Who's Leaking?

That was my immediate reaction, too. However, it doesn't necessarily have to be someone who was in the room. Apparently, the notes on the meeting went out, and that's when the scramble began to do damage control. The leak may have come from someone at that point.

It was not clear whether Mr. Trump wittingly disclosed such highly classified information. He — and possibly other Americans in the room — may have not been aware of the sensitivity of what he was sharing. It was only after the meeting, when notes on the discussion were circulated among National Security Council officials, that it was flagged as too sensitive to be shared, even among many American officials, the former official said.

  NYT

Senior White House officials appeared to recognize quickly that Trump had overstepped and moved to contain the potential fallout.

Thomas Bossert, assistant to the president for homeland security and counterterrorism, placed calls to the directors of the CIA and the NSA, services most directly involved in the intelligence-sharing arrangement with the partner.

One of Bossert's subordinates also called for the problematic portion of Trump's discussion to be stricken from internal memos and for the full transcript to be limited to a small circle of recipients, efforts to prevent sensitive details from being disseminated further or leaked.

  WaPo

The more sensitive details may have been prevented from leaking by this action, but the story of tRump's big mouth got out.  Any number of people in the White House may be rightfully freaking out about the man in the oval office and the risk to the U.S. by him being there.  The leaks aren't likely to stop, the firing of James Comey in an attempt to get someone in that position who will investigate and prosecute them notwithstanding.  Fortunately.