Cis Azure Foundations 7.1

Ensure RDP access from the Internet is evaluated and restricted

Description

Network security groups should be periodically evaluated for port misconfigurations. Where RDP is not explicitly required and narrowly configured for resources attached to a network security group, Internet-level access to Azure resources should be restricted or eliminated.

Rationale

The potential security problem with using RDP over the Internet is that attackers can use various brute force techniques to gain access to Azure Virtual Machines. Once the attackers gain access, they can use a virtual machine as a launch point for compromising other machines on the Azure Virtual Network or even attack networked devices outside of Azure.

Impact

None documented. Restricting RDP access from the Internet is a security best practice with no negative functional impact when properly configured with alternative access methods such as VPN, ExpressRoute, or Azure Bastion.

Audit Procedure

Using Azure Portal

1. Go to Network security groups.
2. Click the name of a network security group.
3. Under Settings, click Inbound security rules.
4. Ensure that no inbound security rule exists that matches the following:
   • Port: 3389 or range including 3389
   • Protocol: TCP or Any
   • Source: 0.0.0.0/0, Internet, or Any
   • Action: Allow
5. Repeat steps 1-4 for each network security group.

Using Azure Resource Graph

1. Go to Resource Graph Explorer.
2. Click New query.
3. Paste the following into the query window:

resources | where type =~ "microsoft.network/networksecuritygroups" | project id, name, securityRule = properties.securityRules | mv-expand securityRule | extend access = securityRule.properties.access, direction = securityRule.properties.direction, protocol = securityRule.properties.protocol, destinationPort = case(isempty(securityRule.properties.destinationPortRange), securityRule.properties.destinationPortRanges, securityRule.properties.destinationPortRange), sourceAddress = case(isempty(securityRule.properties.sourceAddressPrefix), securityRule.properties.sourceAddressPrefixes, securityRule.properties.sourceAddressPrefix) | where access =~ "Allow" and direction =~ "Inbound" and protocol in~ ("tcp", "*") | mv-expand destinationPort | mv-expand sourceAddress | extend destinationPortMin = toint(split(destinationPort, "-")[0]), destinationPortMax = toint(split(destinationPort, "-")[-1]) | where (destinationPortMin <= 3389 and destinationPortMax >= 3389) or destinationPort == "" | where sourceAddress in~ ("*", "0.0.0.0", "internet", "any") or sourceAddress endswith "/0"

4. Click Run query.
5. Ensure that no results are returned.

Using Azure CLI

List network security groups with non-default security rules:

az network nsg list --query [*].[name,securityRules]

Ensure that no network security group has an inbound security rule that matches the following:

"access" : "Allow"
"destinationPortRange" : "3389", "*", or "<range-including-3389>"
"direction" : "Inbound"
"protocol" : "TCP" or "*"
"sourceAddressPrefix" : "0.0.0.0/0", "Internet", or "*"

Using Azure Policy

• Policy ID: 22730e10-96f6-4aac-ad84-9383d35b5917 - Name: 'Management ports should be closed on your virtual machines'

Expected Result

No inbound security rules should allow unrestricted RDP access (port 3389) from the Internet. The Azure Resource Graph query should return zero results.

Remediation

Remediate from Azure Portal

1. Go to Network security groups.
2. Click the name of a network security group.
3. Under Settings, click Inbound security rules.
4. Check the box next to any inbound security rule matching:
   • Port: 3389 or range including 3389
   • Protocol: TCP or Any
   • Source: 0.0.0.0/0, Internet, or Any
   • Action: Allow
5. Click Delete.
6. Click Yes.
7. Repeat steps 1-6 for each network security group requiring remediation.

Remediate from Azure CLI

For each network security group rule requiring remediation, run the following command to delete the rule:

az network nsg rule delete --resource-group <resource-group> --nsg-name <network-security-group> --name <rule>

Default Value

By default, RDP access from internet is not enabled.

References

1. https://learn.microsoft.com/en-us/azure/security/fundamentals/network-best-practices#disable-rdpssh-access-to-virtual-machines
2. https://learn.microsoft.com/en-us/security/benchmark/azure/mcsb-network-security#ns-1-establish-network-segmentation-boundaries
3. Express Route: https://learn.microsoft.com/en-us/azure/expressroute/
4. Site-to-Site VPN: https://learn.microsoft.com/en-us/azure/vpn-gateway/tutorial-site-to-site-portal
5. Point-to-Site VPN: https://learn.microsoft.com/en-us/azure/vpn-gateway/point-to-site-certificate-gateway

CIS Controls

MITRE ATT&CK Mappings

Profile

Level 1 | Automated