AWS: Grant CloudFormation in Another Account S3 Read Access

Table of Contents#

1. Core Concepts
2. Typical Usage Scenarios
3. Common Practice
4. Best Practices
5. Conclusion
6. FAQ
7. References

Core Concepts#

AWS CloudFormation#

AWS CloudFormation is a service that helps you model and set up your AWS resources so that you can spend less time managing those resources and more time focusing on your applications that run in AWS. It uses templates (JSON or YAML files) to describe and provision all the resources in your stack.

Amazon S3#

Amazon Simple Storage Service (S3) is an object storage service that offers industry-leading scalability, data availability, security, and performance. It allows you to store and retrieve any amount of data at any time, from anywhere on the web.

Cross - Account Access#

Cross - account access in AWS enables you to share resources between different AWS accounts. This is achieved through the use of Identity and Access Management (IAM) policies, which define who can access which resources and what actions they can perform.

CloudFormation Service Role#

A CloudFormation service role is an IAM role that CloudFormation assumes when performing stack operations. By using a service role, you can control the permissions that CloudFormation has when interacting with other AWS resources.

Typical Usage Scenarios#

Centralized Template Storage#

You may have a central AWS account where you store all your CloudFormation templates in an S3 bucket. Other accounts can then use these templates to create their own stacks. This approach helps in maintaining a single source of truth for your templates and simplifies version control.

Shared Configuration Data#

If you have configuration data that is used across multiple accounts, you can store it in an S3 bucket in one account and grant other accounts' CloudFormation stacks read access to this data. For example, you might have a common set of security group rules or network configurations that are stored in S3 and used by multiple CloudFormation stacks.

Deployment Automation#

In a large organization, you may have a dedicated DevOps or infrastructure team that manages the deployment of CloudFormation stacks. They can store all the necessary artifacts in an S3 bucket in their account and grant other accounts' CloudFormation stacks access to these artifacts for automated deployment.

Common Practice#

Step 1: Create an IAM Role in the Destination Account#

In the account where the S3 bucket is located, create an IAM role that allows CloudFormation in the other account to assume it. This role should have permissions to read from the S3 bucket.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::SOURCE_ACCOUNT_ID:root"
            },
            "Action": "sts:AssumeRole",
            "Condition": {}
        }
    ]
}

Step 2: Attach an S3 Read Policy to the IAM Role#

Attach a policy to the IAM role that allows read access to the S3 bucket.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "s3:GetObject",
                "s3:ListBucket"
            ],
            "Resource": [
                "arn:aws:s3:::YOUR_BUCKET_NAME",
                "arn:aws:s3:::YOUR_BUCKET_NAME/*"
            ]
        }
    ]
}

Step 3: Use the IAM Role in the Source Account's CloudFormation Stack#

In the CloudFormation template in the source account, specify the ARN of the IAM role created in the destination account as the RoleARN property.

AWSTemplateFormatVersion: '2010-09-09'
Resources:
  MyStack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: https://s3.amazonaws.com/YOUR_BUCKET_NAME/YOUR_TEMPLATE.json
      RoleARN: arn:aws:iam::DESTINATION_ACCOUNT_ID:role/YOUR_ROLE_NAME

Best Practices#

Least Privilege Principle#

Only grant the minimum permissions necessary for CloudFormation to perform its tasks. For example, if CloudFormation only needs to read specific objects in the S3 bucket, limit the permissions to those objects instead of granting full access to the entire bucket.

Regularly Review and Update Permissions#

As your AWS environment evolves, regularly review and update the IAM policies and roles to ensure that they still meet your security requirements. Remove any unnecessary permissions and add new ones as needed.

Use Multi - Factor Authentication (MFA)#

Enable MFA for the IAM users and roles that have access to the S3 bucket. This adds an extra layer of security and helps prevent unauthorized access.

Monitor and Log Access#

Use AWS CloudTrail to monitor and log all access to the S3 bucket and CloudFormation stacks. This allows you to detect and respond to any suspicious activity in a timely manner.

Conclusion#

Granting AWS CloudFormation in another account S3 read access is a powerful feature that enables resource sharing and collaboration across multiple AWS accounts. By understanding the core concepts, typical usage scenarios, common practices, and best practices, software engineers can effectively manage cross - account access to S3 buckets and ensure the security and efficiency of their CloudFormation stack deployments.

FAQ#

Q: Can I grant write access to the S3 bucket as well?#

A: Yes, you can modify the IAM policy to include write actions such as s3:PutObject if you need CloudFormation in the other account to have write access to the S3 bucket. However, be cautious when granting write access as it can pose security risks.

Q: What if the IAM role in the destination account is deleted?#

A: If the IAM role is deleted, CloudFormation in the source account will no longer be able to assume the role and access the S3 bucket. You will need to recreate the role and update the RoleARN in the CloudFormation template.

Q: Can I use a different service principal instead of arn:aws:iam::SOURCE_ACCOUNT_ID:root?#

A: Yes, you can use a more specific IAM user or role in the source account as the principal. This provides more fine - grained control over who can access the S3 bucket.

References#

• AWS CloudFormation Documentation
• Amazon S3 Documentation
• AWS IAM Documentation