- 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.
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?
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.
When this WOULD be correct
This option would be correct if the question required instances in a private subnet to access the internet (e.g., for software updates) and explicitly allowed making the subnet public, or if the subnet was already public and needed internet access for STS.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The SAA-C03 exam frequently reuses these exact scenarios with slightly different constraints.
✓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.Correct answer▾
Why this is correct
Interface endpoints provide private, in-VPC connectivity to AWS APIs like STS without requiring internet access or NAT.
✗Create a gateway VPC endpoint for S3 and route the STS traffic through the S3 endpoint gateway.Wrong answer — click to see why▾
Why this is wrong here
A gateway VPC endpoint only supports S3 and DynamoDB; it cannot route STS traffic, which requires an interface endpoint. STS uses HTTPS traffic that must be directed through an interface endpoint, not a gateway endpoint.
★ When this WOULD be the correct answer
This option would be correct in a scenario where EC2 instances in a private subnet need to access S3 buckets without a NAT gateway or internet gateway, and the question asks for a solution to access S3 specifically.
Why candidates choose this
Candidates may confuse gateway endpoints with interface endpoints, assuming all AWS services can be accessed via a gateway endpoint, or they may think routing STS traffic through an S3 endpoint is possible due to similar naming.
✗Open an inbound rule in the instances’ security group to allow outbound HTTPS to the internet CIDR block directly.Wrong answer — click to see why▾
Why this is wrong here
Opening an inbound rule for outbound HTTPS to the internet CIDR does not provide a route to the internet; the instances are in a private subnet with no NAT gateway, so outbound traffic cannot reach the internet.
★ When this WOULD be the correct answer
This would be correct if the instances were in a public subnet with an internet gateway, and the security group needed to allow outbound HTTPS traffic to the internet for STS calls.
Why candidates choose this
Candidates may think that allowing outbound HTTPS in the security group is sufficient to reach the STS endpoint, overlooking the missing route to the internet from a private subnet.
✗Attach an Internet Gateway to the private subnet route table so the STS API can be reached over public internet.Wrong answer — click to see why▾
Why this is wrong here
Attaching an Internet Gateway to the private subnet route table would make the subnet public, violating the requirement to keep instances private. The instances would have direct internet access, which is not allowed.
★ When this WOULD be the correct answer
This option would be correct if the question required instances in a private subnet to access the internet (e.g., for software updates) and explicitly allowed making the subnet public, or if the subnet was already public and needed internet access for STS.
Why candidates choose this
Candidates may think that an Internet Gateway is necessary for any outbound internet access, not realizing that private subnets should not have direct internet routes, and that VPC endpoints can provide private connectivity.
Analysis generated from the official SAA-C03blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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.
Visual reference
What to study next
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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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Last reviewed: Jun 11, 2026
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