Microsoft was already aware of the issue and working on a fix for a subsequent update, but the accidental publishing of the flaw caused them to move up the timetable for its release.
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The fact that Microsoft had this update ready to go within 48 hours of the vulnerability being exposed shows you how serious they felt it was so you should be sure to update Windows 10 as soon as possible if the update wasn’t applied automatically. In case you missed the news yesterday, the problem again was in the SMBv3 protocol which allows your users to share resources (files, printers, scanners, etc.) over the local network or the internet. The flaw in question could have allowed an attacker to inject malicious code onto a connected system, which could then self-replicate across additional machines. Researchers had already built proof-of-concept attacks based on the flaw, which was apparently trivial to identify given the knowledge that it existed, so again it’s easy to see why Microsoft addressed this as quickly as possible.