“It’s quite literally a rogue executable file that has somehow appeared in c:\windows\temp\mubstemp and is digitally signed by Microsoft.”
The pop-up lands on top of games and apps to push Bing on people who chose Chrome. It skips the notification center and every setting a user could turn off. Microsoft’s statement calls it “unintended behavior,” about a signed executable it shipped to users’ machines. Malware has to sneak in, and Microsoft already holds the signing key.