Quick Answer
The answer is that the KMS key must be in the same AWS Region as the S3 bucket, and a cross-region key failure occurs when they are mismatched. SSE-KMS enforces a strict regional boundary: the KMS key used for encryption must reside in the exact same region as the S3 bucket storing the objects, because AWS KMS does not support cross-region key operations. On the AWS Certified Security Specialty SCS-C02 exam, this concept tests your understanding of KMS regionality and S3 encryption integration—a common trap is assuming KMS keys can be used globally like IAM roles. Remember the memory tip: “Keys and buckets are regional roommates; they cannot live in different houses.” If your application writes to a bucket in us-east-1 but references a KMS key from eu-west-1, the write will fail with a “key not found” error, making region mismatch the most likely cause of the failure.
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 stores sensitive data in an S3 bucket. The security team wants to ensure that all objects are encrypted at rest using server-side encryption with AWS KMS managed keys (SSE-KMS). An application writes objects to the bucket but sometimes fails because the encryption key is not found. 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 KMS key is in a different AWS Region than the S3 bucket.
SSE-KMS requires the key to be in the same region as the bucket.
The bucket policy does not allow SSE-KMS.
Why wrong: The bucket policy allows SSE-KMS if it's configured.
The KMS key has expired.
Why wrong: KMS customer master keys do not expire.
The KMS key does not have automatic rotation enabled.
Why wrong: Rotation is not required for encryption.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The KMS key is in a different AWS Region than the S3 bucket.
Option D is correct because SSE-KMS requires the KMS key to be in the same region as the S3 bucket; cross-region keys are not supported. Option A is wrong because SSE-S3 is not used. Option B is wrong because KMS keys do not expire. Option C is wrong because SSE-KMS does not require customer master key (CMK) rotation to be enabled.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The KMS key is in a different AWS Region than the S3 bucket.
Why this is correct
SSE-KMS requires the key to be in the same region as the bucket.
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.
The bucket policy does not allow SSE-KMS.
Why it's wrong here
The bucket policy allows SSE-KMS if it's configured.
The KMS key has expired.
Why it's wrong here
KMS customer master keys do not expire.
The KMS key does not have automatic rotation enabled.
Why it's wrong here
Rotation is not required for encryption.
Common exam traps
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
Identify which SCS-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Data Protection — study guide chapter
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FAQ
Data Protection — This question tests Data Protection — Read the scenario before looking for a memorised answer..
The correct answer is: The KMS key is in a different AWS Region than the S3 bucket. — Option D is correct because SSE-KMS requires the KMS key to be in the same region as the S3 bucket; cross-region keys are not supported. Option A is wrong because SSE-S3 is not used. Option B is wrong because KMS keys do not expire. Option C is wrong because SSE-KMS does not require customer master key (CMK) rotation to be enabled.
Identify which SCS-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 20, 2026
This SCS-C02 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 SCS-C02 exam.
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