Get the Whitepaper called The Tor Project: Authority “No Check” Weakness. It discusses the fact that exit nodes can be leveraged easily to commit man-in-the-middle attacks.
You can download it from the following link: https://packetstormsecurity.com/files/download/107805/tor-nocheck.pdf
Source: https://packetstormsecurity.com/files/107805/The-Tor-Project-Authority-No-Check-Weakness.html