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 it shouldn't 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.

We recommend using the aws:SourceArn and aws:SourceAccount global condition context keys in resource policies to limit the permissions that AWS CloudFormation gives another service to a specific resource, such as a CloudFormation extension. Use aws:SourceArn if you want only one resource to be associated with the cross-service access. Use aws:SourceAccount if you want to allow any resource in that account to be associated with the cross-service use.

Make sure that the value of aws:SourceArn is an ARN of the resource that CloudFormation stores.

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:cloudformation:*:123456789012:*.

If the aws:SourceArn value doesn't contain the account ID, you must use both global condition context keys to limit permissions.

The following example shows how you can use the aws:SourceArn and aws:SourceAccount global condition context keys in CloudFormation to prevent the confused deputy problem.

Example trust policy that uses aws:SourceArn and aws:SourceAccount condition keys

For registry services, CloudFormation makes calls to AWS Security Token Service (AWS STS) to assume a service role in your account. This role is configured for ExecutionRoleArn in the RegisterType operation and LogRoleArn set in the LoggingConfig operation. For more information, see Configure an execution role with IAM permissions and a trust policy for public extension access.

This example role trust policy uses condition statements to limit the AssumeRole capability on the service role to only actions on the specified CloudFormation extension in the specified account. The aws:SourceArn and aws:SourceAccount conditions are evaluated independently. Any request to use the service role must satisfy both conditions.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": [
          "resources.cloudformation.amazonaws.com"
        ]
      },
      "Action": "sts:AssumeRole",
      "Condition": {
        "StringEquals": {
          "aws:SourceAccount": "123456789012"
        },
        "StringLike": {
          "aws:SourceArn": "arn:aws:cloudformation:us-east-1:123456789012:type/resource/Organization-Service-Resource/*"
        }
      }
    }
  ]
}

Additional information

For example policies that use the aws:SourceArn and aws:SourceAccount global condition context keys for a service role used by StackSets, see Set up global keys to mitigate confused deputy problems.

For more information, see Update a role trust policy in the IAM User Guide.