Schneier, with "Mudge" of L0pht Heavy Industries, found and published security flaws in Microsoft PPTP in 1998; Microsoft quickly fixed these issues with MS-CHAPv2 and MPPE, and Schneier and Mudge published an analysis confirming the improvements in 1999, but they pointed out that the security of Microsoft PPTP still depended on the security of each user's password. Microsoft has addressed this issue by enforcing password strength policies in its operating systems, but Schneier and Mudge still recommend IPsec rather than PPTP for secure VPNs as inherently safer.

An older protocol developed by Cisco Systems Inc., L2TP combines ideas from L2F and PPTP to create a data link layer protocol. This provides a tunnel, but no security or authentication. L2TP can carry PPP sessions within its tunnel. Cisco implements L2TP in its routers. There are several open-source implementations of L2TP for Linux.

L2TP/IPsec combines L2TP's tunnel with IPsec's secure channel, which allows for easier secure Internet Key Exchange than pure IPsec. Microsoft has provided a free L2TP/IPsec VPN client for Windows 98, ME and NT since 2002, and ships an L2TP/IPsec VPN client with Windows XP, 2000, 2003 and Vista. Windows Server 2003 and Windows 2000 Server include L2TP/IPsec servers.

SSL and TLS are protocols for securing data flows at Layer 4 of the OSI model. SSL 3.0 and TLS 1.0, its successor, are commonly used with HTTP to enable secure Web browsing, called HTTPS. However, SSL/TLS can also be used to create a VPN tunnel. For example, OpenVPN is an open-source VPN package for Linux, xBSD, Mac OS X, Pocket PCs and Windows 2000, XP, 2003 and Vista, which uses SSL to provide encryption of both the data and control channels. Several vendors supply SSL VPN servers and clients.

Benefits and security risks of VPNs