The Associated Press reports that some documents appeared to contain personal contact information for Game of Thrones actors.
Submit your Newswire tips here. "So consider us another budget for your advertisements!" the hackers" letter sent to HBO read.
HBO bosses have called in the FBI to investigate the security breach, while also working with police and cybersecurity experts in an attempt to prevent the release of any more information or unaired episodes.
The hackers claim to have taken 1.5TB of data - the equivalent to several TV series box sets or millions of documents - but HBO said that it doesn't believe its email system as a whole has been compromised, although it did acknowledge the theft of "proprietary information".
So far, the group has released the fourth episode of season seven before its official release date and fans did not hesitate to fulfill their curiosity. "So make a wise decision!" While the amount of data is almost seven times that of the 2014 Sony hack, the impact has so far been much less catastrophic.
Although this may constitute HBO's biggest leak to date, it falls short of the cyberattack inflicted on Sony in 2014, when hackers released thousands of personal emails and information including salaries and social security numbers belonging to almost 50,000 past and present employees.
HBO spokesperson Jeff Cusson responded to WIRED's request for comment on the new leak in a statement, saying that the company had been expecting more data to emerge from its breach, but that the company's "forensic review is ongoing". It was reportedly signed: "Mr Smith".
"We continue to work around the clock with outside cybersecurity firms and law enforcement to resolve the incident", Cusson writes.
Now they're back and demanding a ransom. "Its a game for us". Money isn't our main objective.
HBO suffered a colossal hack earlier this month, with 1.5 terabytes of data purportedly stolen from the U.S. company. "We want to be your partner in a tiny part of HBO's huge income".