The correct answer is that the Lambda function’s execution role lacks the `sts:AssumeRole` permission for the target role it is trying to assume. This error occurs because the Lambda authorizer’s code is explicitly calling the AWS Security Token Service (STS) to assume a different IAM role—perhaps to access another AWS service with elevated permissions—but the function’s own execution role has not been granted the `sts:AssumeRole` action on that specific role’s trust policy. On the AWS Certified Developer Associate DVA-C02 exam, this question tests your understanding of IAM role chaining and the principle of least privilege: a Lambda function can only assume another role if its execution role includes an IAM policy allowing `sts:AssumeRole` for that role’s ARN. A common trap is confusing the Lambda invocation permission (which API Gateway needs) with the STS assume-role permission, or assuming the authorizer automatically inherits the target role’s permissions. Remember the mnemonic: “Assume needs Allow”—for any role assumption, the caller’s role must explicitly allow the `sts:AssumeRole` action on the target role.
DVA-C02 Troubleshooting and Optimization Practice Question
This DVA-C02 practice question tests your understanding of troubleshooting and optimization. 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 developer is using Amazon API Gateway with a Lambda authorizer to control access to APIs. The authorizer is failing with a 500 error. The Lambda function logs show 'User: arn:aws:iam::123456789012:role/MyLambdaRole is not authorized to perform: sts:AssumeRole'. 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 Lambda function's execution role does not have sts:AssumeRole permission for the target role.
The Lambda function's execution role needs permission to call sts:AssumeRole for the authorizer's role? Actually, the error indicates that the Lambda function's role is trying to assume another role (perhaps the authorizer's role) but is not allowed. In API Gateway Lambda authorizer, the Lambda function is invoked directly by API Gateway; the function does not need to assume a role unless it calls other services. The error may be from within the Lambda code trying to assume a role. Option A is wrong because invoke permission is for API Gateway to invoke Lambda, not for Lambda to assume role. Option B is wrong because the Lambda authorizer does not need to assume a role by default. Option C is correct: the Lambda function's code is attempting to assume an IAM role (maybe to call another service) but the function's execution role lacks sts:AssumeRole permission for that role. Option D is wrong because the Lambda function's resource-based policy is for cross-account access, not for assuming roles.
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 Lambda authorizer is not returning a valid policy.
Why it's wrong here
Invalid policy would cause a 401 or 403, not a 500 with AssumeRole error.
✗
The Lambda function's resource-based policy is missing.
Why it's wrong here
Resource-based policy is for granting invoke permissions to other accounts, not for assuming roles.
✗
The API Gateway does not have permission to invoke the Lambda function.
Why it's wrong here
Invoke permission is configured separately; the error indicates the Lambda function is running but failing.
✓
The Lambda function's execution role does not have sts:AssumeRole permission for the target role.
Why this is correct
The error shows the Lambda function's role is trying to assume another role but lacks permission.
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.
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 DVA-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.
Troubleshooting and Optimization — This question tests Troubleshooting and Optimization — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Lambda function's execution role does not have sts:AssumeRole permission for the target role. — The Lambda function's execution role needs permission to call sts:AssumeRole for the authorizer's role? Actually, the error indicates that the Lambda function's role is trying to assume another role (perhaps the authorizer's role) but is not allowed. In API Gateway Lambda authorizer, the Lambda function is invoked directly by API Gateway; the function does not need to assume a role unless it calls other services. The error may be from within the Lambda code trying to assume a role. Option A is wrong because invoke permission is for API Gateway to invoke Lambda, not for Lambda to assume role. Option B is wrong because the Lambda authorizer does not need to assume a role by default. Option C is correct: the Lambda function's code is attempting to assume an IAM role (maybe to call another service) but the function's execution role lacks sts:AssumeRole permission for that role. Option D is wrong because the Lambda function's resource-based policy is for cross-account access, not for assuming roles.
What should I do if I get this DVA-C02 question wrong?
Identify which DVA-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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