S3 modify bucket policy using boto/boto3

Question

Hi Team, 
Need help, was looking for a way to modify S3 bucket policy with Lambda (Python3.10) trigger. Since there is no update bucket policy is possible I was looking for option by which I can fetch existing bucket policy then modify that and put the bucket policy back. I was trying the below code but that did not help...

`#s3_client = boto3.client('s3')

bucket_name = 'vpc-flow-log-sudsark'

def lambda_handler(event, context):
    
    # Or use client to get Bucket policy
    s3_client = boto3.client('s3')
    policy = s3_client.get_bucket_policy(Bucket='vpc-flow-log-sudsark')
    
    # assign policy using s3 resource 
    user_policy = {
            "Sid": "AutomatedRestrictiveAccess",
            "Effect": "Allow",
            "Principal": {
                "Service": "delivery.logs.amazonaws.com"
            },
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::vpc-flow-log-sudsark/*",
            "Condition": {
                "StringEquals": {
                    "s3:x-amz-acl": "bucket-owner-full-control",
                    "aws:SourceAccount": "333344445555"
                }
            }
        }
    new_policy =  policy['Statement'].append(user_policy)
    s3_client.put_bucket_policy(Policy=new_policy)
`

Answer

Hello.

I modified the Lambda code as below.  
There are a few changes, but "get_bucket_policy()" does not directly retrieve the bucket policy in json, so it is necessary to retrieve only the bucket policy part from the response.  
The response looks like this:  
```
{'ResponseMetadata': {'RequestId': 'TTTTTTTTTT', 'HostId': 'TTTTTTTTTTTTTT=', 'HTTPStatusCode': 200, 'HTTPHeaders': {'x-amz-id-2': 'TTTTTTTTTTT=', 'x-amz-request-id': 'TTTTTTTTTTTT', 'date': 'Sat, 09 Mar 2024 05:58:03 GMT', 'content-type': 'application/json', 'transfer-encoding': 'chunked', 'server': 'AmazonS3'}, 'RetryAttempts': 0}, 'Policy': '{"Version":"2012-10-17","Statement":[{"Sid":"AWSLogDeliveryAclCheck","Effect":"Allow","Principal":{"Service":"delivery.logs.amazonaws.com"},"Action":"s3:GetBucketAcl","Resource":"arn:aws:s3:::test-s3","Condition":{"StringEquals":{"aws:SourceAccount":"1111111111111"},"ArnLike":{"aws:SourceArn":"arn:aws:logs:ap-northeast-1:111111111111:*"}}}]}'}
```

Also, near the end, a variable called "new_policy" is created, but since it only contained statement, we modified it to contain the bucket policy after creation.  
"put_bucket_policy()" requires a bucket name to be specified as a parameter, so I added it.  
https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/s3/client/put_bucket_policy.html

``` python
import boto3
import json
s3_client = boto3.client('s3')

bucket_name = 'vpc-flow-log-sudsark'

def lambda_handler(event, context):

# Or use client to get Bucket policy
    s3_client = boto3.client('s3')
    res = s3_client.get_bucket_policy(Bucket='vpc-flow-log-sudsark')
    policy_json = res['Policy']
    policy = json.loads(policy_json)

# assign policy using s3 resource 
    user_policy = {
            "Sid": "AutomatedRestrictiveAccess",
            "Effect": "Allow",
            "Principal": {
                "Service": "delivery.logs.amazonaws.com"
            },
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::vpc-flow-log-sudsark/*",
            "Condition": {
                "StringEquals": {
                    "s3:x-amz-acl": "bucket-owner-full-control",
                    "aws:SourceAccount": "333344445555"
                }
            }
        }
    policy['Statement'].append(user_policy)
    new_policy = json.dumps(policy)
    print(new_policy)
    s3_client.put_bucket_policy(Bucket='vpc-flow-log-sudsark', Policy=new_policy)
```

S3 modify bucket policy using boto/boto3

s3_client.put_bucket_policy(Bucket='vpc-flow-log-sudsark', Policy=new_policy)`

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