Hackers have attacked the Web site of top showbiz attorney Allen Grubman, demanding $21 million while threatening to reveal personal details of his clients including Elton John, Lady Gaga and Barbara Streisand.

Grubman Shire Meiselas & Sacks’ site is down, and the hackers claim to have 756 gigabytes of data including contracts and personal e-mails. A screenshot alleged to be a contract for Madonna has already been released.

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Multiple sources tell Page Six that the FBI is investigating, while the NYC-based law firm says it has notified its famous clients.

One source said, “The hackers got into the system while everyone was focused on the coronavirus. We assume, but there is no confirmation, that the hackers are Eastern European. They are demanding a $21 million ransom, and the firm is not negotiating with them.”

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The source added of Grubman — the father of famed public relations pro Lizzie Grubman — “His firm has sensitive details on everything — work contracts, confidential settlements and endorsement deals for the biggest stars in New York and Hollywood.”

The law firm said in a statement: “We can confirm that we’ve been victimized by a cyberattack. We have notified our clients and our staff. We have hired the world’s experts who specialize in this area, and we are working around the clock to address these matters.”

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