Cross-service confused deputy prevention - AWS Transfer Family
Transfer Family user rolesTransfer Family workflow rolesTransfer Family logging/invocation roles

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 the action. In AWS, cross-service impersonation can result in the confused deputy problem. Cross-service impersonation can occur when one service (the calling service) calls another service (the called service). The calling service can be manipulated to use its permissions to act on another customer's resources in a way that it should not otherwise have permission to access. 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. For a detailed description of this problem, see the confused deputy problem in the IAM User Guide.

We recommend using the aws:SourceArn and aws:SourceAccount global condition context keys in resource policies to limit the permissions that AWS Transfer Family has for 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 most effective way to protect against the confused deputy problem is to use the exact Amazon Resource Name (ARN) of the resource you want to allow. If you are specifying multiple resources, use the aws:SourceArn global context condition key with wildcard characters (*) for the unknown portions of the ARN. For example, arn:aws:transfer::region::account-id:server/*.

AWS Transfer Family uses the following types of roles:

  • User role – Allows service-managed users to access the necessary Transfer Family resources. AWS Transfer Family assumes this role in the context of a Transfer Family user ARN.

  • Access role – Provides access to only the Amazon S3 files that are being transferred. For inbound AS2 transfers, the access role uses the Amazon Resource Name (ARN) for the agreement. For outbound AS2 transfers, the access role uses the ARN for the connector.

  • Invocation role – For use with Amazon API Gateway as the server's custom identity provider. Transfer Family assumes this role in the context of a Transfer Family server ARN.

  • Logging role – Used to log entries into Amazon CloudWatch. Transfer Family uses this role to log success and failure details along with information about file transfers. Transfer Family assumes this role in the context of a Transfer Family server ARN. For outbound AS2 transfers, the logging role uses the connector ARN.

  • Execution role – Allows a Transfer Family user to call and launch workflows. Transfer Family assumes this role in the context of a Transfer Family workflow ARN.

For more information, see Policies and permissions in IAM in the IAM User Guide.

Note

In the following examples, replace each user input placeholder with your own information.

Note

In our examples, we use both ArnLike and ArnEquals. They are functionally identical, and therefore you may use either when you construct your policies. Transfer Family documentation uses ArnLike when the condition contains a wildcard character, and ArnEquals to indicate an exact match condition.

AWS Transfer Family user role cross-service confused deputy prevention

The following example policy allows any user of any server in the account to assume the role.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "",
            "Effect": "Allow",
            "Principal": {
                "Service": "transfer.amazonaws.com"
            },
            "Action": "sts:AssumeRole",
            "Condition": {
                "StringEquals": {
                    "aws:SourceAccount": "account-id"
                },
                "ArnLike": {
                    "aws:SourceArn": "arn:aws:transfer:region:account-id:user/*"
                }
            }
        }
    ]
}

The following example policy allows any user of a specific server to assume the role.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "",
            "Effect": "Allow",
            "Principal": {
                "Service": "transfer.amazonaws.com"
            },
            "Action": "sts:AssumeRole",
            "Condition": {
                "StringEquals": {
                    "aws:SourceAccount": "account-id"
                },
                "ArnEquals": {
                    "aws:SourceArn": "arn:aws:transfer:region:account-id:user/server-id/*"
                }
            }
        }
    ]
}

The following example policy allows a specific user of a specific server to assume the role.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "",
            "Effect": "Allow",
            "Principal": {
                "Service": "transfer.amazonaws.com"
            },
            "Action": "sts:AssumeRole",
            "Condition": {
                "ArnLike": {
                    "aws:SourceArn": "arn:aws:transfer:region:account-id:user/server-id/user-name"
                }
            }
        }
    ]
}

AWS Transfer Family workflow role cross-service confused deputy prevention

The following example policy allows any workflow in the account to assume the role.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "",
            "Effect": "Allow",
            "Principal": {
                "Service": "transfer.amazonaws.com"
            },
            "Action": "sts:AssumeRole",
            "Condition": {
                "StringEquals": {
                    "aws:SourceAccount": "account-id"
                },
                "ArnLike": {
                    "aws:SourceArn": "arn:aws:transfer:region:account-id:workflow/*"
                }
            }
        }
    ]
}

The following example policy allows a specific workflow to assume the role.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "",
            "Effect": "Allow",
            "Principal": {
                "Service": "transfer.amazonaws.com"
            },
            "Action": "sts:AssumeRole",
            "Condition": {
                "ArnLike": {
                    "aws:SourceArn": "arn:aws:transfer:region:account-id:workflow/workflow-id"
                }
            }
        }
    ]
}

AWS Transfer Family logging and invocation role cross-service confused deputy prevention

Note

The following examples can be used in both logging and invocation roles.

In these examples, you can remove the ARN details for a workflow if your server doesn't have any workflows attached to it.

The following example logging/invocation policy allows any server (and workflow) in the account to assume the role.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowAllServersWithWorkflowAttached",
            "Effect": "Allow",
            "Principal": {
                "Service": "transfer.amazonaws.com"
            },
            "Action": "sts:AssumeRole",
            "Condition": {
                "StringEquals": {
                    "aws:SourceAccount": "account-id"
                },
                "ArnLike": {
                   "aws:SourceArn": [
                     "arn:aws:transfer:region:account-id:server/*",
                     "arn:aws:transfer:region:account-id:workflow/*"
                   ]
                }
            }
        }
    ]
}

The following example logging/invocation policy allows a specific server (and workflow) to assume the role.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowSpecificServerWithWorkflowAttached",
            "Effect": "Allow",
            "Principal": {
                "Service": "transfer.amazonaws.com"
            },
            "Action": "sts:AssumeRole",
            "Condition": {
                "StringEquals": {
                    "aws:SourceAccount": "account-id"
                },
                "ArnEquals": {
                   "aws:SourceArn": [
                       "arn:aws:transfer:region:account-id:server/server-id",
                       "arn:aws:transfer:region:account-id:workflow/workflow-id"
                   ]
                }
            }
        }
    ]
}