Iranian hackers were able to penetrate the Navy Marine Corps Internet for four months, monitoring the unclassified voice, video, and data communications of the network's 800,000 users around the world, The Wall Street Journal reported.

The breach, which left Iranian spyware lurking throughout the system, was deeper and longer than previously thought, according to the newspaper. Officials said they had initially underestimated the skills of the Iranian hackers.

No email accounts were hacked, the Navy said.

The penetration will probably be probed during the confirmation hearings of Vice Adm. Michael Rogers, whom President Barack Obama has nominated to become director of the National Security Agency. As head of Fleet Cyber Command, Rogers is chief of cybersecurity and oversaw the Navy's response to the Iranian cyberattack, according to the Journal.

Navy sources said it took four months, until November 2013, to drive the invasive hackers from the computer network because it was a significant penetration that capitalized on a weakness in a public-facing website. The sources added that the Navy took its time because it wanted to fix the underlying security weak spots, not just this particular infiltration.

A defense source told the Journal that Rogers did a good job handling the infiltration. "It was a big problem, but it was a success," said the official. The incident is not expected to spoil Rogers' confirmation prospects.

The cyberattack cost the Navy at least $10 million to patch. Broader defenses put in place at the Defense Department will likely cost several hundred million dollars more, the Journal reported.

Obama selected Rogers for the NSA post because of his background in cyberwarfare,  according to The New York Times. It is not known if he was involved in the Stuxnet sabotage operation aimed at thwarting Iran's nuclear weapons program.

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