I've seen a new(?) type of cookie being used by porn AND non-porn sites. It's some sort of doo-hickey that automatically sends you to a specific page with popups everytime you start a browser.
And NO, it's not just the default page setting for the browser. I checked that first.
Took me a while to track the fucker down, but Froggy prevailed.
As fo the problem you described, a simple javascript can change your default start page. Annoying as all hell, but isn't that half the fun?
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I shot the sheriff... but the fucking deputy got away.