Investigators found no evidence that consumers' personal data was taken in the breach, federal officials said. The hacker appears only to have accessed a server used to test code for HealthCare.gov. The Department of Health and Human Services discovered the attack last week.
An HHS official said the attack appears to mark the first successful intrusion into the website, where millions of Americans bought insurance starting last year under the Affordable Care Act. It raised concerns among federal officials because of how easily the intruder gained access and how much damage could have occurred.
"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," the Department of Health and Human Services said in a written statement. "We have taken measures to further strengthen security."
The attack comes as the federal government and insurance companies prepare for open enrollment, which begins Nov. 15. It is likely to be seized on by Republican lawmakers, who oppose the law, in fall campaigns as another sign of the health law's flaws. HealthCare.gov suffered from crippling technology problems when it launched in October, though the government has since improved the site.