Own Idea vs Professors Project for Masters Thesis?
Pursuing a topic beyond a school's core competencies (e.g., security) might result in worse supervision. For instance, 


Due to the inability of staff to provide the required guidance. That said, at your level, competent staff should be capable of supervising you. 
Poorly aligned student-supervisor interests. (Ill-alignment shouldn't matter in theory, but does in reality.)


These risks can be mitigated against, e.g., by finding an external supervisor/mentor.

Beyond your short-term goal (getting a degree), you should consider the longer-term: What do you want to do next? If you want to entertain the idea of pursuing a research career in security, then pursuing a thesis on the topic will help you decide whether that's the right path and will help you get on that path.