An exploit on Microsoft's download page allows users to obtain a free copy of Windows 8 Pro.
Users who attempt the download for the Microsoft Windows Media Center upgrade, which ends on January 31, will consequently see Windows 8 Pro being permanently activated.
For pirates, the exploit is especially useful as it provides an alternative to the company's Key Management Service (KMS). The KMS, which is built for enterprise users, delivers Windows 8 through a KMS host and KMS client, leading to the new operating system becoming fully usable for 180 days before activation.
As opposed to having to input an activation key, businesses can activate Windows copies on a local network. However, setting up pirate KMS servers will see keys traded across the web for free.
That said, Windows is required to be reactivated every 180 days should you use this method. Each individual PC needs a unique key, so multiple users won't be able to activate the OS through the same key.
Reddit user noveleven explained why the method in question is successful:
When you activate Windows via KMS, in the activation window it says "Windows is activated until..." and a date (so if you were to install it today, it would say it's activated until May). After installing the upgrade, the window just says "Windows was activated on..." and the date of activation. That means the activation is permanent. When you install the upgrade key, that replaces the existing product key; only the new upgrade key is used for future checks. Windows won't check the key you used to install because it no longer has it.
