This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
I won't say you are paranoid because we've learn in the (short) history of the web that if 'something might break - it will' :)
IMHO, we don't have the same problem as in C because our JS run inside a sandbox in the browser. There are many limitation that the browser enforce us (=web developers) and we've made some progress in areas like CORS: https://mikewest.org/2011/10/content-security-policy-a-primer It will be interesting to see what are the measurements that browsers will take to make sure this powerful feature won't be abuse.