a) how do they acquire users' web history? Do they purchase it from ISP?
b) how do they know what ISP to ask?
c) how does the ISP know my web history? Last time I checked, almost all websites I visit use https, so the only thing they can see is domain, not /pathname/
d) how does it feel to poke your nose where it doesn't belong?
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Coupled with: https://www.vice.com/en/article/gygx7y/your-anonymous-browsi...
b) Afaik there's data brokers that know this and collect data from many different kinds of sources (isps being only one of them) those then sell to people like private investigators and private intelligence companies. Sadly I don't have a source ready, I learned this on a podcast and completely forgot which one. But the vice article above goes into that direction too.
c) That's correct, but that's also already valuable on it's own. I assume the /pathnames/ can come from third party tracking / ad companies, but dns data alone is also valuable. It's all in the patters of the (meta)data
d) Very likely exciting to many. Voyeurism and the "forbidden apple" aren't exactly new concepts