S3 Bucket Public 'WRITE' ACL Access

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 Write in the Objects column, the selected Amazon S3 bucket is publicly accessible for unrestricted content updates (add, delete, and/or replace S3 objects), 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.

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-production-access-logs",
  "cc-internal-audit-reports"
]

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-access-logs
  --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": "WRITE"
  }
]

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

01 CloudFormation template (JSON):

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Description": "S3 Bucket Public 'WRITE' Access",
  "Resources": {
    "SecureS3Bucket": {
      "Properties": {
        "AccessControl": "Private",
        "BucketName": "cc-production-access-logs",
        "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 'WRITE' Access
Resources:
  SecureS3Bucket:
    Properties:
      AccessControl: Private
      BucketName: cc-production-access-logs
      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.

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-access-logs"
  acl = "private"
}

resource "aws_s3_bucket_public_access_block" "securebucket" {
  bucket = "cc-production-access-logs"
  block_public_acls = true
  ignore_public_acls = true
  block_public_policy = true
  restrict_public_buckets = true
}

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 Write permission checkbox available next to the Everyone (public access) grantee in the Objects column, to remove the WRITE (UPLOAD/DELETE) 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 configuration changes.

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

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 (see Audit section part II to identify the right S3 resource) to deny public WRITE access to the selected S3 bucket by removing the WRITE (UPLOAD/DELETE) permissions set for the Everyone (public access) grantee. The following command request uses the PRIVATE canned ACL to remove the WRITE (UPLOAD/DELETE) permissions for the specified S3 bucket (if successful, the command does not produce an output):

aws s3api put-bucket-acl
  --bucket cc-production-access-logs
  --acl private

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