S3 Bucket Public 'FULL_CONTROL' Access

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Risk level: Very High (act immediately)
Rule ID: S3-005

Ensure that your Amazon S3 buckets are not allowing FULL_CONTROL access to anonymous users (i.e. public access) in order to prevent unauthorized access. A publicly accessible Amazon S3 bucket allows everyone to LIST (READ) the objects within the bucket, UPLOAD/DELETE (WRITE) objects, VIEW (READ_ACP) object permissions, and EDIT (WRITE_ACP) object permissions. Trend Micro Cloud One™ – Conformity strongly recommends against setting all these permissions for the "Everyone (public access)" grantee in production.

This rule can help you with the following compliance standards:

  • PCI
  • 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 FULL_CONTROL access to your Amazon S3 buckets can allow everyone on the Internet to view, upload, modify, and delete S3 objects without any restrictions. Exposing your S3 buckets to the public Internet can lead to data leaks, data loss, and unexpected charges on your AWS bill.


Audit

To determine if your Amazon S3 buckets are exposed to the Internet, perform the following operations:

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). This grantee is a predefined group that allows anonymous users to access your Amazon S3 resources. If the Everyone (public access) grantee has all the permissions enabled, i.e. Objects set to List, Write, and Bucket ACL set to Read, Write, the selected Amazon S3 bucket is publicly accessible, therefore the bucket ACL configuration is not secure and compliant.

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 in 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-production-web-data",
  "cc-project5-audit-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-production-web-data
  --query 'Grants[?(Grantee.URI==`http://acs.amazonaws.com/groups/global/AllUsers`)]'

04 The command output should return the ACL configuration available for the Authenticated users group (anyone with an AWS account) grantee:

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

If the get-bucket-acl command output returns "FULL_CONTROL" for the "Permission" attribute, as shown in the example above, the selected Amazon S3 bucket is publicly exposed to the Internet, therefore the bucket ACL configuration is not secure and compliant.

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 FULL_CONTROL access to your Amazon S3 buckets using Access Control Lists (ACLs), perform the following operations:

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 'FULL_CONTROL' Access",
  "Resources": {
    "SecureS3Bucket": {
      "Properties": {
        "AccessControl": "Private",
        "BucketName": "cc-production-web-data",
        "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 'FULL_CONTROL' Access
Resources:
  SecureS3Bucket:
    Properties:
    AccessControl: Private
    BucketName: cc-production-web-data
    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):

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-production-web-data"
  acl = "private"
}

resource "aws_s3_bucket_public_access_block" "securebucket" {
  bucket = "cc-production-web-data"
  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 bucket.

06 Under Access control list (ACL), deselect all the permissions listed next to the Everyone (public access) grantee, i.e. List, Write from Objects and Read, Write from Bucket ACL, to deny public/anonymous FULL_CONTROL access to 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 publicly accessible Amazon S3 bucket, available in your AWS cloud account.

Using AWS CLI

01 Run put-bucket-acl command (OSX/Linux/UNIX) using the name of the Amazon S3 bucket that you want to reconfigure as the identifier parameter (see Audit section part II to identify the right S3 resource) to deny public FULL_CONTROL access to the selected S3 bucket by removing the permissions set for the Everyone (public access) grantee. The following command request removes all the permissions, i.e. LIST (READ), UPLOAD/DELETE (WRITE), VIEW (READ_ACP), and EDIT (WRITE_ACP), from the Everyone (public access) grantee, for the specified S3 bucket (the command request should not produce an output):

aws s3api put-bucket-acl
  --bucket cc-production-web-data
  --acl private

02 Repeat step no. 1 for each publicly accessible Amazon S3 bucket, available within your AWS cloud account.

References

Publication date May 11, 2016

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S3 Bucket Public 'FULL_CONTROL' Access

Risk level: Very High