Introduction

In this guide, we'll explore how to synchronize AWS Secrets Manager values across two different AWS accounts using Lambda and EventBridge.

Setup

We have two AWS accounts, A and B, each running separate e-commerce sites. Account A stores database information in Secrets Manager, and Account B needs to use this data. The goal is to sync secrets from Account A to Account B whenever they change.

Solution Overview

We'll use AWS EventBridge to detect changes in Secrets Manager in Account A and trigger a Lambda function that updates Secrets Manager in Account B.

Steps

Account B Preparation

Create Secrets Manager Secret: Name it super-top-secretB.

Create IAM Role: Allow switching from Account A to operate Secrets Manager in Account B.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::<AccountA-ID>:role/service-role/AccountB-SecretsManager-change-Lambda-role"
            },
            "Action": "sts:AssumeRole"
        }
    ]
}

Attach Policy: Attach SecretsManagerReadWrite policy to the role.

Account A Preparation

Create Lambda Function: This function will read secrets from Account A and write them to Account B.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "sts:AssumeRole",
            "Resource": "arn:aws:iam::<AccountB-ID>:role/secretsmanager-change-role"
        }
    ]
}

Attach Policies: Attach AWSLambdaBasicExecutionRole, SecretsManagerReadWrite, and AccountB-assumerole-policy.

Lambda Function Code

import boto3
import json

def lambda_handler(event, context):
    source_secret_name = "super-top-secretA"
    destination_secret_name = "super-top-secretB"

    client_source = boto3.client('secretsmanager', region_name='eu-west-2')

    sts_client = boto3.client('sts')
    assumed_role = sts_client.assume_role(
        RoleArn="arn:aws:iam::<AccountB-ID>:role/secretsmanager-change-role",
        RoleSessionName="ReplicateSecretSession"
    )
    credentials = assumed_role['Credentials']

    client_destination = boto3.client(
        'secretsmanager',
        region_name='eu-west-2',
        aws_access_key_id=credentials['AccessKeyId'],
        aws_secret_access_key=credentials['SecretAccessKey'],
        aws_session_token=credentials['SessionToken'],
    )

    secret_value = client_source.get_secret_value(SecretId=source_secret_name)['SecretString']

    response = client_destination.put_secret_value(
        SecretId=destination_secret_name,
        SecretString=secret_value
    )

    return {
        'statusCode': 200,
        'body': json.dumps('Secret replicated successfully.')
    }

Create EventBridge Rule

Create an EventBridge rule to detect changes in Secrets Manager in Account A and set the Lambda function as the target.

{
  "source": ["aws.secretsmanager"],
  "detail-type": ["AWS API Call via CloudTrail"],
  "detail": {
    "eventSource": ["secretsmanager.amazonaws.com"],
    "eventName": ["PutSecretValue"],
    "responseElements": {
      "arn": ["arn:aws:secretsmanager:eu-west-2:<AccountA-ID>:secret:super-top-secretA"]
    }
  }
}

Conclusion

By following these steps, you can automate the synchronization of secrets across AWS accounts using EventBridge and Lambda. This approach ensures that secrets in Account B are always up to date with changes in Account A.

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Syncing AWS Secrets Manager across accounts