Researchers have identified a bug in the way that some third-party VPN services use the PPTP protocol over IPv6. Problem enables eavesdroppers to unmask the specific IP addresses of the VPN service s users. Problem could be a particular concern for users of Web-based VPN services that are popular with users of torrent services. The flaw only seems to affect VPN implementations that are using IPv6, the newest version of the core Internet Protocol, which is meant to have some added security advantages over the current standard.
Source: https://threatpost.com/new-vpn-flaw-can-expose-users-ip-addresses-062110/74134/