The subtle way in which the IPv6 and IPv4 protocols coexist in typical networks, together with the lack of proper IPv6 support in popular Virtual Private Network (VPN) tunnel products, may inadvertently result in VPN tunnel traffic leakages. That is, traffic meant to be transferred over an encrypted and integrity- protected VPN tunnel may leak out of such a tunnel and be sent in the clear on the local network towards the final destination. This document discusses some scenarios in which such VPN tunnel traffic leakages may occur as a result of employing IPv6-unaware VPN software. Additionally, this document offers possible mitigations for this issue.
You can download it from the following link: https://packetstormsecurity.com/files/download/128014/rfc7359.txt
Source: https://packetstormsecurity.com/files/128014/RFC7359-Layer-3-Virtual-Private-Network-VPN-Tunnel-Traffic-Leakages-In-Dual-Stack-Hosts-Networks.html