Back in 1997-1999 it was fairly easy to get access to MS source code on the corp net. Esp if you were in Visual Studio or Windows NT (2000, XP, etc) team. Meaning pretty much anyone could get it. I had a local copy on my workstation I would study. Very readable code the NT 4 codebase. Everyone who wants a copy of codebase X can get a codebase X with a concerted amount of work. It will be impossible to stop this.

Having access to the source can give you insights into the various ways you can attack a system. It saves months if not years of hacking. Yes you can make exploits w/o the source. With the source it is a whole new ballgame.

Firewalls, MAC, separation of concerns, etc. With the proper virus one can bring the whole system down (pick your system).