Can a penetration test write-up be used as a bachelor thesis? I'm doing a Computer Science degree in university. While the program provided by the uni is not focused, I'm personally focusing on studying security, aiming to become a Penetration Tester/Red Teamer. I'd like to do a penetration test (on a real system with permission or a simulated system) and make the write-up my thesis. 

I've emailed the main professor in Computer Security of my university but haven't yet received a reply. I'd like answers besides "just ask your professor" and "wait for a reply".

I'd like opinions from other university professors: Can a pentest write-up be used as a bachelor thesis?
Sorry, but the only valid answer will come from the professor. It is acceptable or not based on their views only. 

But be aware that a penetration test on a real system can come with legal implications. Claiming to be a white hat may hold little weight with authorities. Gaining permission, as you suggest, might be very difficult as institutions running real system have a lot of concerns beyond the obvious. Their own legal department might nix any such project. 

A simulated system is fine, though seldom has sufficient protections in place for a valid test. An iterative, competitive, duel between two teams (Red team, Blue team) might be interesting. 

The answer to the general question, though, is yes, such a test can be used as a thesis, but only with the consent of the advisor. The general answer to the question has exactly zero impact on your case.