Adding a Standard Bucket Policy

It is good practice to deny traffic that is not HTTPS to your S3 bucket. For this reason, I wrote a script that I can use to apply a standard policy to each of my S3 buckets. While the script could be more robust to iterate through each bucket in my account, I decided to write this script to take input of the name of the bucket and then apply the changes.

import boto3
from botocore.exceptions import ClientError
import json
import sys

def check_s3_policy(bucket_name):
    client = boto3.client("s3", region_name='us-west-2')

    # Get existing policy so that we don't overwrite anything
    try:
        result = client.get_bucket_policy(Bucket=bucket_name)
        if result == None:
            return None
        else:
            return result
    except ClientError as e:
        print("failed to retrieve policy")
        print(e)
        return None

if __name__ == "__main__":
    bucket_name = sys.argv[1]
    source_aws_account = boto3.client('sts').get_caller_identity().get('Account')
    print("Our current account number: " + source_aws_account)
    connect_instance_arn = ""
    standard_bucket_policy = {

                "Sid": "AWSHTTPSAccess",
                "Action": [
                    "s3:*"
                ],
                "Effect": "Deny",
                "Resource": [
                    "arn:aws:s3:::"+ bucket_name,
                    "arn:aws:s3:::" + bucket_name + "/*"
                ],
                "Condition": {
                    "Bool": {
                        "aws:SecureTransport": "false"
                    }
                },
                "Principal": "*"

    }
    
    existing_policy = check_s3_policy(bucket_name)
    if existing_policy == None:
        print("No policy exists so lets create a new one")
        print("Applying our standard bucket policy that denies non-HTTPS traffic...")
        try:
            new_bucket_policy = {
                "Version": "2012-10-17",
                "Statement": [standard_bucket_policy]
            }
            client = boto3.client("s3", region_name='us-west-2')
            client.put_bucket_policy(Bucket=bucket_name, Policy=json.dumps(new_bucket_policy))
            
        except ClientError as e:
            print("failed to put bucket policy")
            print(e)
    else:
        print("There is a policy so we need to modify")
        policy_to_modify = json.loads(existing_policy['Policy'])
        policy_to_modify['Statement'].append(standard_bucket_policy)
        try:
            client = boto3.client("s3", region_name="us-west-2")
            client.put_bucket_policy(Bucket=bucket_name, Policy=json.dumps(policy_to_modify))
        except ClientError as e:
            print("Error putting new bucket policy")
            print(e)
        
    print("Our bucket now follows all compliance ...")
    print("Exiting ...")

You can change the policy as needed and use this script to apply changes to your buckets!

I hope that this is helpful to someone. Please share this to your friends!
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