Rule: S3 Buckets Should Prohibit Public Write Access

This rule ensures that S3 buckets restrict public write access to enhance security measures.

Rule	S3 buckets should prohibit public write access
Framework	NIST 800-53 Revision 5
Severity	✔ High

Rule Description

This policy is designed to ensure that the S3 buckets in your environment do not allow public write access. Public write access refers to the ability for anyone, including unauthorized users, to upload or modify objects within the S3 bucket.

Violating this policy can compromise the security and integrity of your S3 buckets, leading to potential data breaches or unauthorized modifications.

Remediation Steps

To remediate this issue and enforce the prohibition of public write access on your S3 buckets, follow the steps below:

1.

Identify the affected S3 buckets - You need to identify which S3 buckets currently allow public write access. This can be done by reviewing the access control policies and permissions associated with each bucket.

Update bucket policies - Modify the bucket policies associated with the affected S3 buckets to explicitly deny public write access. This can be achieved by adding a condition to the bucket policy that denies any request where the principal is "*", indicating any entity.

Example bucket policy snippet:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "DenyPublicWriteAccess",
            "Effect": "Deny",
            "Principal": "*",
            "Action": [
                "s3:PutObject",
                "s3:PutObjectAcl"
            ],
            "Resource": "arn:aws:s3:::your-bucket-name/*"
        }
    ]
}

This policy explicitly denies the actions

s3:PutObject

and

s3:PutObjectAcl

for any principal ("*") on all objects within the bucket.

3.

Test the changes - Verify that the policy modification has successfully restricted public write access by attempting to upload an object to the S3 bucket using a non-authorized account or anonymous access. The upload should be denied.

4.

Repeat for all affected buckets - Apply the updated bucket policy to all the affected S3 buckets in your environment.

5.

Verify and monitor - Regularly review the access control configuration of your S3 buckets to ensure that public write access is still prohibited. Additionally, consider enabling logging and monitoring mechanisms to track any unauthorized access attempts or changes to bucket policies.

Troubleshooting

If you encounter any issues while implementing this policy, consider the following troubleshooting steps:

Check the bucket policy syntax - Ensure that the bucket policy JSON is well-formed and follows the required syntax. Mistakes in the policy structure can lead to unexpected behavior or errors. You can use online JSON validators to validate the policy before applying.
IAM permissions - Verify that the IAM user or role making changes to the bucket policy has the necessary permissions (
```
s3:PutBucketPolicy
```
) to modify the policy. Without proper IAM permissions, the policy update may fail.
Document version compatibility - Make sure you are using the correct syntax and versions for NIST 800-53 Revision 5. Different versions may have slightly different policy structures or conditions. Double-check the documentation for the correct syntax.
Comprehensive testing - Thoroughly test the updated bucket policy to ensure it does not inadvertently block legitimate write access by authorized users or applications. Consider creating a separate test bucket to conduct these tests before applying the policy changes to production buckets.

By following these steps, you can effectively enforce the prohibition of public write access to S3 buckets as required by NIST 800-53 Revision 5, thereby improving the security posture of your AWS environment.

Rule: S3 Buckets Should Prohibit Public Write Access

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