Customer data has been stolen from Ashley Madison, an online dating website for married people who wish to cheat on their spouse.
The hackers said they had obtained information including “all the customers’ secret sexual fantasies and matching credit card transactions”.
The site’s operator confirmed there had been an “intrusion” but not its extent.
Security expert Brian Krebs said a small percentage of the site’s user account data had been published online.
The hackers, who call themselves Impact Team, said they had managed to steal the real names and addresses of the site’s users, including those who had previously paid to “delete” their accounts.
Ashley Madison, which promotes its service with the tagline “life is short, have an affair”, says it has more than 37 million users.
Avid Life Media, which owns the website, said in a statement: “We apologise for this unprovoked and criminal intrusion into our customers’ information.”
“We have been able to secure our sites, and close the unauthorised access points.
“Any and all parties responsible for this act of cyber-terrorism will be held responsible.”
In May, casual dating website Adult Friend Finder was also hacked. Thousands of its customer records were leaked online.