The accounts of several HBO series, including "Last Week Tonight with John Oliver", "True Blood", "Veep" and "Vinyl", tweeted a message Wednesday night purporting to be from the notorious OurMine hackers.
The Twitter account for "Game of Thrones" was also misused.
In two tweets posted to several of HBO's accounts, the hackers wrote "we are just testing your security" and "let's make #HBOHacked trending!"
HBO's summer of cyber-woes continues: Episode six of Game of Thrones season seven has been leaked to internet piracy sites, four days ahead of Sunday's premiere.
Footage from the episode was posted on Reddit via YouTube, Instagram, Twitch and other streaming services, before being taken down.
They also include shooting "diaries" of the seventh season of the phenomena Game of Thrones.
HBO has been the victim of hacking in recent weeks in the U.S. In India, episode four of Game of Thrones was recently leaked, leading to the arrest of four suspects.
HBO knows how to produce hit shows that have changed the television landscape, but its cyber security has some catching up to do.
Later, people started downloading "GoT" Episode 4 and sharing the clip with friends on various social media platforms. The news of hack first broke out on July 31 when the hackers reportedly released a handful of unaired episodes of HBO shows, as well as other internal data, online.