I would have to say "it's not".. anything non-encrypted you risk being manipulated by a node of the TOR network. I bet there are people who act as relays just to try to sniff out any good non-HTTPS traffic.. although, I suppose generally anyone using TOR is mostly aware of this so there's probably not that much to gain.

Complete side note.. but this just reminds me of back when I was in college in the late 90s, and our entire apartment buildings traffic was a hub (not a switch).. so I had a packet sniffer running for fun on linux and could see everything from everyones internet since every single packet was rounting to every machine on the network, and lots of stuff had no encryption... nuts to think how open stuff was back in the day.

A subset of relay operators & other volunteers monitor the network for bad actors and report them to Tor Project who will then direct directory authority operators to blacklist those relays.

One of those volunteers is nusenu: https://medium.com/@nusenu/the-growing-problem-of-malicious-...

It can't be prevented on a technological level. However there are attempts to detect bad nodes and ban them. Since the exit node doesn't know "who" is accessing the website it can't just temper with your content. So it would be detected asymptomatically.