Security flaw found for those using VPN PPTP with IPv6
Security flaw has been discovered in operating systems using IPv6 with VPN PPTP. Right now those are Vista, Windows 7 and Ubuntu. Ipredator, Relakks are few VPN services which this affects.
To fix these issues in Vista and Windows 7 do the following:
Run cmd as administrator and type netsh interface teredo set state disabled. This disables the IPv6 tunneling which can reveal your true IP.
For Ubuntu 10+ paste this into a terminal:
echo “#disable ipv6″ | sudo tee -a /etc/sysctl.conf
echo “net.ipv6.conf.all.disable_ipv6 = 1″ | sudo tee -a /etc/sysctl.conf
echo “net.ipv6.conf.default.disable_ipv6 = 1″ | sudo tee -a /etc/sysctl.conf
echo “net.ipv6.conf.lo.disable_ipv6 = 1″ | sudo tee -a /etc/sysctl.conf
Source: http://torrentfreak.com/huge-security-flaw-makes-vpns-useless-for-bittorrent-100617/