This gets even more involved when you consider things like loading libraries, there's also the impact of calls like OpenProcess/WriteProcessMemory/CreateRemoteThread (Windows-land versions, though I'm sure similar exists elsewhere).

The "good" Windows firewalls like Outpost and Zone Alarm used to have features to catch this, they'd detect when a process tried to invoke or open a running process which had internet access. They'd also do things like detect when a process tried to write a startup item. This went by names like "Leak Control" but it was basically providing near-complete HIDS features with local control.