History sniffingat least the way that YouPorn did itinvolves exploiting a JavaScript vulnerability in the browser so that the site can see which other porn sites you've been viewing. In theory, this information could be used by a competing site in order to figure out what tickles your fancy and offer you something better, or by offering that data to the site's ad network so that you'll get more specifically targeted ads. Turns out that you've been surfing for pterodactyl porn at one site and something equally weird at another? YouPorn might try to use that information to better "serve" you.
