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Perform following to update S3 bucket public and cross account access acl and uploading public object :
Login to the AWS Management Console at https://console.aws.amazon.com.
Navigate to s3 console.
In the navigation pane, select buckets.
Click on the bucket to be modified, click permissions.
Click Public access settings.
Click edit.
In Manage public bucket policies section, check the box for lock public access to buckets and objects granted through new access control lists (ACLs).
Choose Save.
When asked for confirmation, enter confirm. Then choose Confirm to save changes.
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