Around 37 million people will be extremely nervous Monday after the extramarital-affair website Ashley Madison was hacked and the details posted online. The Canadian-based site sells itself with the slogan "Life is short. Have an affair."

Data security expert and blogger Brian Krebs revealed the hack on his site, Krebs On Security, saying a group calling itself The Impact Team was behind the hack and said it had stolen user databases, financial records including salary information, and other records.

Krebs says Ashley Madison's parent company, Avid Life Media (ALM), which also runs Cougar Life and Established Men, acknowledged the breach, with CEO Noel Biderman saying the company was "working diligently and feverishly" to delete the release of IP.

Krebs says it's uncertain how much user account data was posted online, but the hackers have threatened to post more unless ALM takes down its websites, saying its core complaint is that the company charges customers $19 for a "full delete" of profiles, netting $1.7 million in revenue in 2014, but it failed to deliver on the promise.

"It's also a complete lie," The Impact Team says. "Users almost always pay with credit card; their purchase details are not removed as promised, and include real name and address, which is of course the most important information the users want removed."

The group is threatening to release all customer records, including sexual fantasies, credit-card details, and real names and addresses, unless Ashley Madison and Established Men are taken offline permanently.

"Too bad for ALM, you promised secrecy but didn't deliver. We've got the complete set of profiles in our DB dumps, and we'll release them soon if Ashley Madison stays online. And with over 37 million members, mostly from the US and Canada, a significant percentage of the population is about to have a very bad day, including many rich and powerful people," the hackers say.

The attack comes just two months after another relationship site, AdultFriendFinder, was hacked, and it comes as Ashley Madison was considering a $200 million initial public offering on the London exchange later this year, though two weeks ago, Biderman told Business Insider he had found a "better option" to the listing.

In 2014, Ashley Madison had $115 million in revenue off the back of solid growth in female users, and revenue was projected to top $150 million in the coming year.

--This story first appeared on Business Insider Australia