Heads up, Always On VPN administrators! This month’s patch Tuesday includes fixes for critical security vulnerabilities affecting Windows Server Routing and Remote Access Service (RRAS). Crucially there are remote code execution (RCE) vulnerabilities in the Point-to-Point Tunneling Protocol (PPTP) (CVE-2023-28232), the Layer Two Tunneling Protocol (L2TP) (CVE-2023-28219, CVE-2023-28220), the Point-to-Point over Ethernet (PPPoE) protocol (CVE-2023-28224), and the Internet Key Exchange (IKE) protocol (CVE-2023-28238). The vulnerabilities in PPTP and L2TP are especially urgent as they allow an unauthenticated attacker to exploit them. There is also a denial-of-service (DoS) vulnerability (CVE-2023-28234) in the Secure Socket Tunneling Protocol (SSTP) protocol.
Exposure and Risk
The RCEs in PPTP, L2TP, and PPPoE should present limited risk as these protocols aren’t commonly used for Always On VPN (PPPoE and PPTP aren’t supported for Always On VPN, in fact). However, organizations may be using these protocols for other purposes. In addition, improperly configured edge firewalls could allow these connections even though administrators may not be actively using them. An attacker could also exploit these vulnerabilities with access to the RRAS server from the internal network.
Attack Surface Reduction
Always On VPN administrators are advised to ensure that only protocols and ports for VPN protocols in use are allowed through the edge firewall. Also, administrators should disable any unused protocols and services in RRAS to reduce the attack surface on their RRAS servers. To do this, open an elevated PowerShell command window on the RRAS server and run the following commands to disable support for the PPTP, L2TP, and PPPoE protocols.
netsh.exe ras set wanports device = “WAN Miniport (L2TP)” rasinonly = disabled ddinout = disabled ddoutonly = disabled maxports = 0
netsh.exe ras set wanports device = “WAN Miniport (PPTP)” rasinonly = disabled ddinout = disabled ddoutonly = disabled maxports = 1
netsh.exe ras set wanports device = “WAN Miniport (PPPOE)” ddoutonly = disabled
Restart-Service RemoteAccess -PassThru
Additional Vulnerabilities
This month’s update also includes fixes for other vulnerabilities that may impact Always On VPN deployments. Specifically, there are RCEs in Windows Network Address Translation (NAT) (CVE-2023-28217) and Windows Network Load Balancing (NLB) (CVE-2023-28240), and a DoS vulnerability in Windows Transport Layer Security (TLS) (CVE-2023-28234).
Update Now
Administrators should patch their RRAS servers as soon as possible to avoid potential compromise of the RRAS server in their environments.
