The attack occurred on July 27 and was done by a highly sophisticated culprit. Specific areas of data were targeted, and there is a fear that video, email and internal documents could be leaked. The network notified their employees on Saturday via email and warning them not to open any suspicious emails.
On July 30, a group going by the name of "little.finger66" claimed to pull off "the greatest leak of cyber space era" [sic] and soon after, unaired episodes of Ballers and Room 104 were leaked, as well as a link to the script for the August 6 episode of Game of Thrones. Incidentally, the hacker's name is a Game of Thrones reference to a character on the show.
It still isn't known exactly what data was stolen but the amount of data suggests massive amounts of video. Erik Rasmussen, a former special agent with the Secret Service who now works at the cybersecurity firm Kroll, said, "At 1.5 terabytes, it could be a whole block of TV, or worse, it could be emails, financial documents, employee or customer information. The fact that you have law enforcement and a [cybersecurity] firm involved most likely means this will be a very large incident for HBO."
HBO is said to be working with the FBI and cybersecurity firm Mandiant to get to the bottom of the incident.
