The hackers, who went by the handle little.finger66, claimed to have stolen 1.5 terabytes of data — though only an estimated 300 megabytes have actually leaked on the Internet, according to one person familiar with the matter who insisted on speaking anonymously in order to discuss details of the investigation.
U.S. intelligence officials said the HBO intrusion appears to be much smaller in scale than a similar data breach at Sony in 2014, in which thieves claimed to have made off with roughly 100 TB of data. That number was never fully confirmed, but the intrusion involved the leaking of a trove of documents and emails, many of which were embarrassing in nature. The following year, during the fall-out from those revelations, Amy Pascal, the chairperson of the Motion Pictures Group of Sony Pictures Entertainment, stepped down.
HBO officials said Wednesday that they are continuing to look for evidence that the hackers accessed company communications.
"At this time, we do not believe that our e-mail system as a whole has been compromised, but the forensic review is ongoing," wrote chief executive Richard Plepler in a memo to staff. When reached, HBO declined to comment further.
Thus far, the data breach appears limited to HBO, according to Time Warner, and none of its other subsidiaries has been affected. But it could still take days or weeks of investigating to know the full impact of the leaks, security experts say.
"You don't know what you don't know," said Hemanshu Nigam, a former federal prosecutor and founder of Internet security company SSP Blue.
The HBO incident is the latest this year in a series that has targeted blockbuster entertainment companies, including Netflix. The streaming video giant in May rebuffed the demands of hackers who threatened to release new episodes of "Orange Is the New Black" unless they were paid a ransom. While Netflix successfully weathered the storm — its stock price even increased despite the leaks — the breach at HBO comes at a sensitive time as its parent company, Time Warner, seeks to close an $85 billion acquisition by AT&T. Both companies have said they still expect to complete the transaction by the end of the year.
A wider breach could be risky for Time Warner, which also owns CNN. The news outlet is a frequent object of criticism by President Trump, who in 2016 vowed to block the AT&T-Time Warner deal. Antitrust experts say Trump has little direct say in the outcome. Still, AT&T-Time Warner and other recently announced acquisitions, such as Amazon's proposed purchase of Whole Foods, have led to increased popular scrutiny of large, consolidated businesses.
In its earnings report Wednesday, Time Warner said subscriptions to HBO jumped by $106 million, an increase of 8 percent.
As a dwindling number of players in entertainment becomes responsible for ever more of the nation's cultural output, the remaining companies stand out as major targets, some analysts say. Each passing leak of Hollywood data emboldens copycats and organized criminal actors who view hacking firms such as HBO as a financial opportunity, said Nigam. And it increases the amount of time and money companies must spend on forensic investigations.
Ellen Nakashima contributed reporting.