On .NET Framework 4.6.1 and Exchange compatibility

Update (October 2016): The information in this blog post is now outdated. Blocking of .NET 4.6.1 is not needed anymore, as long as relevant updates are installed. Please see this article for more information as well as Exchange Supportability Matrix for versions of .NET that Exchange Server supports.

We wanted to post a quick note to call out that since yesterday, the .NET Framework 4.6.1 has been made a recommended update on WU (Windows Update). As we have already stated in the Exchange Supportability Matrix, at this time, this version of .NET framework is not supported by Exchange. In fact, we know of some issues if it is installed. We are working with the .NET team to ensure that Exchange customers have a smooth transition to .NET Framework 4.6.1, but in the meantime, delay this particular .NET update on your Exchange servers (information on how this can be accomplished can be found in the KB article 3133990, How to temporarily block the installation of the .NET Framework 4.6.1).

Update 2/12/2016:

We had several questions on how our customers should go about rolling back to .NET Framework 4.5.2 if 4.6.1 was already automatically installed. Here are the steps:

1. If the server has already automatically updated to 4.6.1 and has not rebooted yet, do so now to allow the installation to complete
2. Stop all running services related to Exchange.  You can run the following cmdlet from Exchange Management Shell to accomplish this:  (Test-ServiceHealth).ServicesRunning | %{Stop-Service $_ -Force}
3. Go to add/remove programs, select view installed updates, and find the entry for KB3102467.  Uninstall the update.  Reboot when prompted.
4. Check the version of the .NET Framework and verify that it is showing 4.5.2.  If it shows a version prior to 4.5.2 go to windows update, check for updates, and install .NET 4.5.2 via the KB2934520 update.  Do NOT select 4.6.1/KB3102467.  Reboot when prompted.  If it shows 4.5.2 proceed to step 5.
5. Stop services using the command from step 2.  Run a repair of .NET 4.5.2 by downloading the offline installer, running setup, and choosing the repair option.  Reboot when setup is complete.
6. Apply the February security updates for .NET 4.5.2 by going to Windows update, checking for updates, and installing KB3122654 and KB3127226.  Do NOT select KB3102467.  Reboot after installation.
7. After reboot verify that the .NET Framework version is 4.5.2 and that security updates KB3122654 and KB3127226 are installed.
8. Follow the steps here to block future automatic installations of .NET 4.6.1.

Thanks to Marc, Nasir and Brett for testing the above to make sure it all works as expected! Nino Bilic