In an email sent to employees on Monday and shared with CNN Tech, chairman and CEO Richard Plepler said a cyber intrusion resulted in "some stolen proprietary information, including some of our programming."
"Any intrusion of this nature is obviously disruptive, unsettling, and disturbing for all of us," Plepler said.
It's unclear what information hackers stole and potentially leaked. According to Entertainment Weekly, which first reported the intrusion, hackers published one episode each of "Ballers" and "Room 104," as well as the alleged script to next week's "Game of Thrones."
Related: A hacker says it has the new season of Netflix's 'Orange is the New Black'
HBO, which like CNN is owned by Time Warner, declined to comment on the specifics of what content was stolen or leaked due to an ongoing investigation. A spokesperson said the firm is working with law enforcement and outside cybersecurity companies to investigate the hack.
HBO is the latest entertainment firm to be hit by a cyber breach. Earlier this year, a hacker leaked the fifth season of Netflix's "Orange is the New Black" online. A hacker stole copies of the show through a third-party production company. The hacker claimed Netflix (NFLX, Tech30) did not pay a request for ransom to keep the show offline.
The hit series "Game of Thrones" has experienced leaks ahead of schedule before. In 2015, the first four episodes of the fifth season were leaked shortly after the season premiere. The leaked episodes originated "from within a group approved by HBO to receive them," HBO said at the time.