We've been having the somewhat common problem of not being able to add the .NET 3.5 feature on our Windows Server 2012 R2 machines, and up until recently it was easy to resolve. After mounting an OS ISO, we could either point to the sources directory in the Add/Remove Features wizard or run a nifty little PowerShell command:
Install-WindowsFeature NET-Framework-Core -Source E:\Sources\SxS.
But this week I ran into the problem where adding the
Source parameter didn't work, but I was lucky enough to stumble on a solution. Windows Update KB2966828 has something to do with an ASLR vulnerability in .NET, and it also seems to break the installation of .NET 3.5.
So to get the install working again, I just uninstalled KB2966828 and adding the feature worked as expected.