Update: It seems the Internet might have gotten a little carried away on this story. (Weird, that never happens on anything related to Bieber!) It turns out the man "attacking" Bieber was in fact a plainsclothes policeman trying to restrain an unruly crowd, though neither the singer nor his security detail, which fought back, realized it in the moment. [TMZ]
Earlier: According to the bros narrating this YouTube video, it was an old guy who hopped the barricade to attack Bieber outside of Macy's in New York today — and the (unharmed) teenybopper swung back.