In this blog article, we will cover how to deny the creation of a subnet in a Virtual Network if the subnet does not have a Network Security Group associated with it, using a custom Azure Policy.

You can follow the steps below to create a custom policy:

1) From the Azure portal, access Azure policy, then definitions blade.
2) Create a new policy definition.

3) Add the definition location (which subscription will be hosting this policy), Name, and description.
4) Set the category to use existing and select Networking (as below):

5) Then add the below policy definition into the rule field:

Note: you can adjust the below parameters as needed, also the below example excludes the following subnets. You can add more subnets of your choice.
“GatewaySubnet”,
“AzureFirewallSubnet”,
“AzureBastionSubnet”,
“AzureFirewallManagementSubnet”

{
“mode”: “All”,
“parameters”: {
“excludedSubnets”: {
“type”: “Array”,
“metadata”: {
“displayName”: “Excluded subnets”,
“description”: “The list of subnet names to exclude from the policy”
},
“defaultValue”: [
“GatewaySubnet”,
“AzureFirewallSubnet”,
“AzureBastionSubnet”,
“AzureFirewallManagementSubnet”,
“YourCustomSubnet”
]
}
},
“policyRule”: {
“if”: {
“anyOf”: [
{
“allOf”: [
{
“field”: “type”,
“equals”: “Microsoft.Network/virtualNetworks”
},
{
“not”: {
“field”: “Microsoft.Network/virtualNetworks/subnets[*].networkSecurityGroup.id”,
“exists”: true
}
},
{
“field”: “Microsoft.Network/virtualNetworks/subnets[*].name”,
“notIn”: “[parameters(‘excludedSubnets’)]”
}
]
},
{
“allOf”: [
{
“field”: “type”,
“equals”: “Microsoft.Network/virtualNetworks/subnets”
},
{
“not”: {
“field”: “Microsoft.Network/virtualNetworks/subnets/networkSecurityGroup.id”,
“exists”: true
}
},
{
“field”: “name”,
“notIn”: “[parameters(‘excludedSubnets’)]”
}
]
}
]
},
“then”: {
“effect”: “deny”
}
}
}

6) Then save the policy.

Now you can assign this policy as per your requirements.

1) From Azure policies page, and access definitions blade -> select the created custom policy, and click assign policy (you can assign on the Subscription level or a specific resource group depending on your business requirements).

2) To update the excluded subnet list at time of policy assignment. Go to Parameters tab, then uncheck the box “Only show parameters that need input or review” and select of the three dots next to the “Excluded subnets” box.

3) It will open the editor; update the subnet name you want to exclude and click save.

4) Click Next, and Next, update the “Non-compliance message” as per your requirement.

5) Click review + create and review the output. Once verified create the policy assignment. Policy assignment usually takes around 5-15 minutes to take effect.

To update the list of excluded subnets after the policy assignment.

1) From the Azure portal, access Azure policy, then Assignments blade and search the assignment.
2) Open the assignment by clicking on the name

3) Select Edit assignment

4) Go to Parameters tab, then uncheck the box “Only show parameters that need input or review” and select of the three dots next to the “Excluded subnets” box.

5) It will open the editor; update the subnet name you want to exclude and click save.

Disclaimer

Please note that products and options presented in this article are subject to change. This article reflects custom policy for Azure Subnets in September 2024.
If users have the required permissions, they can create exemptions for their resources, which makes this policy ineffective for those resources.
Some subnets managed by Azure services may not require an NSG. Ensure these subnets are added to the excluded subnet list or use a policy exception as needed.
It is highly recommended to test this policy in a non-production environment before applying it to your production environment to avoid any unintended disruptions and to make sure it meets your requirements.

References

Tutorial: Create a custom policy definition – Azure Policy | Microsoft Learn

Programmatically create policies – Azure Policy | Microsoft Learn

Troubleshoot common errors – Azure Policy | Microsoft Learn

Overview of Azure Policy – Azure Policy | Microsoft Learn

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