Setting Up Cross-Account S3 Upload with Lambda

📝 Introduction

💡 Brief overview of what you’re setting up and its purpose

This guide outlines how to configure an AWS Lambda function in Account A to upload objects into an S3 bucket located in Account B. This setup enables secure cross-account data transfer, leveraging IAM roles for access control and eliminating the need for hardcoded credentials or sts:AssumeRole calls within the Lambda function code.

👥 Target audience

This guide is intended for AWS administrators and developers who need to implement cross-account S3 uploads using AWS Lambda and IAM roles.

Prerequisites and requirements

AWS Account A (source) and Account B (target).
An S3 bucket named “your-s3-bucket” in Account B.
An AWS Lambda function in Account A.
Basic understanding of IAM roles, policies, and S3 permissions.

🚀 Step-by-Step Setup

Step 1: Configure S3 Bucket Policy in Account B

1. Navigate to the S3 console in Account B.
2. Select the “your-s3-bucket” bucket.
3. Go to the “Permissions” tab.
4. Edit the “Bucket policy.”
5. Replace the existing policy (if any) with the following, replacing [AccountA-Lambda-Execution-Role-ARN] with the ARN of your lambda execution role.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowLambdaAccountA",
            "Effect": "Allow",
            "Principal": {
                "AWS": "[AccountA-Lambda-Execution-Role-ARN]"
            },
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::your-s3-bucket/*"
        }
    ]
}

6. Save the policy.

Step 2: Configure Lambda Execution Role in Account A

1. Navigate to the IAM console in Account A.
2. Select the IAM role attached to your Lambda function.
3. Attach an inline policy or a managed policy with the following:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::your-s3-bucket/*"
        }
    ]
}

4. Ensure the trust relationship of the role includes:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "Service": "lambda.amazonaws.com"
            },
            "Action": "sts:AssumeRole"
        }
    ]
}

Save the role.

Step 3: Update Lambda Function Code

Update your Lambda function code in Account A with the following Python example:

import boto3

def lambda_handler(event, context):
    try:
        account_b_bucket_name = 'your-s3-bucket'
        object_key = 'payload.json'
        file_content = 'This file was uploaded from Lambda in Account A.'

        s3_client = boto3.client('s3')

        s3_client.put_object(
            Bucket=account_b_bucket_name,
            Key=object_key,
            Body=file_content
        )

        return {
            'statusCode': 200,
            'body': 'File uploaded successfully to Account B S3 bucket.'
        }

    except Exception as e:
        print(f"Error: {e}")
        return {
            'statusCode': 500,
            'body': f'Error uploading file: {e}'
        }

Deploy the updated Lambda function.

🧪 Verification and Testing

✅ How to confirm the setup is successful

Check the “your-s3-bucket” S3 bucket in Account B for the uploaded file.
Review Lambda function logs in CloudWatch for successful execution.

🔬 Basic testing procedures

Create a test event in the Lambda console.
Invoke the Lambda function.
Verify the file upload in the S3 bucket.
Check CloudWatch logs for any errors.

Post-Setup Configuration

⚙️ Optional configuration steps

Configure S3 lifecycle policies in Account B for object versioning or deletion.
Set up S3 event notifications in Account B to trigger other processes.

🔒 Security hardening

Follow the principle of least privilege for IAM roles and policies.
Enable S3 encryption.

📈 Performance tuning

Optimize Lambda function code for efficiency.
Consider S3 transfer acceleration if needed.

🎉 Conclusion

Summary of the setup process

This guide successfully configured a Lambda function in Account A to upload objects to an S3 bucket in Account B using IAM roles for secure cross-account access.

⏭️ Next steps and further resources

Explore advanced S3 features and configurations.
Implement error handling and retry mechanisms in the Lambda function.
Monitor Lambda function performance and S3 usage.