If your smart lightbulb, baby monitor, or old router has default passwords, a worm like Mirai has likely recruited it. Your device is now a proxy in someone else's farm.
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The operator rents a cloud server (often using stolen credit cards or cryptocurrency). They then purchase a list of proxies. The cheapest are "datacenter proxies" (easily detected), while the best are "residential proxies" harvested from infected IoT devices like routers and smart fridges.