# Cross-service confused deputy prevention
<a name="cross-service-confused-deputy-prevention"></a>

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 Amazon, 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 it should not otherwise have permission to access. To prevent this, Amazon 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 [https://docs.amazonaws.cn/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-sourcearn](https://docs.amazonaws.cn/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-sourcearn) and [https://docs.amazonaws.cn/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-sourceaccount](https://docs.amazonaws.cn/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-sourceaccount) global condition context keys in resource policies to limit the permissions that Amazon Backup 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 a Amazon Backup vault when using Amazon Backup to publish Amazon SNS topics on your behalf.

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. If you don't know the full ARN of the resource or if you are specifying multiple resources, use the `aws:SourceArn` global context condition key with wildcards (`*`) for the unknown portions of the ARN. For example, `arn:aws-cn::{{servicename}}::{{123456789012}}:*`. 

The following example shows how you can use the `aws:SourceArn` and `aws:SourceAccount` global condition context keys in Amazon Backup to prevent the confused deputy problem. Add the following statement to your *KMS key policy* to deny the service principal `backup-storage.amazonaws.com` from performing KMS actions unless the request originates from your specified backup vaults and account: 

```
{
  "Sid": "Deny Backup Storage confused deputy",
  "Effect": "Deny",
  "Principal": {
    "Service": "backup-storage.amazonaws.com"
  },
  "Action": [
    "kms:Decrypt",
    "kms:RetireGrant",
    "kms:GenerateDataKey"
  ],
  "Resource": "*",
  "Condition": {
    "StringNotEquals": {
      "aws:SourceAccount": "{{123456789012}}"
    },
    "ArnNotLike": {
      "aws:SourceArn": "arn:aws-cn::backup:{{us-east-1}}:{{123456789012}}:backup-vault:*"
    }
  }
}
```

Replace {{us-east-1}} with your Amazon Web Services Region and {{123456789012}} with your Amazon account ID. This policy denies the Amazon Backup storage service principal from using your KMS key unless the request comes from a backup vault in your specified account and Region.