Episode six of the blockbuster fantasy series "Game of Thrones" season seven has been leaked to internet piracy sites, four days ahead of Sunday's premiere.

It is the second episode of the season to be leaked online ahead of its airdate.

This leak wasn't caused by hackers or rogue employees at a production facility but evidently it was inadvertently published by HBO Nordic and HBO España services. 

According to HBO, the leak appears to have stemmed from a mistake by a third-party vendor in those territories.

"We have learned that the upcoming episode of 'Game of Thrones' was accidentally posted for a brief time on the HBO Nordic and HBO España platforms," Tom Krogsgaard Nielsen, VP of public relations for HBO Europe, said in a statement. He added, "This is not connected to the recent cyber-incident at HBO in the U.S."

Links to the leaked "GOT" episode were posted to Reddit, with the copy shared via YouTube, Twitch and other streaming services. The Reddit posts were swiftly taken down.

In a separate incident two weeks ago, the fourth episode of the current season of "Game of Thrones" leaked out. That came from a data-management vendor enlisted by 21st Century Fox-owned Sky India. 

Police in Mumbai have arrested four people, who are current or former employees of Prime Focus Technologies, in connection with the crime.