This flaw lets attackers assume control over a victim machine by sending an SMB request to Windows Search.
"Attackers are mostly relying on unpatched systems," says Amol Sarwate, director of engineering at Qualys.
The problem is that some systems – particularly older ones – may not be able to handle the Creators Update, Microsoft said.
The US giant has been focusing on updating newer kit first, since it feels that hardware is less likely to run into difficulties, and has stopped automatically offering the latest Windows 10 upgrade to computers it suspects will suffer from breakdowns.
The update that is causing the problem, KB3194496, is not installing correctly for users.