Law firm representing Drake, Lady Gaga, Madonna and more hit by cyber attack as hackers claim to have stolen personal information and contracts

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A law firm representing many of the world's most famous celebrities has been hacked.

The website of Grubman Shire Meiselas & Sacks has been taken offline, and hackers claim to have stolen some 756GB of data relating to its clients.

Singers, actors and other stars have worked with the law firm, according to old versions of its website, with more than 200 very high-profile celebrities and companies said to have used its services.

They include Madonna, Lady Gaga, Elton John and Drake.

The hackers behind the attack claim to have person information on celebrities including letters, as well as official contracts.

Hackers have already released a purported screenshot of a Madonna contract in an attempt to prove they have access to personal files.

It is not known what the hackers are demanding in return for the files, or whether negotiations are ongoing.

"We can confirm that we've been victimised by a cyber-attack," the firm said in a media statement. "We have notified our clients and our staff.

"We have hired the world's experts who specialise in this area, and we are working around the clock to address these matters."

The hack used a piece of software known as REvil or Sodinokibi. Similar software took foreign exchange company Travelex offline in January, as part of a major hack.

Traditionally, such ransomware has been used to lock down computers and demand money from their owners to unlock them again, and grant access to files.

Increasingly, hackers threaten to release those files to the public if their demands are not met.