From a security perspective this is not really a best practice, but sometimes you get into this horrible situation where you cannot logon to a server using RDP, and you don’t have access to the server console… sometimes necessity knows no law…
When you try to logon to a remote server using RPD an authentication error occurs, and you are not able to logon the following error is shown:
An authentication error has occurred.
The function requested is not supported
This could be due to CredSSP encryption oracle remediation
Unfortunately, the link provided in the error message points to a non-existing page on the Microsoft website…
In March 2018 Microsoft released a fix that addresses a CredSSP, “Remote Code Execution” vulnerability (CVE-2018-0886) that could impact RDP connections. If the host you are working on has this fix, and the server you are connecting to does not have this fix (can occur when deploying new VM’s remotely) the error shown above pops-up.
The best solution is to update the host you’re connecting to, but if it’s not possible to get access to the console for whatever reason, you can also lower the security on your own host (ouch!).
To do this, add the following registry entry to your own host:
REG ADD HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters\ /v AllowEncryptionOracle /t REG_DWORD /d 2
I strongly recommend raising security again when you have updated the remote server.