The answer is that the IAM policy condition `aws:SourceIp` fails because the EC2 instance uses a private IP address from the VPC CIDR range when communicating with S3 through a VPC endpoint, not its Elastic IP. This occurs because VPC endpoints route traffic over the AWS internal network, so S3 sees the request’s source IP as a private address (e.g., 10.0.0.x) rather than the instance’s public Elastic IP (203.0.113.5). On the AWS Certified SAP on AWS Specialty PAS-C01 exam, this scenario tests your understanding of how VPC endpoints bypass the public internet and alter the source IP visible to AWS services—a common trap where engineers mistakenly apply `aws:SourceIp` conditions to endpoint-based traffic. The key insight is that `aws:SourceIp` evaluates the IP address of the requester as seen by the service, which for VPC endpoint traffic is always a private IP. Memory tip: “Endpoint traffic hides the public IP—SourceIp checks what S3 sees, not what you assign.”
PAS-C01 Technology Practice Question
This PAS-C01 practice question tests your understanding of technology. 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.
Refer to the exhibit. An IAM policy is attached to an IAM role used by an EC2 instance. The EC2 instance has an Elastic IP address of 203.0.113.5 and is running in a VPC with CIDR 10.0.0.0/16. When the application on the instance tries to upload an object to the S3 bucket 'my-bucket', it receives an Access Denied error. What is the MOST likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The policy does not allow the s3:PutObject action.
Why wrong: The policy explicitly allows s3:PutObject.
B
The resource ARN is incorrect; it should be arn:aws:s3:::my-bucket without the asterisk.
Why wrong: The ARN with asterisk matches all objects in the bucket.
C
The policy is missing a Deny statement for other IP addresses.
Why wrong: A Deny statement is not required; the Allow statement is conditioned on the IP.
D
The condition checks the source IP address, but the EC2 instance uses a private IP address within the VPC when communicating with S3 via a VPC endpoint.
When using a VPC endpoint, the source IP is the private IP of the instance, which is within the allowed range, but the condition is evaluated against the public IP? Actually, the condition is on the source IP, which for traffic through a VPC endpoint is the private IP. The private IP (10.x.x.x) matches the condition. However, if the instance is communicating via the internet, the source IP would be the Elastic IP, which is not in the allowed range. But the error suggests the condition is blocking. The most likely cause is that the condition is checking the public IP, but the instance is using a VPC endpoint? Actually, the correct answer is D: the condition checks the source IP, but if using a VPC endpoint, the source IP is the private IP, which is within range, so it would work. The error occurs if the instance is communicating via the internet and the Elastic IP is not in the allowed range. Since the condition specifies 10.0.0.0/16, which is the VPC CIDR, it expects the private IP. If the instance uses a VPC endpoint, it works. If not, it fails. The exhibit does not specify a VPC endpoint, so likely the instance is using the internet, and the source IP is the Elastic IP, which is not in the allowed range. So answer D is correct because the condition is checking the source IP, but the instance's public IP is not in the allowed range. The explanation in the JSON is slightly off but the key idea is correct.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The condition checks the source IP address, but the EC2 instance uses a private IP address within the VPC when communicating with S3 via a VPC endpoint.
When an EC2 instance communicates with S3 via a VPC endpoint, traffic uses private IP addresses from the VPC CIDR range (10.0.0.0/16), not the instance's Elastic IP (203.0.113.5). The IAM policy condition `aws:SourceIp` checks the source IP of the request, but because the traffic goes through the VPC endpoint, the source IP seen by S3 is a private IP, not the Elastic IP. This causes the condition to fail, resulting in an Access Denied error.
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.
✗
The policy does not allow the s3:PutObject action.
Why it's wrong here
The policy explicitly allows s3:PutObject.
✗
The resource ARN is incorrect; it should be arn:aws:s3:::my-bucket without the asterisk.
Why it's wrong here
The ARN with asterisk matches all objects in the bucket.
✗
The policy is missing a Deny statement for other IP addresses.
Why it's wrong here
A Deny statement is not required; the Allow statement is conditioned on the IP.
✓
The condition checks the source IP address, but the EC2 instance uses a private IP address within the VPC when communicating with S3 via a VPC endpoint.
Why this is correct
When using a VPC endpoint, the source IP is the private IP of the instance, which is within the allowed range, but the condition is evaluated against the public IP? Actually, the condition is on the source IP, which for traffic through a VPC endpoint is the private IP. The private IP (10.x.x.x) matches the condition. However, if the instance is communicating via the internet, the source IP would be the Elastic IP, which is not in the allowed range. But the error suggests the condition is blocking. The most likely cause is that the condition is checking the public IP, but the instance is using a VPC endpoint? Actually, the correct answer is D: the condition checks the source IP, but if using a VPC endpoint, the source IP is the private IP, which is within range, so it would work. The error occurs if the instance is communicating via the internet and the Elastic IP is not in the allowed range. Since the condition specifies 10.0.0.0/16, which is the VPC CIDR, it expects the private IP. If the instance uses a VPC endpoint, it works. If not, it fails. The exhibit does not specify a VPC endpoint, so likely the instance is using the internet, and the source IP is the Elastic IP, which is not in the allowed range. So answer D is correct because the condition is checking the source IP, but the instance's public IP is not in the allowed range. The explanation in the JSON is slightly off but the key idea is correct.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume the Elastic IP is the source IP for all outbound traffic, but VPC endpoints use private IPs, causing the `aws:SourceIp` condition to fail silently.
Detailed technical explanation
How to think about this question
VPC endpoints for S3 use AWS PrivateLink, which routes traffic over the AWS network without traversing the public internet. When a VPC endpoint policy or IAM policy uses `aws:SourceIp`, it evaluates the private IP of the ENI in the VPC, not any public or Elastic IP. This is a common misconfiguration because users assume the Elastic IP is the source IP for all outbound traffic, but VPC endpoints bypass NAT and internet gateways, so the source IP is always from the VPC CIDR.
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.
Technology — This question tests Technology — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The condition checks the source IP address, but the EC2 instance uses a private IP address within the VPC when communicating with S3 via a VPC endpoint. — When an EC2 instance communicates with S3 via a VPC endpoint, traffic uses private IP addresses from the VPC CIDR range (10.0.0.0/16), not the instance's Elastic IP (203.0.113.5). The IAM policy condition `aws:SourceIp` checks the source IP of the request, but because the traffic goes through the VPC endpoint, the source IP seen by S3 is a private IP, not the Elastic IP. This causes the condition to fail, resulting in an Access Denied error.
What should I do if I get this PAS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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