Always On VPN Security Updates July 2025

Patch Tuesday has arrived, and, unlike last month, it’s a busy month for Always On VPN administrators. The June 2025 Microsoft security updates address a whopping 16 (!) vulnerabilities in the Windows Routing and Remote Access Service (RRAS). Notably, DirectAccess administrators are once again impacted by a critical vulnerability in the Windows KDC Proxy Service (KPSSVC) this month.

RRAS

As stated previously, this month’s update addresses 16 unique CVEs in Windows Server RRAS. All are memory-related buffer overflows and out-of-bounds reads, indicating that a security researcher was recently probing for vulnerabilities in RRAS.

While all the above CVEs are Remote Code Execution (RCE) and Information Disclosure vulnerabilities, none are rated as Critical; all are rated as Important. This means exploitation is unlikely, but administrators are encouraged to update as soon as possible.

KDC Proxy

This month’s security update includes another Critical RCE in the Windows KDC Proxy Service (KPSSVC).

The KDC Proxy is enabled by default when DirectAccess is configured. By design, this means the service is exposed to the public Internet, posing a significant risk to organizations using DirectAccess for secure remote access. Administrators are urged to update their systems immediately to avoid compromise.

Additional Information

Microsoft July 2025 Security Updates

DirectAccess and CVE-2024-38063

With the August 2024 Windows security updates, Microsoft released a fix to address a Remote Code Execution vulnerability in the Windows TCP/IP stack (CVE-2024-38063). Critically, this vulnerability affects IPv6 only and does not require authentication or user interaction to exploit. An attacker would only need to send specially crafted IPv6 packets to a Windows host, which could allow them to run arbitrary code on the server. This vulnerability presents some unique challenges for organizations that have deployed DirectAccess.

Exposure

DirectAccess servers are deployed to provide secure remote access and are, necessarily, exposed to the public Internet. Sometimes, this is a direct connection (not recommended) or behind an edge firewall or load balancer. In either case, anyone can establish a TCP connection from the Internet to the DirectAccess server by default. If the DirectAccess server has a global unicast IPv6 address assigned to its external interface, that presents a worst-case scenario for exposure. Administrators should update their DirectAccess servers immediately. There are some other mitigation options, though. See below for more details.

IPv6 Transition

DirectAccess servers are usually reachable on the public Internet via IPv4 only. The lack of direct IPv6 connectivity significantly reduces the attack vector for this vulnerability. However, DirectAccess servers use various IPv6 transition technologies that could present additional risks.

Tunnel Establishment

Clients on the Internet can establish an IPv6 transition tunnel to the DirectAccess server without authentication. Once the tunnel is established, the client will receive a router advertisement (RA) and establish an IPv6 address on link. However, communication over the link requires IPsec. Although an attacker can obtain an IPv6 address, they require authentication to send TCP and UDP traffic inside the tunnel.

ICMP

It’s important to know that ICMP traffic inside the DirectAccess IPv6 transition tunnel is exempt from IPsec policy processing, by default. It is unclear whether the “specially crafted packets” an attacker must send to exploit this vulnerability can be ICMP packets. If that’s the case, this introduces significant risks and increases exposure exponentially. I will update this post if I learn anything more about this specifically.

Mitigation

The best and most obvious way to mitigate this attack is to immediately apply the Microsoft security updates. However, some additional controls can be effective in mitigating this risk.

Authentication

As mentioned, DirectAccess allows IPv6 transition tunnels to be established by default without authentication. However, it is possible to update the DirectAccess configuration to support authentication, as described here.

https://directaccess.richardhicks.com/2016/06/13/directaccess-ip-https-preauthentication/

Note: Updating the DirectAccess configuration can be impactful for remote users. Be sure to test this change in a lab environment before implementing in production.

Load Balancers

If the DirectAccess server is behind a load balancer, it can be configured to require authentication for DirectAccess IPv6 transition tunnels. Below is published guidance for configuring popular load balancers to support this.

F5 BIG-IP

Citrix ADC (formerly NetScaler)

Additional Information

Microsoft Windows TCP/IP Remote Code Execution Vulnerability

DirectAccess IP-HTTPS Preauthentication

DirectAccess IP-HTTPS Preauthentication using F5 BIG-IP

DirectAccess IP-HTTPS Preauthentication using Citrix ADC (formerly NetScaler)

Always On VPN Security Updates August 2024

Today is the second Tuesday of the month, so Windows Server administrators everywhere know what that means – it’s Update Tuesday! For Always On VPN administrators in particular there are a few security updates that affect Windows Server Routing and Remote Access (RRAS), which is a popular VPN server used to support Always On VPN implementations. While many of these updates address Remote Code Execution vulnerabilities, non are considered critical.

RRAS Updates

This month there are six vulnerabilities disclosed affecting Windows Server RRAS.

CVE-2024-38120 – Windows RRAS Remote Code Execution Vulnerability (Important)

CVE-2024-38121 – Windows RRAS Remote Code Execution Vulnerability (Important)

CVE-2024-38128 – Windows RRAS Remote Code Execution Vulnerability (Important)

CVE-2024-38130 – Windows RRAS Remote Code Execution Vulnerability (Important)

CVE-2024-38154 – Windows RRAS Remote Code Execution Vulnerability (Important)

CVE-2024-38214 – Windows RRAS Remote Code Execution Vulnerability (Important)

Additional Updates

In addition to the updates addressing vulnerabilities in Windows Server RRAS, there are also updates available for the Windows Network Address Translation (NAT), Windows Transport Layer Security (TLS), and Windows IP Routing Management snapin that could potentially impact Always On VPN deployments.

Recommendations

None of the security vulnerabilities disclosed this month are critical. Although the RRAS vulnerabilities are remote code execution, exploitation is unlikely. However, administrators are encouraged to update their systems as soon as possible.

Additional Information

Microsoft August 2024 Security Updates