An Iranian attack on the Navy’s largest unclassified computer network that was first reported on in September managed to get into the “bloodstream” of the system — a much more serious breach of security than previous thought — officials now admit, the Wall Street Journal said.

“It was a real big deal,” a senior U.S. official told the Journal of the attack, which affected the Navy and Marine Corps. “It was a significant penetration that showed a weakness in the system.”

The Navy reports that the compromised network has 800,000 users at 2,500 locations.

A senior defense official said that it took up to four months to “eliminate the bad guys” from the system, the Wall Street Journal said.

The response to the attack, called Operation Rolling Tide, was overseen by Vice Adm. Michael Rogers, President Obama’s choice for the new NSA director.

Adm. Rogers is likely to face questions pertaining to the attack during his upcoming (but as-of-yet unscheduled) confirmation hearing next month, even if most of his answers will have to be given during a classified briefing.

“It was a big problem, but it was a success,” the Wall Street Journal’s source said. “Mike Rogers did a very, very good job handling this.”

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