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Check for Unrestricted RPC Access

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Risk level: High (not acceptable risk)
Rule ID: Network-010

Ensure that Microsoft Azure network security groups (NSGs) do not allow unrestricted access (i.e. 0.0.0.0/0) on TCP port 135 in order to implement the principle of least privilege and effectively reduce the attack surface. Remote Procedure Call (RPC) TCP port 135 is used for client-server communications by Microsoft Message Queuing (MSMQ) as well as other Microsoft Windows/Windows Server software.

This rule resolution is part of the Cloud Conformity Security & Compliance tool for Azure

Security

Allowing unrestricted RPC access on TCP port 135 can increase opportunities for malicious activities such as hacking (backdoor command shell), ransomware attacks and denial-of-service (DoS) attacks.

Audit

To determine if your Azure network security groups allow unrestricted access on TCP port 135 (RPC), perform the following actions:

Using Azure Console

01 Sign in to Azure Management Console.

02 Navigate to All resources blade at https://portal.azure.com/#blade/HubsExtension/BrowseAll to access all your Microsoft Azure resources.

03 From the Subscription filter box, select the Azure account subscription that you want to examine.

04 From the Type filter box, select Network security group to list only the security groups available in the selected Azure subscription.

05 Click on the name of the network security group that you want to examine.

06 In the navigation panel, under Settings, select Inbound security rules to access the list with the inbound rules defined for the selected security group.

07 On the Inbound security rules page, verify the value available in the SOURCE column for any inbound rule with the PORT set to 135 and the PROTOCOL set to TCP. If one or more rules have the SOURCE set to Any (i.e. 0.0.0.0/0), the selected network security group allows unrestricted traffic on TCP port 135, therefore the RPC inbound access to the associated Microsoft Azure virtual machine(s) is not secured.

08 Repeat steps no. 5 – 7 for each network security group available in the selected subscription.

09 Repeat steps no. 3 – 8 for each subscription created in your Microsoft Azure cloud account.

Using Azure CLI

01 Run network nsg list command (Windows/macOS/Linux) using custom query filters to list the names of all network security groups (and the name of their associated resource groups) available in the current Azure subscription:

az network nsg list
    --output table
    --query '[*].{name:name, resourceGroup:resourceGroup}'

02 The command output should return a table with requested information:

Name                     ResourceGroup
----------------------   ------------------------------
cc-windows-server-nsg    cloud-shell-storage-westeurope
cc-project5-server-nsg   cloud-shell-storage-westeurope

03 Run az network nsg rule list command (Windows/macOS/Linux) using the name of the Azure network security group (NSG) that you want to examine and its associated resource group as identifier parameters to describe the RPC inbound rule defined for the selected network security group using custom query filtering:

az network nsg rule list
    --nsg-name cc-windows-server-nsg
    --resource-group cloud-shell-storage-westeurope
    --query "[?direction=='Inbound' && access=='Allow' && protocol=='TCP' && destinationPortRange=='135']"

04 The command output should return the requested security group rule metadata or an empty array, i.e. [], if there are no RPC inbound rules created for TCP port 135:

[
  {
    "access": "Allow",
    "description": null,
    "destinationAddressPrefix": "*",
    "destinationAddressPrefixes": [],
    "destinationApplicationSecurityGroups": null,
    "destinationPortRange": "135",
    "destinationPortRanges": [],
    "direction": "Inbound",
    "etag": "W/\"abcdabcd-abcd-abcd-abcd-abcdabcdabcd\"",
    "id": "/subscriptions/abcd1234-abcd-1234-abcd-1234abcd1234/resourceGroups/cloud-shell-storage-westeurope/providers/Microsoft.Network/networkSecurityGroups/cc-windows-server-nsg/securityRules/RPC",
    "name": "RPC",
    "priority": 420,
    "protocol": "TCP",
    "provisioningState": "Succeeded",
    "resourceGroup": "cloud-shell-storage-westeurope",
    "sourceAddressPrefix": "*",
    "sourceAddressPrefixes": [],
    "sourceApplicationSecurityGroups": null,
    "sourcePortRange": "*",
    "sourcePortRanges": [],
    "type": "Microsoft.Network/networkSecurityGroups/securityRules"
  }
]

If the "sourceAddressPrefix" attribute value is set to "*", "internet" or "any", the selected network security group (NSG) allows unrestricted traffic on TCP port 135, therefore the RPC inbound/ingress access to the associated Microsoft Azure virtual machine(s) is not secured.

05 Repeat step no. 3 and 4 for each Azure network security group created within the selected subscription.

06 Repeat steps no. 1 – 5 for each subscription available in your Microsoft Azure cloud account.

Remediation / Resolution

To update your Azure NSG rule(s) configuration in order to restrict Remote Procedure Call (RPC) access to trusted entities only such as administrator IP addresses, perform the following actions:

Using Azure Console

01 Sign in to Azure Management Console.

02 Navigate to All resources blade at https://portal.azure.com/#blade/HubsExtension/BrowseAll to access all your Microsoft Azure resources.

03 From the Subscription filter box, select the Azure account subscription that you want to access.

04 From the Type filter box, select Network security group to list only the security groups available in the selected Azure subscription.

05 Click on the name of the network security group that you want to reconfigure.

06 In the navigation panel, under Settings, select Inbound security rules to access the list with the inbound rules defined for the selected security group.

07 On the Inbound security rules page, click on the non-compliant RPC security group rule that you want to reconfigure (see Audit section part I to identify the right rule).

08 On the selected security group rule configuration panel, perform the following:

  1. Select IP Addresses from the Source dropdown list to allow inbound traffic on TCP port 135 from trusted IP addresses only.
  2. For Source IP addresses/CIDR ranges, provide the source IP address, IP addresses or IP address ranges that will be allowed to access the virtual machines associated with the selected network security group (NSG). You can specify a single value or comma-separated list of multiple values. An example of multiple values is 192.168.10.70/32, 10.0.10.18/32.
  3. Ensure that Action is set to Allow and leave the rest of the NSG configuration settings unchanged.
  4. Click Save to apply the changes.

09 Repeat steps no. 5 – 8 for each network security group that allows unrestricted inbound access on TCP port 135 (RPC), available in the selected Azure subscription.

10 Repeat steps no. 3 – 9 for each subscription created in your Microsoft Azure cloud account.

Using Azure Console

01 Run network nsg rule update command (Windows/macOS/Linux) using the name of the network security group rule that you want to reconfigure as identifier parameter (see Audit section part II to identify the right NSG rule) to restrict Remote Procedure Call (RPC) access on TCP port 135 to trusted IP address(es) only, by setting the --source-address-prefixes parameter to the IP address, IP addresses or IP address ranges that are allowed to access the virtual machine(s) associated with the selected network security group. You can specify a single value (e.g. 192.168.10.70/32) or a space-separated list of multiple values (e.g. 192.168.10.70/32 10.0.10.18/32):

az network nsg rule update
    --name RPC
    --nsg-name cc-windows-server-nsg
    --resource-group cloud-shell-storage-westeurope
    --source-address-prefixes 192.168.10.70/32

02 The command output should return the metadata for the reconfigured Azure NSG rule:

{
  "access": "Allow",
  "description": null,
  "destinationAddressPrefix": "*",
  "destinationAddressPrefixes": [],
  "destinationApplicationSecurityGroups": null,
  "destinationPortRange": "135",
  "destinationPortRanges": [],
  "direction": "Inbound",

  ...

  "name": "RPC",
  "priority": 420,
  "protocol": "TCP",
  "provisioningState": "Succeeded",
  "resourceGroup": "cloud-shell-storage-westeurope",
  "sourceAddressPrefix": "",
  "sourceAddressPrefixes": "192.168.10.70/32",
  "sourcePortRange": "*",
  "sourcePortRanges": [],
  "type": "Microsoft.Network/networkSecurityGroups/securityRules"
}

03 Repeat step no. 1 and 2 for each network security group (NSG) that allows unrestricted inbound access on TCP port 135 (RPC), available in the current subscription.

04 Repeat steps no. 1 – 3 for each subscription created within your Microsoft Azure cloud account.

References

Publication date Oct 26, 2019

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Check for Unrestricted RPC Access

Risk level: High