I made the "understand the framework" statement in response to some mistakes I have seen introduced by sloppy dev's on some rails projects I have worked on...That obviously wasn't the case here.......
If security is important you should periodically try to hack the system and consider incorporating automated penetration testing software against your application. I haven't had a need to go this far with my own recent apps so I cannot speak authoritatively on this, but I think there are some tools that can help with doing penetration testing etc..
In a past life I had to work on some pretty secure systems and did some crazy testing on things. I saw a lot of good developers introduce pretty big security holes....my favorite was when our /etc/password was served up by an application...and this was a well known team of craftsman that did this on a fairly large project. None of these have been limited to Ruby projects....they have included Java, C etc..
In my view security is a moving target. If the cost of a attack warrants the effort to protect against it then you do. If not then you don't. Even if the developers of the framework concentrate on security, there will always be ways to get around it. Safe's are rated on the amount of time it takes to break into them, if someone wants into bad enough they will get in. The same is true with software.
Should I be more aware of the security on my apps? probably. Should we as a community be better with it? Yup.
But unless I've taken the time to really dig into it, offer constructive feedback and be willing to jump in to fix it I have no business criticizing the state of things.
I'm just saying Ruby is good at enabling DWIM and Rails has taken it and run with it to the point of magical thinking.
It's very hard for an app developer to test for vulnerabilities such as this one which seem to involve a combination of contributory factors. When magical stuff is constantly willing to help the app developer out in the background, it's very difficult to get a handle on what our true attack surface is.
Rails has definitely started to get complex, which is why many have chosen to go towards the simplicity of Sinatra, Rack etc.. Any time a framework get's complex this happens.
Look at big data and AI. There are loads of permutations. How do you prevent stuff from just happening? How do you make sure that things are correct etc.? Trust me I would love to do TDD on an AI / big data analysis project. But the reality is that no-one has figured out how to reliably test things....so TDD is not the right tool at this point. But there is also the potential to introduce vulnerabilities. That doesn't mean I am not going to use AI or do big data.
It's always a balance between tighter security/less complexity, or more complexity and less security etc...obviously there are other factors as well, but my point is to choose the right tool for the job.....sometimes it is Rails, and sometimes not......
And beyond that give the Rails team kudos for a taking care of things like this when they do find them.