But that is an overreaction. The find_by_* dynamic handlers have always accepted strings and integers, by design. What's insecure about User.find_by_id("27")?

What people seem to be reacting to is the idea that User.find_by_id(params[:id]) is exploitable because you can coerce params[:id] into a hash instead of a string (by using a query string like ?id[select]=some_thing_here instead of ?id=27). True, but what most people here are overlooking is that User.find_by_id actually rejects these hashes because their keys are strings and not symbols. Try it out.

This vulnerability can be coupled with other vulnerabilities (like having someone's session secret, which is a much worse vulnerability IMO), but people are talking about it as if you can do something nasty with it by itself alone. That's why it's an overreaction.