AWS DVA-C02 Drill: Lambda and S3 Event Triggers - Preventing Cross-Environment Invocation

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 Lambda resource policies vs. IAM execution roles impact cross-account and cross-environment triggers. In production, this is about knowing exactly which AWS policy element governs event source invocation permissions for Lambda, distinct from what governs the Lambda’s data access permissions. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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BlueWave Studios runs multiple software development environments (dev, staging, prod) all inside a single AWS account. They leverage AWS Lambda functions that process image uploads into different Amazon S3 buckets, each bucket associated with a specific environment.

Recently, after deploying new features to production, developers discovered that changes in the dev environment’s S3 buckets triggered the production environment’s Lambda functions. This caused unintended processing of development files by production Lambdas, raising both data integrity and security concerns.

The development team needs a solution that stops dev S3 bucket events from invoking production Lambda functions while adhering to security best practices.

The Requirement:

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Design a secure and effective way to ensure that only the intended environment’s S3 buckets can invoke their respective Lambda functions, preventing cross-environment trigger invocations.

The Options

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• A) Update the IAM execution role of the production Lambda function to add a policy that allows reading only from the production S3 buckets.

• B) Separate dev and production environments into different AWS accounts. Then add a resource-based policy on each Lambda function that allows invocation only from S3 buckets residing in the same account.

• C) Add a resource-based policy to the production Lambda function to allow invocation only by the production S3 buckets.

• D) Separate dev and production environments into different AWS accounts. Update the Lambda execution role policies to limit read permissions only to S3 buckets within the same account.

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Correct Answer

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C

Quick Insight: The DVA Imperative

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• The fundamental distinction is that which entities can invoke a Lambda function is controlled by the Lambda resource-based policy, not the IAM execution role attached to the function.
• The execution role governs what the function can do, like reading from S3, but does NOT control which event sources can trigger the function.
• So, locking down the invocation permissions at the Lambda function’s resource policy level is necessary to prevent cross-environment triggers.

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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• The Lambda function resource-based policy explicitly controls which principals can invoke the function. In this case, the source S3 bucket is the principal invoking Lambda asynchronously on object events.
• By adding a resource policy that only allows the production S3 bucket ARN to invoke the production Lambda function, you prevent any other bucket — including development buckets — from triggering that function.
• IAM execution roles attached to the Lambda function deal with what the Lambda is allowed to do at runtime (e.g., read content from S3 buckets), but they do not restrict the event source invocation permissions.
• Deploying both envs in the same AWS account is supported, but the resource policy must distinguish triggers to prevent cross-environment interference.
• This method follows security best practices by applying the principle of least privilege to event invocation sources.

The Trap (Distractor Analysis):

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• Why not A?
  Updating the execution role only restricts Lambda’s access to S3 objects post-invocation but does not prevent S3 bucket event notifications from invoking the Lambda in the first place.

• Why not B?
  Moving environments into separate AWS accounts is a valid isolation strategy but is often disruptive operationally and not strictly required here. Also, the key control over invocation still happens at the Lambda resource policy, not just the account boundary.

• Why not D?
  Similar to A, execution roles govern the Lambda’s permissions to read S3 data, but do not influence which S3 buckets can invoke the Lambda on events.

The Technical Blueprint

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aws lambda add-permission \
  --function-name production-function \
  --statement-id AllowExecFromProdBucket \
  --action lambda:InvokeFunction \
  --principal s3.amazonaws.com \
  --source-arn arn:aws:s3:::production-bucket \
  --source-account YOUR_ACCOUNT_ID

This AWS CLI command attaches a resource-based policy to the production-function Lambda permitting invocation only from the production-bucket S3 bucket.

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 remember: Lambda invocation permissions are controlled by resource-based policies, not IAM execution roles.”

Real World

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“In practice, isolating environments with separate accounts can simplify this logically but is not always feasible. Resource policies enable fine-grained control within a single 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.