The answer is that the KMS key policy does not grant the Lambda execution role permission to use the key. Even though the IAM role attached to the Lambda function includes a kms:Decrypt permission, KMS requires that the key policy explicitly allow the IAM role (or the account principal) to call that action; simply allowing the root user does not automatically extend access to all roles in the account. This scenario tests your understanding of the dual-layer authorization model in AWS KMS, where both the key policy and IAM policy must grant the necessary permissions—a common trap on the DOP-C02 exam. A frequent mistake is assuming that root user access in the key policy covers all IAM roles, but in reality, the key policy must list the execution role’s ARN or use a condition to allow it. Memory tip: think of the key policy as the bouncer at the door—even if your IAM badge says “Decrypt,” the bouncer’s list must include your name.
DOP-C02 Security and Compliance Practice Question
This DOP-C02 practice question tests your understanding of security and compliance. 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 runs a critical application on AWS Lambda that processes sensitive data. The security team mandates that all data must be encrypted at rest and in transit. The Lambda function uses an environment variable to store a database password. The DevOps engineer has enabled encryption of environment variables using a KMS CMK. The Lambda function also needs to decrypt the password at runtime. The engineer attaches an IAM role to the Lambda function with permissions to decrypt using the KMS key. However, when the function executes, it fails with an error 'AccessDeniedException' when trying to decrypt the environment variable. The engineer checks the IAM role and confirms that it has kms:Decrypt permission. The KMS key policy allows the root user full access. 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.
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 policy does not grant the Lambda execution role permission to use the key.
To decrypt environment variables, Lambda uses kms:Decrypt on the key. However, Lambda also needs kms:GenerateDataKey? No. The issue might be that the Lambda function's execution role does not have kms:Decrypt on the specific key alias or ARN. But the key policy allows root user, which includes all IAM users and roles? Actually, root user in the account has full access, but IAM roles are not automatically allowed unless the key policy grants them. The condition in the key policy might be missing. Option A is correct. Option B is about VPC, not relevant. Option C is about the key being disabled, but then error would be different. Option D is about the function code, but the error is access denied.
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 KMS key policy does not grant the Lambda execution role permission to use the key.
Why this is correct
Even if IAM policy allows, key policy must also allow.
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 Lambda function code is trying to decrypt the wrong ciphertext.
Why it's wrong here
Would cause InvalidCiphertextException.
✗
The KMS key has been disabled or scheduled for deletion.
Why it's wrong here
Would cause KeyUnavailableException, not AccessDenied.
✗
The Lambda function is not in a VPC, and the KMS key is restricted to VPC endpoints.
Why it's wrong here
No mention of VPC endpoint restriction.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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
How to think about this question
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.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Identify which DOP-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.
Security and Compliance — This question tests Security and Compliance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The KMS key policy does not grant the Lambda execution role permission to use the key. — To decrypt environment variables, Lambda uses kms:Decrypt on the key. However, Lambda also needs kms:GenerateDataKey? No. The issue might be that the Lambda function's execution role does not have kms:Decrypt on the specific key alias or ARN. But the key policy allows root user, which includes all IAM users and roles? Actually, root user in the account has full access, but IAM roles are not automatically allowed unless the key policy grants them. The condition in the key policy might be missing. Option A is correct. Option B is about VPC, not relevant. Option C is about the key being disabled, but then error would be different. Option D is about the function code, but the error is access denied.
What should I do if I get this DOP-C02 question wrong?
Identify which DOP-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.
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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Question Discussion
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