If one (A) connects through a secure proxy (B), beit webbased or not, and you connect through that proxy to another secure website (C), is it possible for B to listen to and decipher the networktraffic?
But, but, but, well, okay. I guess if you have people who don't know anything about what they're doing it's possible to mount MITM attacks, especially when you can buy certificates for like $25 nowadays.