The NYT is reporting that the National Security Council has confirmed the screw up:
“At this time, the message thread that was reported appears to be authentic, and we are reviewing how an inadvertent number was added to the chain,” Brian Hughes, the National Security Council spokesman, said in an emailed statement. He called the thread “a demonstration of the deep and thoughtful policy coordination between senior officials.”
"Several Defense Department officials said that having the type of conversation that Defense Secretary Pete Hegseth and others had in a Signal chat group could be a violation of the Espionage Act. Revealing operational war plans before planned strikes could also put American troops directly into harm’s way, the officials said."
https://www.nytimes.com/live/2025/03/24/us/trump-news#hegseth-classified-war-plans-group-chat