In the course of assessing recent threats, "activity of concern" was found on the unclassified Executive Office of the President network, the official said under condition of anonymity. "Any such activity is something we take very seriously. In this case, we took immediate measures to evaluate and mitigate the activity."
The intrusion, however, led to temporary outages and loss of connectivity for users while cybersecurity teams moved to counter the attack. The disruptions were entirely the result of the measures taken to defend the networks, the official said.
"Our computers and systems have not been damaged, though some elements of the unclassified network have been affected," the official wrote in an email.
The hackers are thought to be working for the Russian government, and the U.S. was alerted about the breach by an ally, the Washington Post said, citing unnamed sources. There is no evidence to date that the classified network was also breached, according to the report.
Security firm FireEye said in a report that Russia could be behind a computer spying campaign, directed at targets of interests to Russia such as European governments, militaries and security organizations. The attacks by the group FireEye dubbed as APT28 leave less clues than other state-backed attacks, it said.
John Ribeiro covers outsourcing and general technology breaking news from India for The IDG News Service. Follow John on Twitter at @Johnribeiro. John's e-mail address is john_ribeiro@idg.com