Fixing Remote Desktop Connection “An authentication error has occurred”

This post talks about how you can resolve Remote Desktop authentication issues due to CredSSP encryption oracle remediation.

Why is the error and why does it occur?

Recently there was an exploit found in the CredSSP module which affected a number of systems including remote desktop. This allowed remote code execution allowing someone to attack and gain control over a system.

The typical error is shown and reproduced in text below.

Remote Desktop Connection
An authentication error has occurred.
The function requested is not supported

Remote computer: X
This could be due to CredSSP encryption oracle remediation,
For more information, see https://go.microsoft.com/fwlink/?linkid=866660

This will pop up when you try and remote to, or from, a system which does not have this patch installed. Typically this will occur if the machine has not been updated for a period of time.

Resolving this error

To resolve this issue you need to ensure both client and server (the machine you are connecting to) has been updated recently.

Specifically you need to have installed the May 2018 update which resolves an exploit in the remote desktop CredSSP system.

Once updates have been performed you should be able to remote into the machine without any issue.