Deny inbound NSG Rule creation via Azure Policy

  • Thread starter Thread starter Ishan_Shukla
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Ishan_Shukla

In this blog article, we will cover how to deny the creation of inbound Network Security Group Rules if the inbound NSG Rule contains Internet, Any, or 0.0.0.0/0 as source and the destination port contains 22, 3389 or *".



Note: If users have the required permissions, they can create exemption for their resources. Which make this policy ineffective for that resource.



Custom Policy Definition creation


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.



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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):



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

Note: you can add more default ports for which this policy will be evaluated. These are the default port used in this policy.
"*",
"22",
"3389",





Code: 
{
    "mode": "All",
    "parameters": {
        "blockedports": {
            "type": "Array",
            "metadata": {
                "displayName": "blocked ports",
                "description": "The list of ports to block"
            },
            "defaultValue": [
                "*",
                "22",
                "3389"
            ]
        }
    },
    "policyRule": {
        "if": {
            "allOf": [
                {
                    "field": "type",
                    "equals": "Microsoft.Network/networkSecurityGroups/securityRules"
                },
                {
                    "allOf": [
                        {
                            "field": "Microsoft.Network/networkSecurityGroups/securityRules/access",
                            "equals": "Allow"
                        },
                        {
                            "field": "Microsoft.Network/networkSecurityGroups/securityRules/direction",
                            "equals": "Inbound"
                        },
                        {
                            "anyOf": [
                                {
                                    "field": "Microsoft.Network/networkSecurityGroups/securityRules/destinationPortRange",
                                    "in": "[parameters('blockedports')]"
                                },
                                {
                                    "not": {
                                        "field": "Microsoft.Network/networkSecurityGroups/securityRules/destinationPortRanges[*]",
                                        "in": "[parameters('blockedports')]"
                                    }
                                }
                            ]
                        },
                        {
                            "anyOf": [
                                {
                                    "field": "Microsoft.Network/networkSecurityGroups/securityRules/sourceAddressPrefix",
                                    "in": [
                                        "*",
                                        "Internet",
                                        "0.0.0.0/0"
                                    ]
                                },
                                {
                                    "field": "Microsoft.Network/networkSecurityGroups/securityRules/sourceAddressPrefixes[*]",
                                    "in": [
                                        "*",
                                        "Internet",
                                        "0.0.0.0/0"
                                    ]
                                }
                            ]
                        }
                    ]
                }
            ]
        },
        "then": {
            "effect": "deny"
        }
    }
}





6) Then save the policy.



Policy Assignment


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).



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2) To update the port 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 "blocked ports" box.



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3) It will open the editor; update the ports you want to include in this policy and click save.



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



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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 ports after the policy assignment. Edit the policy assignment, go to the parameters tab and edit the ports.



Disclaimer
Please note that products and options presented in this article are subject to change. This article reflects custom policy for Azure Network Security Rules in September 2024.

If users have the required permissions, they can create exemption for their resources. Which make this policy ineffective for that resource.


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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