The hackers demanded "six month salary", which, according to the video, would be more than $6 million.
The same hackers have subsequently released two dumps of HBO material and demanded a multimillion-dollar ransom.
"There is a possibility that HBO's pre-emptive bug bounty program may have yielded a better security posture for them, avoiding this situation altogether", he added. "They're being extorted. If it was a bug bounty, it'd be on the up and up".
The message also asks the hackers to extend a ransom deadline for one week in order for the network to secure and transfer the requested bitcoin payment.
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The hack has come at a sensitive time for HBO, whose parent Time Warner is awaiting regulatory approval to sell itself to AT&T Inc in an $85.4 billion deal announced in October.
Beyler's e-mail, sent several days earlier, might have been an attempt to make the problem go away without too much bad publicity for HBO, said Sanjay Goel, a professor at the University at Albany and chairman of its information technology management department.
About a fortnight back, hackers broke into HBO's servers and stole a staggering 1.5 terabytes of data including five draft scripts of the seventh season of Game of Thrones as well as the personal numbers and addresses, home and email both, of the cast members of season 7 including Emilia Clarke, Lena Headey, Peter Dinklage, and Kit Harington.
HBO has said it was working with forensic experts and law enforcement in response to the attack.
In April, a hacker claimed to have released episodes of Netflix Inc's Orange is the New Black ahead of their official launch date.