RedTube.com was down temporarily on Feb. 5, but resumed operations after just a few hours, running on a different server.
The site is currently trying to detect how the hackers bypassed its security features.
The site has an Alexa ranking of 67, making it one of the most visited sites in the world, and reportedly attracted 4 million visitors in January.
In an unrelated but coincidental incident also reported on Feb. 6, Turkish hackers targeted the German town of Ludwigshafen, in apparent retribution for a fire that claimed the lives of nine Turkish immigrants.
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