S3 Bucket Authenticated Users 'WRITE_ACP' Access

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Risk level: Very High (not tolerated)
Rule ID: S3-009

Ensure that your Amazon S3 buckets do not allow authenticated AWS accounts or IAM users to modify access control permissions in order to protect your S3 data against unauthorized access. An Amazon S3 bucket that grants authenticated WRITE_ACP (EDIT) access, can allow anyone with an AWS account to edit the bucket permissions and gain full access to your S3 bucket.

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 authenticated WRITE_ACP access to your Amazon S3 buckets can allow signed AWS users to edit bucket permissions in order to view, upload, modify, and delete S3 objects within your buckets. Allowing this type of access is dangerous and can lead to data breach, data loss, or unexpected Amazon S3 charges on your AWS bill. To meet security and compliance requirements, avoid granting WRITE_ACP (EDIT) permissions to the "Authenticated users group (anyone with an AWS account)" grantee in production.


Audit

To determine if your Amazon S3 buckets allow WRITE_ACP access to AWS authenticated users, 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 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 labeled Authenticated users group (anyone with an AWS account). This grantee is a predefined group that allows anyone with an AWS account to access your Amazon S3 resources. If the Authenticated users group (anyone with an AWS account) grantee is set to Write in the Bucket ACL column, anyone with an AWS account can edit the ACL permissions configured for the selected Amazon S3 bucket, 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-production-api-logs",
    "cc-development-custom-data"
]

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 Authenticated users group (anyone with an AWS account) grantee, available for the selected S3 bucket:

aws s3api get-bucket-acl
  --bucket cc-production-api-logs
  --query 'Grants[?(Grantee.URI==`http://acs.amazonaws.com/groups/global/AuthenticatedUsers`)]'

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/AuthenticatedUsers"
        },
        "Permission": "WRITE_ACP"
    }
]

If the get-bucket-acl command output returns "WRITE_ACP" for the "Permission" attribute value, as shown in the example above, anyone with an AWS account (root or IAM user) can edit the ACL permissions configured for the selected Amazon S3 bucket, 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 remove authenticated WRITE_ACP access permissions from your Amazon S3 bucket ACL, 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 Authenticated 'WRITE_ACP' Access",
  "Resources": {
    "SecureS3Bucket": {
      "Properties": {
        "AccessControl": "Private",
        "BucketName": "cc-production-api-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 Authenticated 'WRITE_ACP' Access
Resources:
  SecureS3Bucket:
    Properties:
      AccessControl: Private
      BucketName: cc-production-api-logs
      PublicAccessBlockConfiguration:
        BlockPublicAcls: true
        IgnorePublicAcls: true
        BlockPublicPolicy: true
        RestrictPublicBuckets: true
    Type: 'AWS::S3::Bucket'

Using Terraform

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" "secure-bucket" {
  bucket = "cc-production-api-logs"
  acl    = "private"
}

resource "aws_s3_bucket_public_access_block" "secure-bucket" {
  bucket = "cc-production-api-logs"
  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 (see Audit section part I to identify the right resource).

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 Authenticated users group (anyone with an AWS account) grantee in the Bucket ACL column, to remove the WRITE_ACP (EDIT) permissions for authenticated 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 authenticated WRITE_ACP 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 (see Audit section part II to identify the right S3 resource) to deny authenticated WRITE_ACP access to the selected S3 bucket by removing the WRITE_ACP (EDIT) permissions set for the Authenticated users group (anyone with an AWS account) grantee. The following command request uses the PRIVATE canned ACL to remove the WRITE_ACP (EDIT) permissions for the specified S3 bucket (the command does not produce an output):

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

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

References

Publication date May 14, 2016

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S3 Bucket Authenticated Users 'WRITE_ACP' Access

Risk level: Very High