"Finding how to code securely or comprehensive list of insecure patterns is much harder."
The problem is not only secure coding, actually, I'd argue that it's not even the biggest concern. You can produce the most secure code on the whole world, but that won't matter if your device has an unprotected or easily brute forced telnet access by default. Loose default security settings and permissive access is the key concern imho.
If you want to learn how to secure software then learn how to hack. It will give you an insight into the tools and techniques used to breach systems and especially give you an insight on how many attack vectors there are.
Kali Linux and the tutorials for that are the Arduino of hacking.
The way it works is that general advice I have seen many times is "don't do it". Finding how to code securely or comprehensive list of insecure patterns is much harder. So, people inclined not to follow advice and the ones with huge ego are the ones attempting to create security related software.
I am not saying that I have instant solution, but strategy of discouraging people from trying does not work well. Promoting good beginer level write ups would work better.
----------
Nevertheless, the original topic was arduino and doing circuits while being unable to read schematic. That is seen as a problem, because they did not learned theory before doing 5V dummy circuits.
And I think it is as good start for learning or getting interest as any.