If design isn’t involved in QC you’re not doing QC very well. If design isn’t plugged into development process enough to understand QC then you’re not doing design very well.
Software security and quality is the responsibility of everyone on the team. A good UX designer should be thinking of ways a user can escape the typical flow or operate in unintended ways and express that to testers. And in decisions where management is forcing untested patches everyone should chime in.
Granted, a company that isn’t focused on the user experience as much as it is on other things might not prioritise this as much in the first place.
I bet my ass UX is not anywhere close to the low-level OS team.
UX is definitely embedded in the App level team but not in low-level.