Earlier quoted context omitted.
UAC is not a security boundary. Its only there to prevent the user from accidentally making changes to the system that they didn't intend, but it provides no security except from your own mistakes. Its so much not considered a security boundary that microsoft doesn't even offer a bug bounty for UAC bypasses.
Oh, user separation is not a security boundary? I was not aware! Let’s all work as root then! /s The fact that this was the norm on single-user Windows workstations until Windows Vista is astonishing, to say the least. How anyone could want that back is beyond me. It enabled malware to compromise the entirety of the computer with ease. No exploits or anything required! Certainly not one of these pesky UAC prompts! Ca…
>One of the common misconceptions about UAC and Same-desktop Elevation in particular is: it prevents malware from being installed, or from gaining administrative rights. First, malware can be written not to require administrative rights. And malware can be written to write just to areas in the user's profile. More important, Same-desktop Elevation in UAC isn't a security boundary. It can be hijacked by unprivileged software that runs on the same desktop. Same-desktop Elevation should be considered a convenience feature. From a security perspective, Protected Administrator should be considered the equivalent of Administrator. By contrast, using Fast User Switching to sign in to a different session by using an administrator account involves a security boundary between the administrator account and the standard user session.
UAC is not a security boundary, it's not the same thing as sudo on Unix. You only have a security boundary in place if Windows asks you for a password when trying to run as Administrator.