An Iranian attack on the U.S. Navy's largest unclassified computer network managed to get into the 'bloodstream' of the system.
Officials have now admitted that the hacking was much more serious breach of security than previous thought.
A senior US official said the attack was a real big deal, which affected the Navy and Marine Corps.
The official added that it was a significant penetration that showed a weakness in the system.
According to the Washington Times, The Navy said 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 response to the attack, called Operation Rolling Tide, was overseen by Vice Adm. Michael Rogers, President Obama's choice for the new NSA director, the report added.
