The subtle way in which the IPv6 and IPv4 protocols co-exist in typical networks, together with the lack of proper IPv6 support in popular Virtual Private Network (VPN) products, may inadvertently result in VPN traffic leaks. That is, traffic meant to be transferred over a VPN connection may leak out of such connection and be transferred in the clear on the local network. This document discusses some scenarios in which such VPN leakages may occur, either as a side effect of enabling IPv6 on a local network, or as a result of a deliberate attack from a local attacker. Additionally, it discusses possible mitigations for the aforementioned issue.
You can download it from the following link: https://packetstormsecurity.com/files/download/119659/draft-ietf-opsec-vpn-leakages-00.txt
Source: https://packetstormsecurity.com/files/119659/VPN-Traffic-Leakages-In-Dual-Stack-Hosts-Networks.html