Updated: July 20 at 2 a.m. EDT with statement from Avid Life Media.

A hacking group is threatening to expose millions of users of AshleyMadison, the extramarital affairs website, unless their demands are met.

The hack, first reported by KrebsOnSecurity, has been claimed by a hacker or group of hackers, calling itself "The Impact Team." The group claims to have invaded the databases of Avid Life Media (ALM), the company that owns AshleyMadison, gaining access to everything from user profiles and financial records to ALM salary information.

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Some amount of ALM internal data has already been posted online. According to Krebs, The Impact Team posted a manifesto alongside the information, which said it had released AshleyMadison information in retaliation for the company's "Full Delete" service that promised to entirely remove member profiles for $19. The group claims this was a lie.

"Full Delete netted ALM $1.7mm in revenue in 2014. It’s also a complete lie," it wrote. "Users almost always pay with credit card; their purchase details are not removed as promised."

The Impact Team reportedly demanded that ALM take AshleyMadison and the company's other dating site, Established Men permanently offline, or else they would release customer records and profiles, including real names and customers "secret sexual fantasies."

"Too bad for those men, they’re cheating dirtbags and deserve no such discretion," the hackers continued. “Too bad for ALM, you promised secrecy but didn’t deliver ... 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."

Avid Life Media confirmed to Mashable that an attempt had been made by an unauthorized party to gain access to its systems.

"We apologize for this unprovoked and criminal intrusion into our customers’ information," the company said in a statement. "We ... have had stringent security measures in place, including working with leading IT vendors from around the world. As other companies have experienced, these security measures have unfortunately not prevented this attack to our system.

"At this time, we have been able to secure our sites, and close the unauthorized access points. We are working with law enforcement agencies, which are investigating this criminal act."

On Monday morning, ALF Media released a statement suggesting the hack was under control. "Using the Digital Millennium Copyright Act (DMCA), our team has now successfully removed the posts related to this incident as well as all Personally Identifiable Information (PII) about our users published online. We have always had the confidentiality of our customers’ information foremost in our minds and are pleased that the provisions included in the DMCA have been effective in addressing this matter.

"Our team of forensics experts and security professionals, in addition to law enforcement, are continuing to investigate this incident and we will continue to provide updates as they become available."

On Sunday, ALM Chief Executive Noel Biderman suggested to Krebs the hack may have been facilitated by someone who once had legitimate access to ALM's internal networks. "We’re on the doorstep of [confirming] who we believe is the culprit," he said. "It was definitely a person here that was not an employee but certainly had touched our technical services."

Founded in 2001, the controversial website bills itself as serving the "darker side of dating." With a reported 36 million members who use the site to find partners for infidelity, the site was always going to be high value target for hackers. As well as the marital cheating website, ALM owns niche dating services CougarLife and Established Men.

If the hackers are true to their word, AshleyMadison, a website that describes itself as a service for "discreet encounters," may not be so discreet anymore.