It's somebody who knows Han. Someone with the ability to hear stuff that they shouldn't.

[ He sighs. How can she be this naive? ]

Are you dense, Vivienne? Do you know who fucking told Hale that I'd punched Han? It was H. Do you know who knew Han and Connor were fucking? H.

So take a wild fucking guess and tell me -- who the fuck do you think is out there outing Han's fucking secrets? Because it sure as fuck ain't Oswald Fucking Hale!