Microsoft has released its security updates for November 2023. For Always On VPN administrators, it’s a light month, with just a single CVE affecting Always On VPN infrastructure.
PEAP
CVE-2023-36028 addresses a remote code execution (RCE) vulnerability in the Microsoft Protected Extensible Authentication Protocol (PEAP). An attacker could exploit this vulnerability by sending a specially crafted PEAP packet to a Windows Network Policy Server (NPS). This attack does not require authentication or user interaction.
Affected Systems
This PEAP vulnerability affects only NPS servers configured to support PEAP authentication explicitly. PEAP authentication is a best practice configuration for Always On VPN deployments and is widely deployed. NPS servers deployed to support other services, such as Wi-Fi or router and switch access that are configured to allow PEAP authentication, are also affected.
Exposure
NPS servers are not (or should not be!) exposed directly to the public Internet. This limits the attack surface to adversaries already on the internal network.
Mitigation
Microsoft suggests disabling PEAP authentication support on NPS servers until the update is applied. However, this would break the majority of Always On VPN deployments today. Since disabling PEAP isn’t a viable option, administrators can reduce their attack surface by updating the NPS firewall rules to restrict access only to authorized VPN servers or other network devices until their systems are fully updated.
In Windows, each network interface identified by the operating system is assigned a metric value. Interface metrics are settings that determine the priority or preference of network interfaces when there are multiple active network connections. The Windows networking stack uses these metrics to determine which network interface should be used for routing traffic when multiple network interfaces are available. Network interface metrics are critical for Always On VPN administrators to understand because they can impact how name resolution requests are processed when an Always On VPN connection is established.
Metric Values
By default, Windows automatically assigns metric values to network interfaces (including VPN interfaces) based on various factors, including the connection speed, link state, and interface type. It tries to select the most suitable interface for general internet connectivity.
Metrics and DNS
Windows will also use the network interface with the lowest metric value as the preferred interface for sending DNS queries by default. This means that DNS queries will be routed through the network interface with the lowest metric value, assuming it is available and connected. When an Always On VPN connection is established, DNS queries may fail or return unexpected results if the network interface metrics are not configured optimally.
Split DNS and Wired Ethernet
Split DNS (sometimes called ‘split brain DNS’) is when the DNS namespace is the same internally and externally. The most common scenario where interface metric settings interfere with DNS operation is when using split DNS and the endpoint is connected to the Internet with a wired Ethernet connection. In this scenario, the Ethernet interface will be assigned the same or lower interface metric value as the Always On VPN interface, which can yield unexpected results.
Viewing Metrics
Always On VPN administrators can view currently assigned interface metric values by running the following PowerShell command.
Most Always On VPN administrators will never have to change interface metric settings. However, if your implementation uses split DNS and some of your endpoints connect using wired Ethernet connections, you may need to update the interface metric settings to ensure proper DNS operation. Choose a setting for the interface metric value that is lower than the wired Ethernet interface. I’ve used a value of ‘3’ without issue for many years. Use one of the following methods to update the interface metric for Always On VPN connections.
PowerShell
Updating interface metric settings in Windows can be accomplished by running the Set-NetIpInterface PowerShell command.
Note: Using PowerShell to assign the interface metric is not persistent! While this method is suitable for local validation testing, you should use one of the following methods to implement this change permanently.
Rasphone.pbk
To assign the interface metric permanently, Always On VPN administrators can edit the following settings in the rasphone.pbk configuration file.
IpInterfaceMetric=3
Ipv6InterfaceMetric=3
Administrators can automate updating this setting using the Update-Rasphone.ps1 PowerShell script. In addition, the following scripts can be used with Microsoft Intune remediation.
Hello, Always On VPN administrators! It’s the second Tuesday of the month, so you know what that means. Yes, it’s Patch Tuesday! This month’s security updates include several fixes for vulnerabilities potentially affecting Microsoft Always On VPN deployments.
RRAS Vulnerabilities
Microsoft’s July 2023 security updates include fixes affecting Windows Servers with the Routing and Remote Access Service (RRAS) role installed. Security vulnerabilities CVE-2023-35365, CVE-2023-35366, and CVE-2023-35367 are all Remote Code Execution (RCE) vulnerabilities with a Critical security rating and a CVSS score of 9.8. These security vulnerabilities in Windows Server RRAS are particularly troublesome due to the nature of the workload. RRAS servers are, by design, exposed to the public Internet. Although there are no known exploits in the wild at this time, this attack requires no special privileges other than network access. Administrators running Windows Server with RRAS installed are encouraged to update as soon as possible.
AD CS Vulnerabilities
Most Always On VPN implementations leverage enterprise PKI certificates for user and device authentication. Administrators commonly deploy Microsoft Active Directory Certificate Services (AD CS) to support this. This month there are two security vulnerabilities in AD CS marked as Important. CVE-2023-35350 and CVE-2023-35351 address RCE vulnerabilities that exploit a race condition on the server. However, AD CS servers are not exposed to untrusted networks. In addition, attackers would require administrative rights on the server to exploit these vulnerabilities.
Network Load Balancing
Finally, of importance to Always On VPN administrators using Windows Network Load Balancing (NLB) to provide load balancing for their RRAS servers, there is a vulnerability in the NLB service. CVE-2023-33163 addresses an RCE vulnerability in NLB identified as Important.
