Windows 8 is only three years old, and with Microsoft's 10-year support policy, you'd assume it would receive free security updates for many more years. But thanks to a quirk in that support lifecycle, time is running out. If you're still using Windows 8, it's time to upgrade. 
Microsoft's support lifecycle for each version of Windows is 10 years from the operating system's release date. This means that even those still running Windows Vista can count on receiving security updates for more than a year until April 11, 2017. If you're running Windows 7, you'll be sure to receive updates until January 14, 2020.
So what happened to Windows 8, which was released in October 2012, should be supported for the next decade, right?
No.
Thanks to a quirk of the support lifecycle, Windows 8 will be cut off from security updates. The first Patch Tuesday, January 12, 2016, will be the last batch of security updates for Windows 8.
So if you're running Windows 8, it's time to update to Windows 8.1 or Windows 10 or else your computer risks being exposed to future zero-days.
But let's see why this particular issue arose with Windows 8..
The reason for this strange situation is because Microsoft considers Windows 8.1 to be a service pack for Windows 8. The company's regulations allow users of the operating systems not to install a service pack, but only for two years.
Below is Microsoft's official position:
Unlike service packs, which are usually just a collection of fixes, Windows 8.1 has new features and improvements. We designed Windows 8.1 to give customers the ability to deploy this update in a way that is similar to the way our customers deploy service packs, so we have applied our existing service pack support policy to Windows 8.1….
.....support for Windows 8 ends 24 months after the service pack is released or at the end of the product support lifecycle, whichever comes first. If you are using the software without the latest service pack, no new security or non-security updates will be offered, although pre-existing updates will continue to be provided.
The good news is that upgrading to Windows 8.1 significantly increases product support time. Windows 8.1 reaches the end of its extended support phase on January 10, 2023.
For those who don't know, upgrading from Windows 8 to 8.1, unlike any other Windows upgrade, requires you to go through the Windows Store, rather than Windows Update. The update is free for almost everyone. The only exception: Any company using a Windows 8 Volume License that doesn't include Software Assurance will have to pay extra.
