AWS DVA-C02 Drill: Lambda Function URLs Authentication - Choosing the Right Access Method

Jeff’s Note

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Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Lead Developer.

For AWS DVA-C02 candidates, the confusion often lies in understanding when to use AWS_IAM authentication versus NONE for Lambda Function URLs, and how to correctly assign permissions so only authorized users can invoke functions.

In production, this means knowing exactly how to configure Lambda function URLs with the right auth type combined with appropriate IAM policies to secure and simplify access for your QA or other developer teams. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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At NebulaSoft, a fast-growing SaaS company, the QA team needs to run integration tests on hundreds of AWS Lambda functions. These functions are exposed via Lambda Function URLs allowing HTTPS access. To ensure security and traceability, the development team wants to enable the QA IAM group to invoke all Lambda functions only through these URLs—but only if properly authenticated.

The Requirement

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You must enable the QA IAM group to invoke all these Lambda functions through their respective public Lambda Function URLs. The authentication mechanism must ensure only members of this QA IAM group have invoke permissions and that the invocation via URL is authorized.

The Options

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• A) Create a CLI script that loops through all Lambda functions to add a function URL with the AWS_IAM auth type. Create an IAM identity-based policy allowing the action lambda:InvokeFunctionUrl on all Lambda function ARNs, and attach this policy to the QA IAM group.

• B) Create a CLI script that loops through all Lambda functions to add a function URL with the NONE auth type. Create an IAM resource-based policy that allows the action lambda:InvokeFunctionUrl on all Lambda function ARNs, and attach the policy to the QA IAM group.

• C) Create a CLI script that loops through all Lambda functions to add a function URL with the AWS_IAM auth type. Then loop through the Lambda functions to create an IAM identity-based policy that allows the lambda:InvokeFunctionUrl action from the QA IAM group’s ARN.

• D) Create a CLI script that loops through all Lambda functions to add a function URL with the NONE auth type. Then loop through the Lambda functions to create an IAM resource-based policy that allows the lambda:InvokeFunctionUrl action from the QA IAM group’s ARN.

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Correct Answer

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A

Quick Insight: The Developer Imperative

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When using Lambda Function URLs with AWS_IAM authentication, your IAM users or groups must have an identity-based IAM policy granting lambda:InvokeFunctionUrl on the Lambda resource ARNs.

Choosing NONE for auth makes the function publicly accessible unless restricted by resource-based policies, which can be harder to manage at scale. Option A aligns best with secure, scalable, and manageable invocation patterns for dev workflows.

Content Locked: The Expert Analysis

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You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?

The Expert’s Analysis

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Correct Answer

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Option A

The Winning Logic

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• Lambda function URLs support two authentication types: AWS_IAM and NONE.
• Using AWS_IAM means IAM credentials authenticate requests, enforcing strict authorization via IAM policies on the caller side.
• The correct and scalable method is to configure the function URLs with AWS_IAM auth and grant the QA IAM group an identity-based IAM policy that allows lambda:InvokeFunctionUrl on the specific Lambda ARNs.
• This ensures only authorized QA users can invoke these functions publicly yet securely through URLs.
• Managing permissions as identity-based policies is simpler and aligned with AWS best practices for access control when using AWS_IAM auth.

The Trap (Distractor Analysis)

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• Why not B or D?
  Using NONE auth effectively makes the Lambda function URL publicly accessible. To secure it, you’d need resource-based policies—these do not attach to IAM groups but must be attached to individual Lambda functions, complicating management for hundreds of functions.
• Why not C?
  The difference between A and C lies in the policy scope. C suggests creating identity-based policies per function (looping) referencing the QA group’s ARN, which is ambiguous and unnecessarily complicated.
  Access is controlled via an identity-based policy on the QA IAM group granting lambda:InvokeFunctionUrl on the functions’ ARNs in bulk—not looping per function for the policy on the group itself.

The Technical Blueprint

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Developer CLI Snippet Example to Add Lambda Function URL with AWS_IAM Auth

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# Loop through all Lambda functions and add Lambda Function URL with AWS_IAM auth
for fn in $(aws lambda list-functions --query 'Functions[].FunctionName' --output text); do
  aws lambda create-function-url-config \
    --function-name "$fn" \
    --auth-type AWS_IAM
done

# IAM policy granting lambda:InvokeFunctionUrl on all Lambda ARNs for QA group
cat > qa-lambda-invoke-policy.json <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "lambda:InvokeFunctionUrl",
      "Resource": "arn:aws:lambda:REGION:ACCOUNT_ID:function:*"
    }
  ]
}
EOF

# Attach this policy to QA IAM group
aws iam put-group-policy --group-name QA-Team --policy-name LambdaInvokeURLAccess --policy-document file://qa-lambda-invoke-policy.json

The Comparative Analysis

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Real-World Application (Practitioner Insight)

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Exam Rule

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When you see Lambda Function URLs with AWS_IAM auth, always control access via identity-based IAM policies attached to user groups or roles.

Real World

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In practice, you might use NONE auth combined with resource policies only when you need truly public functions or integration with external services without AWS credentials. But for internal dev/test teams, always prefer AWS_IAM auth for security and auditing.

(CTA) Stop Guessing, Start Mastering

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Disclaimer

This is a study note based on simulated scenarios for the AWS DVA-C02 exam.