> That would never happen. A network tap would be able to detect a malicious update even if the main PC was implanted very well, and a Microsoft-signed malicious update would be worldwide news.

While I don't know of that specific scenario, Stuxnet used a hardware vendor's key to install infected drivers[1]. There was also a Chinese registrar that allowed a customer to man-in-the-middle Google[2]. Depending on how Windows organizes their driver updates, I could see an adversary doing a man-in-the-middle between Microsoft and their target, and pushing a bad driver update.

1. https://www.welivesecurity.com/2010/07/22/why-steal-digital-... 2. https://www.techdirt.com/articles/20140909/03424628458/china...