AWS S3 403 Access Denied from EC2 instance

Question

I am facing this issue where the S3 Buckets on say Account A are accessible from external IPs(like my laptop), but not from an EC2 instance of another Account B.

I have granted full s3 access to the keys I am using from the laptop and the EC2 instance.Are there any IP level permissions that need to be granted specifically for EC2 instances?

Attached S3 IAM policy for the user :

{
"Version": "2012-10-17",
"Statement": [
    {
        "Effect": "Allow",
        "Action": "s3:*",
        "Resource": "*"
    }
]

}

PS: I am aware of cross account access permissions, but the thing is I want the EC2 instance to be treated like "just another request" from a public IP.

Answer 1

The issue was we had a VPC endpoint setup which had restrictions on bucket access. Adding permissions to access the Account A buckets as a new statement in the VPC endpoint policy fixed the issue.

Example:

{
    "Sid": "StmtXXXXX",
    "Effect": "Allow",
    "Principal": {
    "AWS": "XXXXXX"   (ARN of the IAM user in Account A whose keys are being used)
    },
    "Action": "s3:*",
    "Resource": "arn:aws:s3:::*"
    }