Least privilege IAM policy for Synology Cloud Sync with Amazon S3 in bi-directional syncing mode. Tested on DSM 6.2.4.

synology-cloud-sync-s3-iam-policy.json

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:GetBucketLocation"
      ],
      "Resource": [
        "arn:aws:s3:::*"
      ]
    },
    {
      "Effect": "Allow",
      "Action": [
        "s3:ListBucket"
      ],
      "Resource": [
        "arn:aws:s3:::bucket-name-here"
      ]
    },
    {
      "Effect": "Allow",
      "Action": [
        "s3:PutObject",
        "s3:GetObject",
        "s3:DeleteObject"
      ],
      "Resource": [
        "arn:aws:s3:::bucket-name-here/optional-subfolder/*"
      ]
    }
  ]
}