s3_block_account_public_access

S3 Block Account Public Access

Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved. SPDX-License-Identifier: CC-BY-SA-4.0

Table of Contents

• Introduction
• Deployed Resource Details
• Implementation Instructions
• References

Introduction

The S3 block account public access solution enables the S3 account level settings within each AWS account in the AWS Organization.

The Amazon S3 Block Public Access feature provides settings for access points, buckets, and accounts to help you manage public access to Amazon S3 resources. By default, new buckets, access points, and objects don't allow public access. However, users can modify bucket policies, access point policies, or object permissions to allow public access. S3 Block Public Access settings override these policies and permissions so that you can limit public access to these resources.

With S3 Block Public Access, account administrators and bucket owners can easily set up centralized controls to limit public access to their Amazon S3 resources that are enforced regardless of how the resources are created.

Key solution features:

• Sets S3 block account public access settings for all existing accounts including the management account and future accounts.
• Ability to exclude accounts via provided account tags.
• Triggered when new accounts are added to the AWS Organization, account tag updates, and on account status changes.

Block public access settings

S3 Block Public Access provides four settings. This solution applies the settings to the account, which applies to all buckets and access points that are owned by that account.

• BlockPublicAcls
  • Setting this option to TRUE causes the following behavior:
    • PUT Bucket acl and PUT Object acl calls fail if the specified access control list (ACL) is public.
    • PUT Object calls fail if the request includes a public ACL.
    • If this setting is applied to an account, then PUT Bucket calls fail if the request includes a public ACL.
• IgnorePublicAcls
  • Setting this option to TRUE causes Amazon S3 to ignore all public ACLs on a bucket and any objects that it contains.
• BlockPublicPolicy
  • Setting this option to TRUE for a bucket causes Amazon S3 to reject calls to PUT Bucket policy if the specified bucket policy allows public access, and to reject calls to PUT access point policy for all of the bucket's access points if the specified policy allows public access.
• RestrictPublicBuckets
  • Setting this option to TRUE restricts access to an access point or bucket with a public policy to only AWS service principals and authorized users within the bucket owner's account. This setting blocks all cross-account access to the access point or bucket (except by AWS service principals), while still allowing users within the account to manage the access point or bucket.

---

Deployed Resource Details

1.0 Control Tower Management Account

1.1 AWS CloudFormation

• All resources are deployed via AWS CloudFormation as a StackSet and Stack Instance within the management account or a CloudFormation Stack within a specific account.
• The Customizations for AWS Control Tower solution deploys all templates as a CloudFormation StackSet.
• For parameter details, review the AWS CloudFormation templates.

1.2 IAM Roles

• The Lambda IAM Role is used by the Lambda function to identify existing and future accounts that need S3 Block Account Public Access configured.
• The S3 Block Account Public Access IAM Role is assumed by the Lambda function to set S3 Block Account Public Access for the management account and the member accounts.
• The Event Rule IAM Role is assumed by EventBridge to forward Global events to the Home Region default Event Bus.

1.3 Regional Event Rules

• The AWS Control Tower Lifecycle Event Rule triggers the AWS Lambda Function when a new AWS Account is provisioned through AWS Control Tower.
• The Organization Compliance Scheduled Event Rule triggers the AWS Lambda Function to capture AWS Account status updates (e.g. suspended to active).
  • A parameter is provided to set the schedule frequency.
  • See the Instructions to Manually Run the Lambda Function for triggering the AWS Lambda Function before the next scheduled run time.
• The AWS Organizations Event Rule triggers the AWS Lambda Function when updates are made to accounts within the organization.
  • When AWS Accounts are added to the AWS Organization outside of the AWS Control Tower Account Factory. (e.g. account created via AWS Organizations console, account invited from another AWS Organization).
  • When tags are added or updated on AWS Accounts.

1.4 Global Event Rules

• If the Home Region is different from the Global Region (e.g. us-east-1), then global event rules are created within the Global Region to forward events to the Home Region default Event Bus.
• The AWS Organizations Event Rule forwards AWS Organization account update events.

1.5 Dead Letter Queue (DLQ)

• SQS dead letter queue used for retaining any failed Lambda events.

1.6 AWS Lambda Function

• The AWS Lambda Function contains the logic for configuring the S3 block public access settings within each account.

1.7 Lambda CloudWatch Log Group

• All the AWS Lambda Function logs are sent to a CloudWatch Log Group </aws/lambda/<LambdaFunctionName> to help with debugging and traceability of the actions performed.
• By default the AWS Lambda Function will create the CloudWatch Log Group and logs are encrypted with a CloudWatch Logs service managed encryption key.
• Parameters are provided for changing the default log group retention and encryption KMS key.

1.8 Alarm SNS Topic

• SNS Topic used to notify subscribers when messages hit the Dead Letter Queue (DLQ).

1.9 SNS Topic

• SNS Topic used to fanout the Lambda function for setting the S3 block account public access configuration.

1.10 S3 Account Settings

• The AWS Lambda Function configures the block public access settings for the account.

---

2.0 All Existing and Future Organization Member Accounts

2.1 AWS CloudFormation

• See 1.1 AWS CloudFormation

2.2 S3 Block Account Public Access IAM Role

• The S3 Block Account Public Access IAM Role is assumed by the Lambda function within the management account to set S3 Block Account Public Access for the account.

2.3 S3 Account Settings

• See 1.10 S3 Account Settings

---

Implementation Instructions

Prerequisites

1. Download and Stage the SRA Solutions. Note: This only needs to be done once for all the solutions.
2. Verify that the SRA Prerequisites Solution has been deployed.
3. No AWS Organizations Service Control Policies (SCPs) are blocking the s3:GetAccountPublicAccessBlock and s3:PutAccountPublicAccessBlock API actions

Solution Deployment

Choose a Deployment Method:

• AWS CloudFormation
• Customizations for AWS Control Tower

AWS CloudFormation

In the management account (home region), launch the sra-s3-block-account-public-access-main-ssm.yaml template. This uses an approach where some of the CloudFormation parameters are populated from SSM parameters created by the SRA Prerequisites Solution.

aws cloudformation deploy --template-file $HOME/aws-sra-examples/aws_sra_examples/solutions/s3/s3_block_account_public_access/templates/sra-s3-block-account-public-access-main-ssm.yaml --stack-name sra-s3-block-account-public-access-main-ssm --capabilities CAPABILITY_NAMED_IAM

Verify Solution Deployment

How to verify after the pipeline completes?

1. Log into an account and navigate to the S3 console page
2. Select the Block Public Access settings for this account in the side menu and verify the settings match the parameters provided in the configuration

Solution Update Instructions

1. Download and Stage the SRA Solutions. Note: Get the latest code and run the staging script.
2. Update the existing CloudFormation Stack or CFCT configuration. Note: Make sure to update the SRA Solution Version parameter and any new added parameters.

Solution Delete Instructions

1. In the management account (home region), delete the AWS CloudFormation Stack (sra-s3-block-account-public-access-main-ssm or sra-s3-block-account-public-access-main) created above.
2. In the management account (home region), delete the AWS CloudWatch Log Group (e.g. /aws/lambda/<solution_name>) for the Lambda function deployed.

Instructions to Manually Run the Lambda Function

1. In the management account (home region).
2. Navigate to the AWS Lambda Functions page.
3. Select the checkbox next to the Lambda Function and select Test from the Actions menu.
4. Scroll down to view the Test event.
5. Click the Test button to trigger the Lambda Function with the default values.
6. Verify that the updates were successful within the expected account(s).

---

References

• Blocking public access to your Amazon S3 storage