Microsoft updates have historically been published on both both the Microsoft Download Center and the Microsoft Update Catalog and Security Bulletins linked directly to update packages on the Microsoft Download Center.
Microsoft yesterday notified users via an e-mail that all Windows updates will be available only via the Microsoft Update Catalog (http://catalog.update.microsoft.com/), and will no longer be available on the Microsoft Download Center (http://download.microsoft.com/) from May 2016 security bulletin release.
Security bulletins will continue to link directly to the updates, but will now point to the packages on the Microsoft Update Catalog for updates not available on the Microsoft Download Center. Customers that use tools linking to the Microsoft Download Center should follow the links provided in the Security Bulletins or search directly on the Microsoft Update Catalog.
Microsoft is doing this change because making the updates available from only one location simplifies the process for customers of finding and downloading security updates.