances private.

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.

✓
Create an interface VPC endpoint for STS (com.amazonaws.<region>.sts) and associate it with the instance subnets and a security group that allows HTTPS.
Why this is correct
Interface endpoints provide private, in-VPC connectivity to AWS APIs like STS without requiring internet access or NAT.
Related concept
Read the scenario before looking for a memorised answer.
✗
Create a gateway VPC endpoint for S3 and route the STS traffic through the S3 endpoint gateway.
Why it's wrong here
Gateway endpoints are for specific services (like S3/DynamoDB) and cannot be used to route STS traffic to the correct API.
✗
Open an inbound rule in the instances’ security group to allow outbound HTTPS to the internet CIDR block directly.
Why it's wrong here
Security groups control allowed traffic, but the subnet route table still needs a path. Without NAT/endpoint routing, traffic cannot reach STS.
✗
Attach an Internet Gateway to the private subnet route table so the STS API can be reached over public internet.
Why it's wrong here
Adding an Internet Gateway to private subnets contradicts the requirement to keep instances private and increases exposure.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often confuse gateway endpoints (for S3/DynamoDB) with interface endpoints (for most other AWS services like STS), or they mistakenly think security group rules alone can enable internet access without a proper routing path.

Detailed technical explanation

How to think about this question

Interface VPC endpoints use AWS PrivateLink to provide private connectivity to AWS services via elastic network interfaces (ENIs) in your VPC, with traffic staying within the AWS backbone and never traversing the internet. The STS endpoint requires an interface endpoint (not gateway) because it uses HTTPS API calls, and the security group must allow inbound HTTPS from the instances and outbound HTTPS to the endpoint. In real-world scenarios, this pattern is common for workloads that need to assume roles (e.g., cross-account access) without exposing instances to the public internet, and it also avoids NAT gateway data transfer costs.

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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.

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.

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FAQ

Questions learners often ask

What does this SAA-C03 question test?

Design Secure Architectures — This question tests Design Secure Architectures — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: Create an interface VPC endpoint for STS (com.amazonaws.<region>.sts) and associate it with the instance subnets and a security group that allows HTTPS. — The error indicates that the EC2 instances in private subnets cannot reach the STS public endpoint over the internet because there is no NAT gateway or internet gateway attached to the private subnets. Creating an interface VPC endpoint for STS (com.amazonaws.<region>.sts) allows the instances to communicate with the STS API privately using AWS PrivateLink, without requiring internet access. Associating the endpoint with the instance subnets and a security group that allows HTTPS (port 443) ensures that traffic stays within the AWS network, resolving the connectivity error while keeping the instances private.

What should I do if I get this SAA-C03 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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Same concept, more angles

1 more ways this is tested on SAA-C03

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. Your EC2 instances run in private subnets with no NAT gateway. The instances use the AWS SDK to call STS AssumeRole to obtain temporary credentials for other services. Application logs show errors like: "EndpointConnectionError: Could not connect to https://sts.<region>.amazonaws.com". Which change most directly resolves this while keeping instances private?

medium

✓ A.Create an interface VPC endpoint for STS (com.amazonaws.<region>.sts) and associate it with the instance subnets and a security group that allows HTTPS.
B.Create a gateway VPC endpoint for S3 and route the STS traffic through the S3 endpoint gateway.
C.Open an inbound rule in the instances’ security group to allow outbound HTTPS to the internet CIDR block directly.
D.Attach an Internet Gateway to the private subnet route table so the STS API can be reached over public internet.

Why A: The error indicates the EC2 instances cannot reach the STS public endpoint over the internet because they are in private subnets without a NAT gateway. An interface VPC endpoint for STS (com.amazonaws.<region>.sts) allows private, direct connectivity to the STS API using AWS PrivateLink, without requiring internet access. Associating the endpoint with the instance subnets and a security group that allows HTTPS (port 443) resolves the connectivity issue while keeping the instances private.

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