Cross-service confused deputy prevention

In AWS, cross-service impersonation can occur when one service (the calling service) calls another service (the called service). The calling service can be manipulated to act on another customer's resources even though it shouldn't have the proper permissions, resulting in the confused deputy problem.

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 Rekognition gives another service to the resource.

If the value of aws:SourceArn does not contain the account ID, such as an Amazon S3 bucket ARN, you must use both keys to limit permissions. If you use both keys and the aws:SourceArn value contains the account ID, 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.

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.

The value of aws:SourceArn must be the ARN of the resource used by Rekognition, which is specified with the following format: arn:aws:rekognition:region:account:resource.

The value of arn:User ARN should be the ARN of the user that will call the video analysis operation (the user that assumes a role).

The recommended approach to the confused deputy problem is to use the aws:SourceArn global condition context key with the full resource ARN.

If you don't know the full ARN of the resource or if you are specifying multiple resources, use the aws:SourceArn key with wildcard characters (*) for the unknown portions of the ARN. For example, arn:aws:rekognition:*:111122223333:*.

In order to protect against the confused deputy problem, carry out the following steps:

1. In the navigation pane of the IAM console choose the Roles option. The console will display the roles for your current account.

2. Choose the name of the role that you want to modify. The role you modify should have the AmazonRekognitionServiceRole permissions policy. Select the Trust relationships tab.

3. Choose Edit trust policy.

4. On the Edit trust policy page, replace the default JSON policy with a policy that utilizes one or both of the aws:SourceArn and aws:SourceAccount global condition context keys. See the following example policies.

5. Choose Update policy.

The following examples are trust policies that show how you can use the aws:SourceArn and aws:SourceAccount global condition context keys in Amazon Rekognition to prevent the confused deputy problem.

If you are working stored and streaming videos, you could use a policy like the following in your IAM role:

{
   "Version":"2012-10-17",
   "Statement":[
      {
         "Effect":"Allow",
         "Principal":{
            "Service":"rekognition.amazonaws.com",
            "AWS":"arn:User ARN"
         },
         "Action":"sts:AssumeRole",
         "Condition":{
            "StringEquals":{
               "aws:SourceAccount":"Account ID"
            },
            "StringLike":{
               "aws:SourceArn":"arn:aws:rekognition:region:111122223333:streamprocessor/*"
            }
         }
      }
   ]
}

If you are working exclusively with stored video, you could use a policy like the following in your IAM role (note that you don't have to include the StringLike argument that specifies the streamprocessor):

{
   "Version":"2012-10-17",
   "Statement":[
      {
         "Effect":"Allow",
         "Principal":{
            "Service":"rekognition.amazonaws.com",
            "AWS":"arn:User ARN"
         },
         "Action":"sts:AssumeRole",
         "Condition":{
            "StringEquals":{
               "aws:SourceAccount":"Account ID"
            }
         }
      }
   ]
}