HBO's Facebook and Twitter accounts have been compromised in the latest cyber security breach to hit the firm.
A group called OurMine appeared to take control of the main HBO accounts, as well as those for the network's shows including Game of Thrones.
One posts said "OurMine are here. we are just testing your security".
It is the latest cyber security headache for the entertainment firm after hackers released Game of Thrones scripts and company data.
Some of the social media posts were removed quickly afterwards.
HBO did not immediately respond to the BBC's request for comment.
OurMine has a reputation for hacking high profile Twitter accounts.
Last year it compromised Netflix, as well as Wikipedia co-founder Jimmy Wales, Facebook co-founder Mark Zuckerberg and Google chief executive Sundar Pichai.
The hack is the latest in a string of security setbacks for HBO in recent weeks.
Unidentified hackers claim to have stolen 1.5TB of data from the company in July.
Out of the haul they released Game of Thrones scripts, company documents and unbroadcast episodes of HBO's other shows including Curb Your Enthusiasm and Insecure.
Separately, four people have been arrested for leaking an episode of Game of Thrones before it aired.The accused were current and former employees of a Mumbai-based company that stores and processes the series for an online streaming service.
Next came a leak in Europe. An episode of the fantasy show was mistakenly released on to its broadcaster's Spanish and Nordic streaming platforms days before it was scheduled to be broadcast.
The episode, titled Death is the Enemy, has since been withdrawn, but not before it was copied and circulated on several file-sharing platforms.