Using ARM templates to create multiple federated credentials for UMIs

I have a list of UMIs that I am creating with the Microsoft.ManagedIdentity/userAssignedIdentities ARM template resource. Let’s say I have ten of these in my list.

For each UMI, I want to create federated credentials to allow tokens from several OIDC providers.

I planned to use resource iteration, but it doesn’t appear I can nest the fed creds child resource when using an iterator.

So my question is: How do I either a) create federated credentials that allow tokens from multiple OIDC providers or b) write an ARM template that can do a nested for loop? i.e. “for each UMI create x fed creds”

Here’s the template I started with

{
  "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "managedIdNames": {
      "type": "array"
    }
  },
  "resources": [
    {
      "type": "Microsoft.ManagedIdentity/userAssignedIdentities",
      "copy": {
        "name": "managedIdentityIterator",
        "count": "[length(parameters('managedIdNames'))]"
      },
      "apiVersion": "2023-01-31",
      "name": "[parameters('managedIdNames')[copyIndex()].name]",
      "location": "[resourceGroup().location]",
      "resources": [
        {
          "type": "Microsoft.ManagedIdentity/userAssignedIdentities/federatedIdentityCredentials",
          "copy": {
            "name": "fedCredsIterator",
            "count": "[length(parameters('oidcProviders'))]"
          },
          "apiVersion": "2023-01-31",
          "name": "string",
          "properties": {
            "audiences": [ "api://AzureADTokenExchange" ],
            "issuer": "[parameters('oidcProviders')[copyIndex()].url]",
            "subject": "system:serviceaccount:[parameters('k8sNamespace')]:[parameters('managedIdNames')[copyIndex()].name]"
          }
        }
      ]
    }
  ]
}

Here’s the error that gave me

Code: InvalidSchema
Message: The template is invalid. Error: 'The template resource 'string' at line '22' column '19' is not valid. Copying nested resources is not supported. Please see https://aka.ms/arm-multiple-instances for usage details.'
2023-10-23 14:10:02,593 - MainProcess - ERROR - (azurehelper:upload_all_templates_ts:1677) - Template upload failed - Exception raised in a thread

Here’s the template I am stuck with now

{
  "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "managedIdNames": {
      "type": "array"
    }
  },
  "resources": [
    {
      "type": "Microsoft.ManagedIdentity/userAssignedIdentities",
      "name": "[parameters('managedIdNames')[copyIndex()].name]",
      "apiVersion": "2023-01-31",
      "location": "[resourceGroup().location]",
      "copy": {
        "name": "managedIdentityIterator",
        "count": "[length(parameters('managedIdNames'))]"
      }
    },
    {
      "type": "Microsoft.ManagedIdentity/userAssignedIdentities/federatedIdentityCredentials",
      "name": "[format('???/fedCreds{0}', copyIndex())]",
      "apiVersion": "2023-01-31",
      "properties": {
        "audiences": [
          "api://AzureADTokenExchange"
        ],
        "issuer": "[parameters('oidcProviders')[copyIndex()].url]",
        "subject": "system:serviceaccount:[parameters('k8sNamespace')]:[parameters('managedIdNames')[???].name]"
      },
      "dependsOn": [
        "???"
      ],
      "copy": {
        "name": "fedCredsIterator",
        "count": "[length(parameters('oidcProviders'))]"
      }
    }
  ]
}