I have seen a fare share of terribly written django apps, riddled with security holes that weren't so obvious at the first glimpse.
> But flask gives those profile a false sense of easiness.
It allows you to prototype very fast, not everyone needs nor wants the hand-holding that django provides.
Don't get me wrong I think django is fantastic at what it gives out of the box but saying that flask is insecure is just fud.
Also - try Pyramid, you'll never know when it suits your needs https://trypyramid.com/
Have you ever written a flask app yourself? It provides identical protections against xss etc. that you mentioned, as well as an auth plugin and tutorials on doing auth right...
I think it's unfair to spread a negative opinion of Flask because you had to work on an app that used it badly.
When using flask, you have to be able to make decisions about those. Many devs are not able to.
I'm ok with it: not everybody has to be an expert to be a great element in a company. But flask gives those profile a false sense of easiness.