One more time, Microsoft has released an emergency patch to plug a hole in the software that is supposed to protect you from the bad guys.
The Windows Defender family of products (including Microsoft Security Essentials, as well as Endpoint Protection, Forefront Endpoint Protection, and Exchange Server 2013 and 2016) has to open the files that you receive via email or text or the network in order to scan them for bad stuff. Unfortunately, the software is less than bug free and at least 3 times in the last few months, Microsoft has had to release emergency patches.
Apparently Microsoft felt it important enough to release an emergency patch just a few days before the monthly patch release. Something got their attention. Probably something GCHQ or the NSA told them.
The bug was discovered by GCHQ, the British version of the NSA. In an unusual move, they decided to tell Microsoft rather than hoarding the bug and letting the bad guys use it. Sometimes this happens because, as a result of their spying, they have discovered that the bad guys already know about the hole. When this happens, they usually decide that plugging it is the wiser course of action.
The end result of this bug, if a bad guy is able to exploit it, is to execute arbitrary code on your computer – not a great situation.
The good news is that for many users the patch will automatically be installed, but you should check to make sure that you are one of those lucky users.
Information for this post came from the Register.