Easy MITM against check.tpo (not SSL-related)
check.tpo is there to give users with a bad setup a fighting chance of getting warned when they aren't actually proxying their requests through the the Tor network, right?
So it would be in the interest of a malicious network operator to make it seem like the broken Tor setup is just fine, filling those users with false confidence.
What would stop such a malicious network operator from looking for TCP requests to one of the IP addresses of check.tpo and transparently proxying just those requests through Tor?
Note that SSL can do nothing at all to stop that, as the content of the request is never inspected or modified.
I've never seen that particular attack discussed anywhere, and it would be very easy to implement.