Nothing (assuming the MITM is someone who can break HTTPS or Peer5 themselves), which is why I proposed this:

https://github.com/Peer5/ShareFest/issues/30#issuecomment-20...

You can download ShareFest, audit it and store it locally so you can run your known-good version. It's just an HTML page and a few JS files (or at least it was, last time I looked).

I was going to create a version of this that was a single webpage and had as little code as possible (for auditability and ease of saving/deployment), but other projects got in the way (plus it's easier to improve ShareFest than write it from scratch).