AWS DVA-C02 Drill: Lambda S3 Trigger Security - Prevent Cross-Environment Invocations

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 how Lambda resource policies govern invocation permissions vs. the Lambda execution role’s resource access policies. In production, this is about knowing exactly which policies control event source permissions and which control runtime permissions within the function’s context. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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Imagine you are a lead developer at a digital marketing startup, BrightPixel Labs. BrightPixel Labs uses AWS Lambda functions triggered by Amazon S3 events to process image assets uploaded to various S3 buckets. To streamline development, the team set up multiple environments—development, staging, and production—all within a single AWS account. Recently, after deploying new code to production, the team noticed an alarming issue: S3 buckets associated with the development environment are triggering the production Lambda functions. This causes unintended processing of development files in production workflows.

The Requirement:

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Prevent S3 buckets in development from invoking production Lambda functions, strictly following AWS security best practices and minimizing operational overhead.

The Options

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• A) Update the Lambda execution role for the production Lambda function by adding a policy statement that allows the execution role to read only from the production environment S3 bucket.

• B) Separate the development and production environments into different AWS accounts. Add resource-based policies to each Lambda function so that only S3 buckets within the same AWS account can invoke that function.

• C) Add a resource-based policy to the production Lambda function that explicitly allows invocation only from the designated production S3 bucket.

• D) Move development and production environments into separate AWS accounts. Update the Lambda execution role policies for each function to allow reading only from the S3 bucket within the same AWS account.

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Correct Answer

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C

Quick Insight: The Lambda Resource Policy Imperative

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• Lambda function invocation needs to be controlled at the resource policy level (who can invoke the Lambda), not just at the execution role level (what resources Lambda can access at runtime).
• For AWS DVA-C02, understanding this distinction between invocation permissions (resource policies) and runtime permissions (execution role) is crucial.

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 C

The Winning Logic

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This solution addresses the core problem by adding a resource-based policy on the production Lambda function. This policy explicitly allows invocations only from the designated production S3 bucket’s ARN. This approach leverages Lambda’s resource policy to restrict which AWS principals (in this case S3 buckets via SNS event invocation) can trigger the function.

• The Lambda execution role governs what the function can do once invoked (e.g., read from certain S3 buckets), but does not limit who can trigger the Lambda function.
• By adding a resource policy that allows invocation only from the production S3 bucket, you ensure that events from development buckets cannot invoke production Lambdas.
• This method follows AWS security best practices by implementing least privilege and clear separation within the same account environment.

The Trap (Distractor Analysis):

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• Why not A?
  Updating the execution role only controls runtime permissions, not invocation permissions. Development S3 buckets can still trigger the Lambda; the role just may not have permission to read from dev buckets, which causes failures but does not prevent invocation.

• Why not B?
  Splitting environments across accounts is a heavy-handed operational change that may not be feasible. Also, setting resource policies by account does not inherently prevent identical bucket ARNs from triggering Lambdas unless strictly controlled. While viable, it is an overhead-heavy solution.

• Why not D?
  This combines the complexity of account segregation (which is a large operational change) with execution role policies affecting runtime access only. It doesn’t directly address invocation permissions where the problem lies.

The Technical Blueprint

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# Example Lambda resource policy restricting invocation to a specific S3 bucket ARN
aws lambda add-permission \
  --function-name ProductionImageProcessor \
  --statement-id AllowProdBucketInvoke \
  --action lambda:InvokeFunction \
  --principal s3.amazonaws.com \
  --source-arn arn:aws:s3:::production-images-bucket \
  --source-account 123456789012

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 Lambda resource policies to manage who or what can invoke your Lambda function, especially in multi-environment setups sharing an AWS account.

Real World

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In reality, many teams keep dev and prod in separate AWS accounts (Option B or D) for stricter isolation and compliance, but resource-based policies (Option C) are the simplest and fastest method to restrict S3 event invocation sources within the same account.

(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.