The answer is that you must update both the bucket policy and the KMS key policy to grant cross-account access. This is required because S3 bucket policies control access to the S3 object itself, while KMS key policies govern who can decrypt the underlying encryption key; without both, the target account can see the object but cannot decrypt it. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of the layered permission model where S3 and KMS policies must align for cross-account sharing of encrypted objects. A common trap is assuming that a bucket policy alone is sufficient, or that object ACLs can bypass KMS restrictions—neither works because KMS key policies are separate and must explicitly include the external account as a principal. Memory tip: think “Bucket for the door, Key for the lock”—you need permission to both open the door and turn the key.
SCS-C02 Data Protection Practice Question
This SCS-C02 practice question tests your understanding of data protection. 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 company needs to share an encrypted Amazon S3 object with another AWS account. The object is encrypted with an AWS KMS customer managed key. Which steps are required?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Update both the bucket policy and the KMS key policy to grant cross-account access.
Option D is correct because both bucket policy and key policy need to grant cross-account access. Option A is wrong because only bucket policy is not enough; the key policy must also allow. Option B is wrong because key policy alone is insufficient; the bucket policy must allow s3:GetObject. Option C is wrong because object ACLs cannot grant cross-account access when KMS encryption is used.
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.
✗
Use an object ACL to grant the other account read access.
Why it's wrong here
Object ACLs are not sufficient with KMS encryption; also key policy is needed.
✓
Update both the bucket policy and the KMS key policy to grant cross-account access.
Why this is correct
Both policies must allow the external account to access the object and decrypt it.
Related concept
Standard ACLs match source addresses.
✗
Update the bucket policy to allow the other account to access the object.
Why it's wrong here
The key policy must also allow the other account to use the key.
✗
Update the KMS key policy to allow the other account to decrypt.
Why it's wrong here
The bucket policy also needs to allow s3:GetObject.
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 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 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 SCS-C02 ACL questions on filtering logic and placement.
Data Protection — This question tests Data Protection — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Update both the bucket policy and the KMS key policy to grant cross-account access. — Option D is correct because both bucket policy and key policy need to grant cross-account access. Option A is wrong because only bucket policy is not enough; the key policy must also allow. Option B is wrong because key policy alone is insufficient; the bucket policy must allow s3:GetObject. Option C is wrong because object ACLs cannot grant cross-account access when KMS encryption is used.
What should I do if I get this SCS-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 SCS-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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Question Discussion
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