HBO Hit by Hackers

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HBO continued to assess the extent of a cyberattack on its systems on Wednesday, working with law enforcement agencies and seeking to provide assurance to its employees.

In a memo to staff, Richard Plepler, the premium cable network’s chief executive, said that the company did not believe its “email system as a whole has been compromised” but that a forensic review was being conducted.

Mr. Plepler wrote in the memo that the network was trying to hire an outside firm “to work with our employees to provide credit monitoring.”

HBO acknowledged on Monday that it had been victimized by a cyberattack, after an anonymous hacker boasted about leaking full episodes of upcoming shows like “Ballers” along with written material from next week’s episode of “Game of Thrones.”

The hacker or hackers claimed to have stolen an estimated 1.5 terabytes of data, according to Entertainment Weekly, which first reported the breach on Monday. The hacker said more material would be “coming soon.

The attack is not expected to have any impact on Time Warner’s deal with AT&T, according to two people familiar with the matter, who requested anonymity to discuss private company matters. Only a large material change in Time Warner’s business would prevent the deal from going through, one of the people said.

Unlike Yahoo, which disclosed last year that hackers had stolen the credentials of hundreds of millions of users in two breaches that went undetected for years, HBO does not appear to have sensitive personal information on a similar scale. Because the network typically sells its service through cable or satellite operators, it has not traditionally collected credit card information and other details about its customers. That has changed recently, however, as HBO introduced a stand-alone streaming service in 2015.

Much remains unclear about how much information was obtained in the cyberattack, which can be difficult to determine, said one of the people familiar with the breach. The person said the stolen content included a few show episodes, scripts and internal documents, like human resources records.

There is no evidence yet that any episodes of “Game of Thrones” have been stolen, which would be a significant blow to the network. “Game of Thrones,” currently in its penultimate season, is the most popular show in HBO’s history and has long been a target of digital piracy. There are four highly anticipated episodes left in the season.

The network said in a statement on Monday that it was investigating the incident and was “working with law enforcement and outside cybersecurity firms.”

In an email to employees this week, Mr. Plepler wrote, “Any intrusion of this nature is obviously disruptive, unsettling and disturbing for all of us.”

He added: “The problem before us is unfortunately all too familiar in the world we now find ourselves a part of.”

The breach of HBO, a major entertainment company, evokes the 2014 hacking of Sony Pictures. Hackers released tens of thousands of emails, some of them embarrassing, as well as scripts and financial data. United States officials placed responsibility for the attack on North Korea, and the Obama administration imposed sanctions on some North Korean officials. In the aftermath of the attack, the Sony Pictures chief, Amy Pascal, was ousted.