Amazon S3 Bucket Policies for Amazon AppFlow

By default, all Amazon S3 buckets and objects are private. Only the resource owner, the AWS account that created the bucket, can access the bucket and any objects that it contains. However, the resource owner can choose to grant access permissions to other resources and users by writing an access policy.

If you want to create or modify an Amazon S3 bucket to be used as a source or destination in a flow, you must further modify the bucket policy. To read from or write to an Amazon S3 bucket, Amazon AppFlow must have the the following permissions. Amazon AppFlow automatically attaches the required permissions to a bucket when you select an Amazon S3 bucket as either the source or destination in a flow in the Amazon AppFlow console. If using the Amazon AppFlow SDK these policies must be added manually.

Amazon AppFlow Required Amazon S3 Policies

Amazon AppFlow requires a permission policy with the following attributes:

• The statement SID

• The bucket name

• The service principal name for Amazon AppFlow.

• The resources required for Amazon AppFlow: the bucket and all of its contents

• The required actions that Amazon AppFlow needs to take, which varies depending on if the bucket is used as a source or destination

The following policy allows Amazon AppFlow to access an Amazon S3 bucket used as the source in a flow. It contains all of the necessary actions Amazon AppFlow needs to read objects from the specified bucket.

Amazon S3 bucket policy

{
    "Statement": [
        {
            "Effect": "Allow",
            "Sid": "AllowAppFlowSourceActions",
            "Principal": {
                "Service": "appflow.amazonaws.com"
            },
            "Action": [
                "s3:ListBucket",
                "s3:GetObject"
            ],
            "Resource": [
                "arn:aws:s3:::myBucketName",
                "arn:aws:s3:::myBucketName/*"
            ]
        }
    ]
}
            

The following policy allows Amazon AppFlow to access an Amazon S3 bucket used as the destination in a flow. It contains all of the necessary actions Amazon AppFlow needs to put objects into an Amazon S3 bucket.

{
    "Statement": [
        {
            "Effect": "Allow",
            "Sid": "AllowAppFlowDestinationActions",
            "Principal": {
                "Service": "appflow.amazonaws.com"
            },
            "Action": [
                "s3:PutObject",
                "s3:AbortMultipartUpload",
                "s3:ListMultipartUploadParts",
                "s3:ListBucketMultipartUploads",
                "s3:GetBucketAcl",
                "s3:PutObjectAcl"
            ],
            "Resource": [
                "arn:aws:s3:::myBucketName",
                "arn:aws:s3:::myBucketName/*"
            ]
        }
    ]
}
            

Cross-service confused deputy prevention

The Confused Deputy problem is a security issue where an entity that doesn't have permission to perform an action can coerce a more-privileged entity to perform that action in AWS. Cross-service impersonation is one means of creating a confused deputy problem. Cross-service impersonation can occur when one service (the calling service) calls another service (the called service). The called service can be manipulated to use its permissions to act on another customer's resources in a way it should not otherwise have permission to do. To prevent this, AWS provides tools that help you protect your data for all services with service principals that have been given access to resources in your account.

We recommend using the aws:SourceArn and aws:SourceAccount global condition context keys in resource policies to limit the permissions that Amazon AppFlow gives another service to the resource. If you use both global condition context keys, the aws:SourceAccount value and the account in the aws:SourceArn value must use the same account ID when used in the same policy statement.

The value of aws:SourceArn must be the resource that is accessing the Amazon S3 bucket. The most effective way to protect against the confused deputy problem is to use the aws:SourceArn global condition context key with the full ARN of the resource. For Amazon AppFlow, these will be the ARNs of the flows created with Amazon S3 as a source or destination. If you would like to specify multiple different flows, you may use a list of different ARNs for the aws:SourceArn context key. Additionally, you may use the aws:SourceArn global context condition key with wildcards (*). For example, arn:aws:servicename::123456789012:*. The following example shows how you can use the aws:SourceArn and aws:SourceAccount global condition context keys in Amazon S3 to prevent the confused deputy problem when Amazon S3 is the destination. (Note that, when Amazon AppFlow creates the policy for you during flow creation, it automatically populates the aws:SourceAccount condition key.)

{
    "Statement": [
        {
            "Effect": "Allow",
            "Sid": "AllowAppFlowDestinationActions",
            "Principal": {
                "Service": "appflow.amazonaws.com"
            },
            "Action": [
                "s3:PutObject",
                "s3:AbortMultipartUpload",
                "s3:ListMultipartUploadParts",
                "s3:ListBucketMultipartUploads",
                "s3:GetBucketAcl",
                "s3:PutObjectAcl"
            ],
            "Resource": [
                "arn:aws:s3:::myBucketName",
                "arn:aws:s3:::myBucketName/*"
            ],
            "Condition" : {
                "StringEquals" : {
                    "aws:SourceAccount" : "myAccountId"
                },
                "ArnLike" : {
                    "aws:SourceArn": ["arn:aws:appflow::myAccountId:flow/flow-name-1",
                                      "arn:aws:appflow::myAccountId:flow/flow-name-2"]
                }
            }
        }
    ]
}
            

Cross-service confused deputy prevention for DescribeConnectorEntity

As part of its DescribeConnectorEntity API, Amazon AppFlow will make calls to Amazon S3 in order to get information about specific objects in a customer’s Amazon S3 bucket. The DescribeConnectorEntity API is invoked either through the direct usage of the Amazon AppFlow SDK, or via the Amazon AppFlow console when using an Amazon S3 bucket as the source during flow creation. This API requires the following permissions:

• S3:GetObject

• S3:ListBucket

These calls are not associated with a particular resource. As such, when using aws:SourceArn in a bucket policy granting these permissions to Amazon AppFlow, one should use the global context condition key with wildcard if planning to use Amazon AppFlow's console or DescribeConnectorEntity API with the Amazon S3 bucket the policy is attached to.