Teredo is a platform-independent protocol developed by Microsoft, which is enabled by default in Windows Vista. Teredo provides a way for nodes located behind an IPv4 NAT to connect to IPv6 nodes on the Internet. However, by tunneling IPv6 traffic over IPv4 UDP through the NAT and directly to the end node, Teredo raises some security concerns.
Primary concerns include bypassing security controls, reducing defense in depth, and allowing unsolicited traffic. Additional security concerns associated with the use of Teredo include the capability of remote nodes to open the NAT for themselves, benefits to worms, ways to deny Teredo service, and the difficulty in finding all Teredo traffic to inspect.
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We have completed an analysis of the Teredo protocol based on a reading of the RFC (and apart from any implementation). In this section, we highlight some of the more significant security implications of the protocol; that is, ways in which Teredo positively or negatively impacts the IPv4 and IPv6 portions of the Internet.