IAM Access Analyzer flags unintended access to S3 buckets shared through access points

Customers use Amazon Simple Storage Service (S3) buckets to store critical data and manage access to data at scale. With Amazon S3 Access Points, customers can easily manage shared data sets by creating separate access points for individual applications. Access points are unique hostnames attached to a bucket and customers can set distinct permissions using access point policies. To help you identify buckets that can be accessed publicly or from other AWS accounts or organizations, AWS Identity and Access Management (IAM) Access Analyzer mathematically analyzes resource policies. Now, Access Analyzer analyzes access point policies in addition to bucket policies and bucket ACLs. This helps you find unintended access to S3 buckets that use access points. Access Analyzer makes it easier to identify and remediate unintended public, cross-account, or cross-organization sharing of your S3 buckets that use access points. This enables you to restrict bucket access and adhere to the security best practice of least privilege.

In this post, first I review Access Analyzer and how to enable it. Then I walk through an example of how to use Access Analyzer to identify an S3 bucket that is shared through an access point. Finally, I show you how to view Access Analyzer bucket findings in the S3 Management Console.

IAM Access Analyzer overview

Access Analyzer helps you determine which resources can be accessed publicly or from other accounts or organizations. Access Analyzer determines this by mathematically analyzing access control policies attached to resources. This form of analysis, called automated reasoning, applies logic and mathematical inference to determine all possible access paths allowed by a resource policy. This is how IAM Access Analyzer uses provable security to deliver comprehensive findings for unintended bucket access. You can enable Access Analyzer by navigating to the IAM console. From there, select Access Analyzer to create an analyzer for an account or an organization.

How to use IAM Access Analyzer to identify an S3 bucket shared through an access point

Once you’ve created your analyzer, you can view findings for resources that can be accessed publicly or from other AWS accounts or organizations. For your S3 bucket findings, the Shared through column indicates whether a bucket is shared through its S3 bucket policy, one of its access points, or the bucket ACL. Looking at the Shared through column in the image below, we see the first finding is shared through an Access point.

Figure 1: IAM Access Analyzer report of findings for resources shared outside of my account

If you use access points to manage bucket access and one of your buckets is shared through an access point, you will see the bucket finding indicate ‘Access Point’. In this example, I select the first finding to learn more. In the detail image below, you can see that the Shared through field lists the Amazon Resource Name (arn) of the access point that grants access to the bucket and the details of the resources and principals. If this access wasn’t your intent, you can review the access point details in the S3 console. There you can modify the access point policy to remove access.

Figure 2: IAM Access Analyzer finding details for a bucket shared through an access point

How to use Access Analyzer for S3 to identify an S3 bucket shared through an access point

You can also view Access Analyzer findings for S3 buckets in the S3 Management Console with Access Analyzer for S3. This view reports S3 buckets that are configured to allow access to anyone on the internet or other AWS accounts. This includes accounts outside of your AWS organization. For each public or shared bucket, Access Analyzer for S3 displays whether the bucket is shared through the bucket policy, access points, or the bucket ACL. In the example below, we see the my-test-public-bucket is set to public access using a Bucket policy and bucket ACL. Additionally, the my-test-bucket is shared access to other AWS accounts using a Bucket policy and one or more access points. After you identify a bucket with unintended access using Access Analyzer for S3, you can Block Public Access to the bucket. Amazon S3 block public access settings override the bucket policies that are applied to the bucket. The settings also override the access point policies applied to the bucket’s access points.

Figure 3: Access Analyzer for S3 findings report in the S3 Management Console

Next steps

To turn on IAM Access Analyzer at no additional cost, head over to the IAM console. IAM Access Analyzer is available in the IAM console and through APIs in all commercial AWS Regions and AWS GovCloud (US). To learn more about IAM Access Analyzer, visit the feature page.

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