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AWS S3 Bucket Authenticated 'READ' Access

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Security

Risk level: Very High (not tolerated)

Ensure that your AWS S3 buckets content cannot be listed by AWS authenticated accounts or IAM users in order to protect your S3 data against unauthorized access. An S3 bucket that allows READ (LIST) access to authenticated users will provide AWS accounts or IAM users the ability to list the objects within the bucket and use the information acquired to find objects with misconfigured ACL permissions and exploit them.

This rule resolution is part of the Cloud Conformity Base Auditing Package

Granting authenticated “READ” access to S3 buckets can allow unauthorized users to list all the objects within the buckets and use this information to gain access to your S3 data. Cloud Conformity strongly recommends against setting READ (LIST) permission for the "Any Authenticated AWS User" ACL predefined group in production.

Audit

To determine if your S3 buckets allow READ access to AWS authenticated users, perform the following:

Using AWS Console

01 Sign in to the AWS Management Console.

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

03 Select the S3 bucket that you want to examine and click the Properties tab from the S3 dashboard top right menu:

Properties tab from the S3 dashboard

04 In the Properties panel, click the Permissions tab and check the Access Control List (ACL) for any grantee named "Any Authenticated AWS User". A grantee can be an AWS account or an AWS S3 predefined group. The grantee called "Any Authenticated AWS User" is an AWS predefined group that allows any authenticated AWS user (root account or IAM user) to access the S3 bucket. If the bucket ACL configuration does specify the "Any Authenticated AWS User" predefined group with the List (READ) permissions enabled:

predefined group with the List (READ) permissions enabled

the selected S3 bucket is accessible to other AWS accounts and IAM users for content listing and is rendered as insecure.

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

Using AWS CLI

01 Run list-buckets command (OSX/Linux/UNIX) to list all S3 buckets within your account:

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

02 The command output should return the name of each S3 bucket available across all AWS regions. The following output example returns one S3 bucket named annual-internal-financial-reports:

[
    "annual-internal-financial-reports"
]

03 Run get-bucket-acl command (OSX/Linux/UNIX) to return the Access Control Policy (ACP) for selected S3 bucket:

aws s3api get-bucket-acl
	--bucket annual-internal-financial-reports

04 The command output should display the bucket policy document which contains the AWS users and groups that have access to the bucket and their level of permissions. If the Grantee group URI is equal to “http://acs.amazonaws.com/groups/global/AuthenticatedUsers” (Any Authenticated AWS User) and the permission associated with the grantee is READ, the selected S3 bucket is accessible to other AWS accounts and IAM users for content listing. The following example displays an S3 bucket ACP that allows READ access to any AWS user that can send valid signed (authenticated) requests to list objects:

{
    "Owner": {
        "DisplayName": "john.doe",
        "ID": "441f3e58089ec3bd00296f84056525d78415fd5e56dcfda3f8309358e9989756"
    },
    "Grants": [
        {
           "Grantee": {
               "Type": "Group",
               "URI": "http://acs.amazonaws.com/groups/global/AuthenticatedUsers"
           },
           "Permission": "READ"
        }
    ]
}

05 Repeat steps no. 3 and 4 for each AWS S3 bucket that you want to examine.

Remediation / Resolution

To remove authenticated READ access to your S3 buckets, you need to perform the following:

Using AWS Console

01 Sign in to the AWS Management Console.

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

03 Select the S3 bucket that you want to examine and click the Properties tab from the S3 dashboard top right menu:

Properties tab from the S3 dashboard

04 In the Properties panel, click Permissions to expand the bucket Access Control List (ACL) configuration tab and search for the grantee (predefined group) named "Any Authenticated AWS User".

05 Uncheck the List (READ) permission applied to "Any Authenticated AWS User":

Uncheck the List (READ) permission applied to 'Any Authenticated AWS User'

or delete the predefined group using the x button next to the group settings:

delete the predefined group using the x button next to the group settings

06 Click Save to apply the new ACL configuration and remove the bucket READ (LIST) access for AWS authenticated users.

07 Repeat steps no. 3 – 6 for each authenticated “READ” accessible S3 bucket available in your AWS account.

Using AWS CLI

01 Run list-buckets command (OSX/Linux/UNIX) to list all S3 buckets available in your account:

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

02 The command output should return the name of each existing S3 bucket (just one bucket name is returned in this example):

[
    "annual-internal-financial-reports"
]

03 Run put-bucket-acl command (OSX/Linux/UNIX) using the bucket name as command parameter, to update the ACL permissions and remove the authenticated READ access for the selected S3 bucket (if successful, the command should not return any output):

aws s3api put-bucket-acl
	--bucket annual-internal-financial-reports
	--acl private

04 Repeat step no. 3 for each authenticated “READ” accessible S3 bucket within your AWS account.

References

Publication date May 13, 2016