What is the user principal of a cognito preauth function?

Question

When you have to give IAM permissions to a cognito preauth trigger, is the role you assign assumed by  'lambda.amazonaws.com' like any other lambda?  Or are triggers run by the cognito service principle, 'cognito-idp.amazonaws.com' ?

I need to give my lambda permission to do dynamodb:GetItem on a specific table (by ARN) and it's not working.  It might be not working for some other reason than this.  I think the answer is it's still lambda.amazonaws.com but wanted to double check, mainly because I can't find what cognito-idp.amazonaws.com is used for.

Accepted Answer

The Lambda Policy has a resource policy that allows it to be accessed by the Congito user pool in the form of:

```
{
  "Version": "2012-10-17",
  "Id": "default",
  "Statement": [
    {
      "Sid": ",",
      "Effect": "Allow",
      "Principal": {
        "Service": "cognito-idp.amazonaws.com"
      },
      "Action": "lambda:InvokeFunction",
      "Resource": "arn:aws:::function:",
      "Condition": {
        "ArnLike": {
          "AWS:SourceArn": "arn:aws:cognito-idp:::userpool/"
        }
      }
    }
  ]
}
```
But the Lambda function still executes as `lambda.amazonaws.com` and must be authorized as such through the Lambda Execution Role associated to the Lambda function.

What is the user principal of a cognito preauth function?

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