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S3 Bucket Public 'READ' Access

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Risk Level: Very High (not tolerated)
Rule ID: S3-001

Ensure that the content of your Amazon S3 buckets can't be listed by anonymous users in order to protect your S3 data against unauthorized access. An Amazon S3 bucket that allows READ (LIST) access to everyone on the Internet will provide anonymous users the ability to list the objects within the bucket and use the information acquired to find potential objects with misconfigured permissions and exploit them.

This rule can help you with the following compliance standards:

  • PCI
  • GDPR
  • APRA
  • MAS
  • NIST4

For further details on compliance standards supported by Conformity, see here.

This rule can help you work with the AWS Well-Architected Framework.

This rule resolution is part of the Conformity Security & Compliance tool for AWS.

Security

Granting public READ access to your Amazon S3 buckets can allow unauthorized users to list all the objects within your buckets and use this information to gain access to your data. To meet security and compliance requirements, avoid granting READ (LIST) permissions to the "Everyone (public access)" grantee in production.


Audit

To determine if your Amazon S3 buckets are configured to allow public READ access, perform the following actions:

Using AWS Console

01 Sign in to the AWS Management Console.

02 Navigate to Amazon S3 console at https://console.aws.amazon.com/s3/.

03 Click on the name (link) of the S3 bucket that you want to examine to access the bucket configuration settings.

04 Select the Permissions tab from the console menu to access the bucket permissions.

05 In the Access control list (ACL) section, check the Access Control List (ACL) configuration settings available for the grantee named Everyone (public access). A grantee can be an AWS account or an S3 predefined group. This grantee is a predefined group that allows anonymous users to access your Amazon S3 resources. If the Everyone (public access) grantee is set to List in the Objects column, the selected Amazon S3 bucket is publicly accessible for content listing, therefore the bucket ACL configuration is not secure.

06 Repeat steps no. 3 – 5 for each Amazon S3 bucket that you want to examine, available in your AWS cloud account.

Using AWS CLI

01 Run list-buckets command (OSX/Linux/UNIX) using custom query filters to list the names of all Amazon S3 buckets available within your AWS cloud account:

aws s3api list-buckets
  --query 'Buckets[*].Name'

02 The command output should return an array with the requested bucket names:

[
  "cc-internal-audit-reports",
  "cc-development-data-logs"
]

03 Run get-bucket-acl command (OSX/Linux/UNIX) using the name of the Amazon S3 bucket that you want to examine as the identifier parameter to describe the Access Control List (ACL) configuration set for the Everyone (public access) grantee, available for the selected S3 bucket:

aws s3api get-bucket-acl
  --bucket cc-internal-audit-reports
  --query 'Grants[?(Grantee.URI==`http://acs.amazonaws.com/groups/global/AllUsers`)]'

04 The command output should return the ACL configuration available for the Everyone (public access) grantee:

[
  {
    "Grantee": {
      "Type": "Group",
      "URI": "http://acs.amazonaws.com/groups/global/AllUsers"
    },
    "Permission": "READ"
  }
]

If the get-bucket-acl command output returns "READ" for the "Permission" attribute value, as shown in the example above, the selected Amazon S3 bucket is publicly accessible for content (object) listing, therefore the bucket ACL configuration is not secure.

05 Repeat steps no. 3 and 4 for each Amazon S3 bucket that you want to examine, available within your AWS cloud account.

Remediation / Resolution

To deny public READ access to your Amazon S3 buckets using Access Control Lists (ACLs), perform the following actions:

Note: An S3 bucket can be deemed compliant if implements either "AccessControl": "Private" or sets the "PublicAccessBlockConfiguration" feature options to true. The following CloudFormation template uses both for added security.

Using AWS CloudFormation

01 CloudFormation template (JSON):

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Description": "S3 Bucket Public 'READ' Access",
  "Resources": {
    "SecureS3Bucket": {
      "Properties": {
        "AccessControl": "Private",
        "BucketName": "cc-internal-audit-reports",
        "PublicAccessBlockConfiguration": {
          "BlockPublicAcls": true,
          "IgnorePublicAcls": true,
          "BlockPublicPolicy": true,
          "RestrictPublicBuckets": true
        }
      },
      "Type": "AWS::S3::Bucket"
    }
  }
}

02 CloudFormation template (YAML):

AWSTemplateFormatVersion: '2010-09-09'
Description: S3 Bucket Public 'READ' Access
Resources:
  SecureS3Bucket:
    Properties:
      AccessControl: Private
      BucketName: cc-internal-audit-reports
      PublicAccessBlockConfiguration:
        BlockPublicAcls: true
        IgnorePublicAcls: true
        BlockPublicPolicy: true
        RestrictPublicBuckets: true
    Type: 'AWS::S3::Bucket'

Note: An S3 bucket can be deemed compliant if implements either acl = "private" or sets the "aws_s3_bucket_public_access_block" feature options to true. The following Terraform configuration file uses both for added security.

Using Terraform

01 Terraform configuration file (.tf):
Note: A bucket can be deemed compliant if it uses either "acl": "private" or "aws_s3_bucket_public_access_block" settings set to true. The example template uses both for added security.

terraform {
  required_providers {
    aws = {
      source = "hashicorp/aws"
      version = "~> 3.27"
    }
  }

  required_version = ">= 0.14.9"
}

provider "aws" {
  profile = "default"
  region = "us-east-1"
}

resource "aws_s3_bucket" "securebucket" {
  bucket = "cc-internal-audit-reports"
  acl = "private"
}

resource "aws_s3_bucket_public_access_block" "securebucket" {
  bucket = "cc-internal-audit-reports"
  block_public_acls = true
  ignore_public_acls = true
  block_public_policy = true
  restrict_public_buckets = true
}

Using AWS Console

01 Sign in to the AWS Management Console.

02 Navigate to Amazon S3 console at https://console.aws.amazon.com/s3/.

03 Click on the name of the S3 bucket that you want to reconfigure.

04 Select the Permissions tab from the console menu to access the bucket permissions.

05 In the Access control list (ACL) section, choose Edit to modify the Access Control List (ACL) configuration available for the selected S3 bucket.

06 Under Access control list (ACL), deselect the List permission checkbox available next to the Everyone (public access) grantee, to remove the LIST (READ) permissions for anonymous users, from the selected Amazon S3 bucket. Select I understand the effects of these changes on my objects and buckets checkbox for confirmation, then choose Save changes to apply the changes.

07 Repeat steps no. 3 – 6 for each Amazon S3 bucket that allows public READ access, available in your AWS cloud account.

Using AWS CLI

01 Run get-bucket-acl command (OSX/Linux/UNIX) using the name of the Amazon S3 bucket that you want to reconfigure as the identifier parameter, to deny public/anonymous READ access to the selected S3 bucket by removing the LIST (READ) permissions set for the Everyone (public access) grantee. The following command request uses the PRIVATE canned ACL to remove the LIST (READ) permissions for the specified S3 bucket (if successful, the command does not produce an output):

aws s3api put-bucket-acl
  --bucket cc-internal-audit-reports
  --acl private

02 Repeat step no. 1 for each Amazon S3 bucket that allows public READ access, available within your AWS cloud account.

References

Publication date May 11, 2016

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S3 Bucket Public 'READ' Access

Risk Level: Very High