The answer is the VPC endpoint policy for the S3 endpoint restricting access to the bucket. Even when an S3 bucket policy explicitly allows access from a specific external IP range (203.0.113.0/24) and requires HTTPS via the `aws:SecureTransport` condition, a separate VPC endpoint policy can override that permission by denying all traffic that does not originate from the VPC. In this scenario, external users satisfy both bucket policy conditions, but the request is routed through the VPC endpoint, where the endpoint policy acts as an additional, restrictive gate—blocking non-VPC sources. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this tests your understanding that S3 bucket policies and VPC endpoint policies are evaluated independently, and a deny in the endpoint policy always wins. A common trap is focusing only on the bucket policy’s IP and HTTPS conditions while overlooking the endpoint’s separate policy. Memory tip: “Endpoint policy is the bouncer—even if the bucket’s guest list says you’re allowed, the bouncer can still turn you away.”
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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.
A security engineer created the S3 bucket policy shown. After applying it, users from the 10.0.0.0/16 network (VPC CIDR) are able to access objects, but external users from 203.0.113.0/24 receive 'Access Denied' even though they are using HTTPS. What is the likely cause?
The VPC endpoint policy for the S3 endpoint restricts access to the bucket.
Even if the bucket policy allows, the VPC endpoint policy can deny access, especially for external IPs routed through the endpoint?
B
The policy only allows 's3:GetObject' but the users are trying to list objects.
Why wrong: The stem says they are trying to access objects, not list.
C
The policy does not include a 'Deny' statement for non-IP ranges.
Why wrong: The policy is permissive; it allows specified IPs, but other traffic is implicitly denied by default, so this is not the issue.
D
The S3 bucket has 'Block public access' enabled, which overrides the policy.
Why wrong: If public access is blocked, the policy would be ignored, but the internal users are accessing, so public access is likely not fully blocked.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VPC endpoint policy for the S3 endpoint restricts access to the bucket.
The policy requires both conditions: source IP from either range AND SecureTransport true. For external users, the IP condition is satisfied, but the problem states they are using HTTPS, so SecureTransport is true. However, the 'Access Denied' suggests the policy is not being evaluated as expected. Actually, the policy allows access only if both conditions are met. For external users, both conditions are met, so they should be allowed. But the external users get denied, meaning the policy might not be the only one. However, the most likely cause is that the policy is missing a statement to explicitly deny all other traffic, or there is a bucket policy that denies. But given options, the most plausible is that the condition 'aws:SecureTransport' requires the request to be made over HTTPS; external users might be using HTTP? The stem says they use HTTPS. So perhaps the issue is that the VPC endpoint adds a source IP that is not 10.0.0.0/16? Actually, the policy allows from 10.0.0.0/16 and 203.0.113.0/24. The external users are from 203.0.113.0/24, which is allowed. So why denied? Maybe the policy is attached to the bucket but there is also a VPC endpoint policy that denies. Option D is most likely: the VPC endpoint policy is blocking access. Option A is wrong because all actions are allowed. Option B is wrong because the policy allows GetObject. Option C is wrong because public access could be blocked but the policy allows. So D is correct.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 VPC endpoint policy for the S3 endpoint restricts access to the bucket.
Why this is correct
Even if the bucket policy allows, the VPC endpoint policy can deny access, especially for external IPs routed through the endpoint?
The policy only allows 's3:GetObject' but the users are trying to list objects.
Why it's wrong here
The stem says they are trying to access objects, not list.
✗
The policy does not include a 'Deny' statement for non-IP ranges.
Why it's wrong here
The policy is permissive; it allows specified IPs, but other traffic is implicitly denied by default, so this is not the issue.
✗
The S3 bucket has 'Block public access' enabled, which overrides the policy.
Why it's wrong here
If public access is blocked, the policy would be ignored, but the internal users are accessing, so public access is likely not fully blocked.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The VPC endpoint policy for the S3 endpoint restricts access to the bucket. — The policy requires both conditions: source IP from either range AND SecureTransport true. For external users, the IP condition is satisfied, but the problem states they are using HTTPS, so SecureTransport is true. However, the 'Access Denied' suggests the policy is not being evaluated as expected. Actually, the policy allows access only if both conditions are met. For external users, both conditions are met, so they should be allowed. But the external users get denied, meaning the policy might not be the only one. However, the most likely cause is that the policy is missing a statement to explicitly deny all other traffic, or there is a bucket policy that denies. But given options, the most plausible is that the condition 'aws:SecureTransport' requires the request to be made over HTTPS; external users might be using HTTP? The stem says they use HTTPS. So perhaps the issue is that the VPC endpoint adds a source IP that is not 10.0.0.0/16? Actually, the policy allows from 10.0.0.0/16 and 203.0.113.0/24. The external users are from 203.0.113.0/24, which is allowed. So why denied? Maybe the policy is attached to the bucket but there is also a VPC endpoint policy that denies. Option D is most likely: the VPC endpoint policy is blocking access. Option A is wrong because all actions are allowed. Option B is wrong because the policy allows GetObject. Option C is wrong because public access could be blocked but the policy allows. So D is correct.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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