https://www.vice.com/en_us/article/akgj5z/microsoft-tries-fails-to-patch-critical-windows-vulnerability-chaos-ensues

In early June, Microsoft patched a Windows vulnerability that it initially classified as low risk. But security researchers say that patch didn't actually fix a closely related, much worse vulnerability. Researchers have since published proof-of-concept code that shows exactly how to exploit the bug, a sign that the bug is likely to be exploited if users don't ultimately update their systems once the vulnerability is fixed. 

The bug is called PrintNightmare, as it exploits a flaw in a legacy Windows' printing service that is reported to be used by default by several versions of Windows and Windows Server.