Rails Security is apparently "omakase" - meaning 'leave it to someone else', apparently.
Disclaimer: I'm a java programmer.
There is plenty to security that is not in the Rails documentation, though, and it is by no means bulletproof by any stretch of the imagination. If you don't take security seriously, then you increase the chance of getting stung. From exploits that you have to watch out for in your code, to setting up the server(s) and infrastructure in the most secure way that makes sense, it is a lot of work.