Researchers published proof of concept code that exploits a recently patched vulnerability in the Microsoft Windows operating system (OS) The vulnerability, named CurveBall, impacts the components that handle the encryption and decryption mechanisms in the Windows OS. When this vulnerability is exploited, CurveBall could allow a hacker to launch man-in-the-middle attacks, which is when a hacker secretly relays and possibly alters the communications between two unsuspecting users. With three popular operating systems afflicted, patching is more important than ever.
Source: https://www.mcafee.com/blogs/consumer/what-is-the-curveball-bug/