AWS DVA-C02 Drill: Secure Lambda Deployment Across Accounts - Least Privilege Access to S3

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 DVA-C02 candidates, the confusion often lies in how to grant cross-account access to Lambda code stored in S3 without exposing the bucket unnecessarily. In production, this boils down to precisely controlling permissions using least privilege—knowing which IAM roles or service principals require which S3 permissions and how bucket policies enforce that. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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TechNova Solutions is deploying a set of AWS Lambda functions across multiple AWS accounts within the same Region. All Lambda deployment packages (code archives) are stored in a central Amazon S3 bucket owned by the “master” account. Deployment is automated using AWS CloudFormation stacks executed in each target account.

The Requirement:

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Determine the MOST secure method to allow the CloudFormation service role in each target account to access the Lambda code stored in the centralized S3 bucket, ensuring limited and auditable access consistent with best security practices.

The Options

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  - A) Grant the CloudFormation service role both the S3 `ListBucket` and `GetObject` permissions. Attach a bucket policy to the S3 bucket granting permissions to the specific target account IDs.
  - B) Grant the CloudFormation service role only the S3 `GetObject` permission. Attach a bucket policy to the S3 bucket that allows `"Principal": "*"` (public access).
  - C) Use AWS resource-based policies (service-linked roles) to give the Lambda function `ListBucket` and `GetObject` permissions, explicitly referencing the bucket owner’s account ID in the resource ARNs.
  - D) Use AWS resource-based policies (service-linked roles) to give the Lambda function only `GetObject` permission, specifying `"Resource": "*"` in the S3 bucket policy.

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Correct Answer

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A

Quick Insight: The Developer Imperative

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• Least privilege means granting only the necessary permissions — here, GetObject and sometimes ListBucket to help CloudFormation locate the artifact.
• Tight bucket policies scoped to the specific target accounts prevent unintended exposure.
• Avoid "Principal": "*" in bucket policies unless it’s a public bucket (which should be avoided for code artifacts).

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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This approach adheres strictly to the principle of least privilege by:

• Granting the CloudFormation service role in each target account the minimal necessary S3 permissions (ListBucket and GetObject) to read the Lambda code artifact.
• Restricting bucket access via an explicit bucket policy granting permissions only to the known AWS account principals, preventing broader or public access.
• Ensuring the Lambda function deployment is secure and auditable without exposing the bucket to unwanted actors.

The Trap (Distractor Analysis):

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• Why not B?
  Allowing "Principal": "*" makes the bucket publicly accessible, which exposes the deployment code to everyone — a massive security risk.

• Why not C?
  Granting Lambda functions direct S3 permissions is not sufficient because the deployment is done by CloudFormation stacks, which need access during deployment. Also, Lambda does not inherently need ListBucket permission.

• Why not D?
  Using "Resource": "*" in a bucket policy opens too broad access and violates least privilege.

The Technical Blueprint

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# Example S3 Bucket Policy allowing only specified target accounts to GetObject and ListBucket
aws s3api put-bucket-policy --bucket technova-lambda-code \
--policy '{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowCloudFormationAccess",
      "Effect": "Allow",
      "Principal": {
        "AWS": [
          "arn:aws:iam::111122223333:role/CloudFormationServiceRole",
          "arn:aws:iam::444455556666:role/CloudFormationServiceRole"
        ]
      },
      "Action": [
        "s3:GetObject",
        "s3:ListBucket"
      ],
      "Resource": [
        "arn:aws:s3:::technova-lambda-code",
        "arn:aws:s3:::technova-lambda-code/*"
      ]
    }
  ]
}'

The Comparative Analysis

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Real-World Application (Practitioner Insight)

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Exam Rule

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For the exam, always pick bucket policies scoped to specific accounts when securing artifacts for cross-account pulls.

Real World

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In production, teams often automate bucket policy updates using CI/CD pipelines to reflect target account principals dynamically, maintaining strict least privilege while onboarding new accounts.

(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.