Kane Gamble, now 18, was aged 15 and 16 when, from his bedroom in Coalville, central England, he managed to impersonate his targets to gain highly sensitive information.
"Kane Gamble gained access to the communications accounts of some very high-ranking US intelligence officials and government employees," prosecutor John Lloyd-Jones told England's Old Bailey central criminal court.
"He also gained access to US law enforcement and intelligence agency networks."
Gamble has admitted 10 offences against the computer misuse act, between June 2015 and February 2016, and is awaiting sentencing.
The court heard how the teenager founded the group Crackas With Attitude (CWA), who used "social engineering" -- manipulating call centres and help desks into divulging confidential information -- which they then exploited.
Gamble impersonated Brennan in calls to the telecommunications companies Verizon and AOL, although in one attempt, he stumbled on a question about Brennan's first pet.
Several sensitive documents were reportedly obtained from Brennan's private email inbox and Gamble managed to get information about military and intelligence operations in Iran and Afghanistan.
"It also seems he was able to successfully access Mr Brennan's iCloud account," the prosecutor said.
Gamble called AOL and initiated a password reset, took control of Brennan's wife's iPad.
'I own you'
Gamble also targeted the then US secretary of homeland security Jeh Johnson and made calls to his phone number.
Other targets included the then US president Barack Obama's deputy national security adviser Avril Haines, his senior science and technology adviser John Holdren, and FBI special agent Amy Hess.
Gamble gained extensive unauthorised access to the US Department of Justice network and was able to access court case files, including on the Deepwater oil spill.
He boasted that he had a list of all Homeland Security employees.
Gamble gave some of the material he managed to access to WikiLeaks.
He was arrested at his home on February 9 last year at the request of the FBI.
He claimed he was motivated to act out of support for the Palestinians, and due to the United States "killing innocent civilians", the prosecutor said.
Gamble, wearing a black jacket, spoke only to confirm his name, mumbling "yes", and sat in the court next to his mother.
He will be sentenced at a date yet to be fixed.
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