Ryan Cleary, 20, and Jake Davis, 19, pleaded guilty to conspiring with other members of LulzSec to attack government, media, and law enforcement websites last year, according to Gryff Waldron, an official at London's Southwark Crown Court.
LulzSec an offshoot of the loose-knit movement known as Anonymous has claimed responsibility for assaults on sites run by the Central Intelligence Agency, the U.S. Public Broadcasting Service, and media mogul Rupert Murdoch's News International. Other targets included media and gaming giants Nintendo Co. and Sony Inc., security company HBGary Inc., Britain's National Health Service, and Arizona State Police.
Waldron said two other defendants 25-year-old Ryan Ackroyd and an unnamed 17-year-old have pleaded not guilty to the same charges and will face trial in April of next year.
All four defendants have denied two counts of encouraging or assisting others to commit computer offenses and fraud. Waldron said prosecutors were still weighing whether to take Cleary and Davis to court on the remaining charges.
LulzSec, whose name draws on Internet-speak for "laugh out loud," shot to prominence in mid-2011 with an eye-catching attack on PBS, whose website it defaced with a bogus story claiming that the late rapper Tupac Shakur had been discovered alive in New Zealand.
It was an opening shot in what became a months-long campaign of data theft, online vandalism and denial-of-service attacks, which work by jamming target websites with bogus traffic.
The hackers repeatedly humbled law enforcement stealing data from FBI partner organization InfraGard, briefly jamming the website of Britain's Serious and Organized Crime Agency, and publishing a large cache of emails from the Arizona Department of Public Safety.
The cybercrime spree focused attention on Anonymous, a loose-knit collection of Web-savvy activists and Internet pranksters many of whom have turned their online guns on governments, officials or corporations over a variety of political grievances.
LulzSec and its reputed leader, known as Sabu, had some of the highest profiles in the movement. But in March U.S. officials unmasked Sabu as FBI informant Hector Xavier Monsegur and officials on both sides of the Atlantic swooped in on his alleged collaborators, making roughly half a dozen arrests.
Cleary, who had been nabbed in an earlier raid, also pleaded guilty to providing the hackers with illegally hijacked computer networks for use in denial-of-service attacks and breaching the Pentagon's cyberdefenses by installing or altering files on U.S. Air Force Agency computers.
Cleary faces a U.S. federal indictment in relation to his cyberattacks, but his attorney says her client is autistic and that she would "fiercely contest" any move to extradite him to America.