Hackers who have claimed to be in possession of 756 gigabytes of data from top celeb attorney Allen Grubman's law firm website are threatening to sell confidential information about Madonna.

It comes after the hackers claimed that they have sold all their "dirty laundry" on US President Donald Trump to a secret buyer after their demands weren't completed by the Allen Grubman's NYC firm "Grubman Shire Meiselas & Sacks."

The Eastern European hackers known as REvil or Sodinokibi have asked for at least $1 million for details on Madonna, reports Page Six. The hackers had earlier also released a screenshot purported as a contract for the pop-star when announcing the cyberattack, warning that they would start releasing personal information about the A-list stars if they are not paid $21 million. They doubled their demand to $42 million last week after Grubman refused to agree to the terms.

The hackers then threatened to publish "a ton of dirty laundry" about Donald Trump, even though the US president has never been a client of the law firm. Federal Bureau of Investigation which is working on the case called it an "act of terrorism" and prevented Grubman from taking any step. The hackers followed it up with another post on the dark web on Monday where they said the "presidency-ending material" about Trump is off the market as they have sold them to a secret buyer.

"We are preparing to auction Madonna data ... The buyer has the right to do whatever he sees fit with the data," they added.

Brett Callow, from cybersecurity firm Emsisoft, told the outlet about the hacker's threats, "What information REvil had on Trump we may likely never know. But they were almost certainly bluffing about it being 'presidency-ending material.'"

Madonna Dimitrios Kambouris/Getty Images

Allen Grubman's law firm represents several A-list stars including musicians Lady Gaga, Elton John, Lizzo, Barry Manilow, Rod Stewart, Lil Nas X, The Weeknd, U2, and Drake. Priyanka Chopra, Robert De Niro, Sofía Vergara, LeBron James, Mariah Carey, Bruce Springsteen, Bette Midler, and Mike Tyson are also among their prominent clients.