Auction house Christie’s disclosed a data breach after the ransomware group RansomHub threatened to leak stolen data. The security breach occurred earlier this month.

The website of the auction house was unreachable after the attack.

According to BBC, Christie had problems in selling art and other high-value items worth an estimated $840 million due to a cyberattack. The spring auctions include a Vincent van Gogh painting valued at $35 million and rare wine, among other lots.

Some sales have been delayed due to the cyber attack.

RansomHub claimed responsibility for the attack and added the company to its Tor leak site. The extortion group said they had stolen 2GB of sensitive information, including personal information belonging to at least 500,000 Christie’s clients.