Github should set up a little warning for people when they log in to their account if their repos match any of a set of obvious security anti-patterns, because this would be fertile ground for exploits. They could expand it later to some kind of code-review bot which does automatic code reviews looking for common antipatterns, off-by-one errors, and anything else that is easy to detect automatically. There are quite a few tools like this for analysis of desktop apps, but code for web apps are not checked as much in a systematic way.
Are there any external services which do this already for public github repos?