Hector Xavier Monsegur, better known by his screen name "Sabu", was first arrested in 2011 and had already served seven months in jail before being released on bail in December 2012.
The New Yorker was a prominent hacker with the activist group Anonymous, which has staged cyber assaults on MasterCard, PayPal and other commercial and government targets.
After his arrest he became a Federal Bureau of Investigation (FBI) informant, helping US agents identify other members of the collective.
He pleaded guilty to nine counts of computer hacking and one count each of credit card fraud, conspiring to commit bank fraud and aggravated identity theft.
Sabu and his co-conspirators carried out hacks that cost companies tens of millions of dollars, defaced websites and stole personal information of customers or employees, court papers said.
The original charges could have landed him in prison for 124 years but the government asked for him to be exempt from even a mandatory minimum sentence given his "extraordinary co-operation."
He had admitted to being involved in cyber attacks on MasterCard, PayPal and Visa and on the Algerian, Tunisian, Yemeni and Zimbabwean governments.
He set up Anonymous offshoot Lulz Security, or LulzSec, which in 2011 engaged in major hacks into and theft from computer servers of US and foreign corporations.
Its victims included Fox Television, compromising a database of contestants in the reality TV show "X-Factor," the website of US public broadcaster PBS, the US Senate and an affiliate of the FBI.
The New York Times reported last month that he directed hundreds of cyber attacks against the websites of governments in Brazil, Iran, Pakistan, Syria and Turkey.
It was unclear who ordered the attacks, but the newspaper said court documents and interviews suggest the government "may have" used hackers to gather intelligence overseas.
Monsegur instructed fellow hacker Jeremy Hammond, who was sentenced to 10 years in jail by a US federal judge last November, to extract data from foreign government websites.
That information, including bank records and login details, was uploaded to a server "monitored" by the FBI, the Times reported.
Source: AFP via I-Net Bridge
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