+1. The reason only 4 people submitted is that the challenge seems so, well, intimidating. Not hard intimidating, but "I wouldn't be able to think up & code a decent entry in a few minutes" intimidating. Nobody really wants to make a whole toy payment processing system like this, just to come up with one underhanded bug. It also comes with a lot of tooling: with underhanded rust, it is very clear that this is a project, with multiple files, cargo, dependencies, etc. All of the underhanded C contests are single-file, often single- or two-function solutions.