I know this is a prototype but it looks like the current state allows the client to execute arbitrary code. A <textarea>'s content is sent right to (dynamic-require) under the assumption the code is scribble code.

If I were writing this I would either:

1. Force use of a harmless, strict subset of scribble and replace the #lang line on each submission to use this subset. Forbid use of Racket from within Scribble, or at least forbid access to ports, threads, file-system checks, etc.
2. Allow the full power of Racket (since scribble already does), but never execute what users submit right off. The code would be funneled into a sandbox VM (Vagrant?) off any sensitive network and be subject to human editorial review.