Is teredo UPNP in my router a backdoor?

All this is quite simple to work around, just turn Teredo off. From 'Accessories', run 'Command Prompt' with a right-click 'run as Administrator', then when you get the command prompt, type in:

netsh interface teredo set state disable then exit and close the CMD prompt window. You will need a reboot to effect the change completely. If you find you now can't go to your favourite website, chat club, MMORPG, etc., then you know what Teredo was actually doing, but why it is on your Win7 or higher machine is not really a mystery - it is 'down talking' to IP4 systems using it (in theory -however, in reality, since this functionality is already built-in to Windows without Teredo, chances are, it's some junky crawler you picked up....ewww....iccck!) If you need to turn Teredo back on for some legitimate reason, just enter: netsh interface teredo set state enable at that same CMD prompt. SIMPLE! (oh, and reboot...of COURSE).