- A
Add kms:Decrypt permissions to the identity policy in account B only, without modifying the CMK key policy in account A.
Why wrong: Identity permissions alone are often insufficient when cross-account KMS access is involved. KMS key policies control whether principals can use the key.
- B
Update the CMK key policy in account A to allow the account B role principal to call kms:Decrypt (and kms:DescribeKey if needed).
KMS customer-managed keys rely on key policies (especially for cross-account access). Granting kms:Decrypt to the exact account B role principal in the key policy enables successful decrypt operations for SSE-KMS objects.
- C
Disable SSE-KMS on the S3 bucket so objects use SSE-S3 instead, eliminating the need for KMS permissions.
Why wrong: This removes KMS-based control rather than correcting permissions. It also may violate the requirement to use the customer-managed CMK.
- D
Attach a broad permissions boundary to the account B role allowing all kms:* actions to override the key policy.
Why wrong: Permissions boundaries are a control on identity policies, not a way to override a KMS key policy. Broad kms:* permissions would still be blocked if the key policy denies.
Quick Answer
The answer is to update the CMK key policy in account A to allow the account B role principal to call kms:Decrypt. This is necessary because when an S3 bucket uses SSE-KMS with a customer-managed key, the KMS decrypt operation is a separate authorization step from the S3 GetObject call; even if the bucket policy and IAM role permit the read, the cross-account KMS key policy acts as a hard gate that must explicitly grant the external principal decrypt permission. On the SAA-C03 exam, this scenario tests your understanding that KMS key policies are resource-based and must explicitly include cross-account principals—a common trap is assuming the IAM role’s permissions alone suffice, or forgetting that the key policy in the owning account is the sole mechanism for cross-account KMS access. Remember the memory tip: “S3 opens the door, but KMS holds the key—you need both permissions for cross-account access.”
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.
An S3 bucket in account A uses default server-side encryption with an AWS KMS customer-managed key (CMK) in account A. A team created an IAM role in account B that is allowed by IAM policy to perform s3:GetObject on the bucket. When the account B role tries to read objects, it fails with: AccessDeniedException: 'User is not authorized to perform kms:Decrypt'. Which change is most likely to fix the issue?
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.
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
Update the CMK key policy in account A to allow the account B role principal to call kms:Decrypt (and kms:DescribeKey if needed).
When an S3 bucket uses SSE-KMS with a customer-managed key (CMK) in account A, the account B role must have explicit kms:Decrypt permission on that CMK. The key policy in account A controls access to the CMK, so adding the account B role principal to the key policy with kms:Decrypt (and kms:DescribeKey if needed) is required. Without this, even if the S3 bucket policy and IAM role allow s3:GetObject, the KMS decrypt call will fail.
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.
- ✗
Add kms:Decrypt permissions to the identity policy in account B only, without modifying the CMK key policy in account A.
Why it's wrong here
Identity permissions alone are often insufficient when cross-account KMS access is involved. KMS key policies control whether principals can use the key.
- ✓
Update the CMK key policy in account A to allow the account B role principal to call kms:Decrypt (and kms:DescribeKey if needed).
Why this is correct
KMS customer-managed keys rely on key policies (especially for cross-account access). Granting kms:Decrypt to the exact account B role principal in the key policy enables successful decrypt operations for SSE-KMS objects.
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.
- ✗
Disable SSE-KMS on the S3 bucket so objects use SSE-S3 instead, eliminating the need for KMS permissions.
Why it's wrong here
This removes KMS-based control rather than correcting permissions. It also may violate the requirement to use the customer-managed CMK.
- ✗
Attach a broad permissions boundary to the account B role allowing all kms:* actions to override the key policy.
Why it's wrong here
Permissions boundaries are a control on identity policies, not a way to override a KMS key policy. Broad kms:* permissions would still be blocked if the key policy denies.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume IAM permissions in account B are sufficient for cross-account KMS operations, forgetting that the KMS key policy in the owning account must explicitly grant access to the external principal.
Detailed technical explanation
How to think about this question
AWS KMS uses a dual authorization model: the key policy (resource-based policy) must grant access to the principal, and the principal must have matching IAM permissions. For cross-account access, the key policy must explicitly specify the external account or role ARN, as IAM policies in the external account cannot grant access to a CMK in another account. Additionally, the S3 service calls kms:Decrypt on behalf of the requester, so the role in account B must be authorized both by the S3 bucket policy (or IAM) and the KMS key policy.
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.
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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: Update the CMK key policy in account A to allow the account B role principal to call kms:Decrypt (and kms:DescribeKey if needed). — When an S3 bucket uses SSE-KMS with a customer-managed key (CMK) in account A, the account B role must have explicit kms:Decrypt permission on that CMK. The key policy in account A controls access to the CMK, so adding the account B role principal to the key policy with kms:Decrypt (and kms:DescribeKey if needed) is required. Without this, even if the S3 bucket policy and IAM role allow s3:GetObject, the KMS decrypt call will fail.
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.
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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Last reviewed: Jun 11, 2026
This SAA-C03 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SAA-C03 exam.
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