The Sony Music archive has been infiltrated by cyber-crooks, who have illegally downloaded more than 50,000 digital files.
Record company bosses paid $250 million (£156 million) to Jackson's estate in 2010 for the catalogue, including unheard material from studio sessions when the superstar recorded Off The Wall, Thriller and Bad.
A source tells Britain's Sunday Times, "Everything Sony purchased from the Michael Jackson estate was compromised.
"It caused them to check their systems and they found the breach. There was a degree of sophistication. Sony identified the weakness and plugged the gap."
The haul is also said to include a duet with the late Freddie Mercury and Black Eyed Peas rapper will.i.am.
The attack was discovered weeks after hackers targeted Sony's PlayStation Network in April (11), but was only confirmed by a Sony Music representative on Saturday (03Mar12).