Posted in case this impacts you too.
Security Update for SQL Server 2008 R2 Service Pack 2 (KB2977320) repeatedly failed to install today on a 2012.R2 server (SQL Express instance which runs Kaspersky Security Center). Error 0x80070643 is issued. Most pointers on the web suggest this may be .NET error related rather than SQL.
I ran the installer manually (extracted from c:\Windows\SoftwareDistribution\Install after failed Windows Update) and the .NET 3.5 feature "couldn't be installed", as expected with 2012.R2 I guess. Installed the .NET 3.5 package via 2012 Server Manager and reran the standalone installer successfully.
Not sure how Kaspersky originally installed the 2008 SSE instance as .NET 3.5 appears to be a requirement.