As it turns out, pretty much everyone on the thread is right. Localhost is special on a network: it's the only machine that that you're guaranteed actually exists, and is therefore qualified as a high-value target for the bad guys.

I hadn't heard about the printer creating a local HTTP server, but it sure doesn't surprise me one bit.

At the same time, we could identify companies with real products that we wanted in the WinRT ecosystem but where the company already had an architecture that assumed that they could connect to localhost. It was a very painful call, but in the end we decided that security was more important than these companies.

We decided against a checkbox because users already have too many checkboxes, and mostly don't understand any of them.

If knowing the current local IP address gives you access to localhost, that would be a bug. More likely, you see that it works because you're running in a debugger, where the access to localhost is restriction isn't currently enforced.

And for goodness sakes everyone -- stop restricting yourself to IPv4! IPv6 is a real and growing thing! We made a bunch of fixes in Windows just so that the WinRT network APIs would work perfectly without ever knowing or caring whether you were going over IPv4 or IPv6.

Using an external IP address is a terrible solution for communicating with another process on the same machine. As soon as you disconnect from the network, it will stop working.