In what was the first-ever high-level exercise testing the U.S. military's ability to defend itself against a cyberattack, the NSA in 1997 hacked into the DoD's entire network in just four days, using nothing but commercially available equipment and soft­ware, according to a new book by Pulitzer Prize-winning journalist Fred Kaplan.

In an excerpt published by Slate, the book, Dark Territory: The Secret History of Cyber War, reports that the secret NSA “Red Team” behind the clandestine exercise, dubbed “Eligible Receiver,” was able to hack into the National Military Command Center on the very first day of the operation. Many of the compromised DoD's systems didn't require log-in credentials or the passwords were exceedingly simplistic, the book continued.

NSA succeeded in proving they could disrupt America's command-control systems, shutting down and intercepting lines of communications, deleting files, and reformat­ting hard drives. The agency even found evidence that outside hackers had already penetrated some systems.