WASHINGTON — Hackers breached security at the website of the government’s health insurance marketplace, HealthCare.gov, but did not steal any personal information on consumers, Obama administration officials said Thursday.

The administration informed Congress of the violation, which it described as “an intrusion on a test server” supporting the website.

“Our review indicates that the server did not contain consumer personal information, data was not transmitted outside the agency and the website was not specifically targeted,” said Aaron Albright, a spokesman at the Centers for Medicare and Medicaid Services, which runs the website. “We have taken measures to further strengthen security.”

Mr. Albright said the hacking was made possible by several security weaknesses. The test server should not have been connected to the Internet, he said, and it came from the manufacturer with a default password that had not been changed.