Azure Policy to add a specified Network Security Group to a Subnet if none is specified at creation time

ApplyNSGIfMIssing.json

{
	"properties": {
        "displayName": "Append NSG if missing",
        "description": "Sets default NSG if none is specified",
		"policyType": "Custom",
        "mode": "All",
        "parameters": {
            "nsgResourceId": {
                "type": "String",
                "metadata": {
                    "description": "Full resource ID path for the NSG"
                }
            }
        },
		"policyRule": {
			"if": {
				"allOf": [
					{
						"field": "type",
						"equals": "Microsoft.Network/virtualNetworks/subnets"
					},
					{
                        "field": "Microsoft.Network/virtualNetworks/subnets/networkSecurityGroup.id",
                        "exists": false
                    }
				]
			},
			"then": {
				"effect": "append",
				"details": [
					{
						"field": "Microsoft.Network/virtualNetworks/subnets/networkSecurityGroup",
						"value": {
							"id": "[parameters('nsgResourceId')]"
						}
					}
				]
			}
		}
	}
}

@vg22, an append policy will only effect resources at creation time. If you want to remediate existing resources, you will first need to change the effect to "modify" and then when you assign the policy, create a remediation task and identity to run the task. This is very straightforward via the Azure portal but can also be done via PowerShell of Azure CLI as per the link. Hope this helps!

@mmckechney I tried the modify action and I was able to see all the non compliant subnets, but remediate action seems to complain about

Reason
Failed to remediate resource: '/subscriptions/<sub-id>/resourceGroups/<rg_name>/providers/Microsoft.Network/virtualNetworks/v-net-name/subnets/subnet-name'. The 'PUT' request failed with status code: 'BadRequest'. Inner Error: 'Cannot parse the request.', Correlation Id: '87565527-4247-474f-b101-0dffd8606e78

Can you please review my policy here. This is my policy

{

	"if": {
		"allOf": [
			{
				"field": "type",
				"equals": "Microsoft.Network/virtualNetworks/subnets"
			},
			{
				"field": "Microsoft.Network/virtualNetworks/subnets/networkSecurityGroup.id",
				"exists": false
			}
		]
	},
	"then": {
		
		"effect": "modify",
		"details": {
		  "roleDefinitionIds": [
			"/providers/microsoft.authorization/roleDefinitions/4d97b98b-1d4f-4787-a291-c67834d212e7"
		  ],
		  "conflictEffect": "audit",
		  "operations": [
			
			{
			  "operation": "addOrReplace",
			  "field": "Microsoft.Network/virtualNetworks/subnets/networkSecurityGroup",
			  "value": "[parameters('nsgResourceId')]"
					}]
		}
	}
}