How the U.S. thinks Russians hacked the White House

Russian hackers behind the damaging cyber intrusion of the State Department in recent months used that perch to penetrate sensitive parts of the White House computer system, according to U.S. officials briefed on the investigation.

While the White House has said the breach only affected an unclassified system, that description belies the seriousness of the intrusion. The hackers had access to sensitive information such as real-time non-public details of the president's schedule. While such information is not classified, it is still highly sensitive and prized by foreign intelligence agencies, U.S. officials say.

The White House in October said it noticed suspicious activity in the unclassified network that serves the executive office of the president. The system has been shut down periodically to allow for security upgrades.

The FBI, Secret Service and U.S. intelligence agencies are all involved in investigating the breach, which they consider among the most sophisticated attacks ever launched against U.S. government systems. ​The intrusion was routed through computers around the world, as hackers often do to hide their tracks, but investigators found tell-tale codes and other markers that they believe point to hackers working for the Russian government.

National Security Council spokesman Mark Stroh didn't confirm the Russian hack, but he did say that "any such activity is something we take very seriously."

"In this case, as we made clear at the time, we took immediate measures to evaluate and mitigate the activity," he said. "As has been our position, we are not going to comment on [this] article's attribution to specific actors."

Neither the U.S. State Department nor the Russian Embassy immediately responded to a request for comment.

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