This isn't hard to figure out. The information about the special counsel's office has been pretty much non-stop since Mueller took the job. If you think there will be somebody in that office stand up and say "I am providing information to the press" think again. My conclusion that it MUST be the SC's is solely based on the nature information--there is no other source that is privy to all of it.
Goat and Marv put out arguments about the defense counsel, or marshal's service leaking stuff. That's tin-foil hat territory. When one considers ALL the information, the SC office is the only logical an reasonable source.
If an attorney is providing the leaks, I think that attorney needs to have his or her chops busted big time. As I said, this crap jeopardizes the prosecution's case, it is unprofessional, and frankly it is shabby bs. If the person is identified as an attorney, Mueller needs to report the matter to the federal court committee on conduct and to what ever state issued the license.