If you’re like me, you have encountered this error remoting into one of your servers.

An Authentication error has occurred. The function requested is not supported.
Remote computer: <servername> This could be due to CredSSP encryption oracle remediation

The quick solution is to patch your host from one of the patches here

If you are unable to patch and then issue the mandatory reboot of the remote server then you can apply the following registry fix

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters]
“AllowEncryptionOracle”=dword:00000002

Workaround Warning

After you change the following setting, an unsecured connection is allowed that will expose the remote server to attacks. Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case of problems occurring.

 

Scenario 1: Updated clients cannot communicate with non-updated servers

The most common scenario is that the client has the CredSSP update installed, and the Encryption Oracle Remediationpolicy setting does not allow an insecure RDP connection to a server that does not have the CredSSP update installed.

To work around this issue, follow these steps:

  1. On the client has the CredSSP update installed, run gpedit.msc, and then browse to Computer Configuration > Administrative Templates > System > Credentials Delegation in the navigation pane.
  2. Change the Encryption Oracle Remediation policy to Enabled, and then change Protection Level to Vulnerable.

If you cannot use gpedit.msc, you can make the same change by using the registry, as follows:

  1. Open a Command Prompt window as Administrator.
  2. Run the following command to add a registry value:
    REG ADD HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters\ /v AllowEncryptionOracle /t REG_DWORD /d 2

Scenario 2: Non-updated clients cannot communicate with patched servers

If the Azure Windows VM has this update installed, and it is restricted to receiving non-updated clients, follow these steps to change the Encryption Oracle Remediation policy setting:

  1. On any Windows computer that has PowerShell installed, add the IP of the VM to the “trusted” list in the host file:
    Set-item wsman:\localhost\Client\TrustedHosts -value <IP>
  2. Go to the Azure portal, locate the VM, and then update the Network Security group to allow PowerShell ports 5985 and 5986.
  3. On the Windows computer, connect to the VM by using PowerShell:
    For HTTP:
    $Skip = New-PSSessionOption -SkipCACheck -SkipCNCheck Enter-PSSession -ComputerName "<<Public IP>>" -port "5985" -Credential (Get-Credential) -SessionOption $SkipFor HTTPS:
    $Skip = New-PSSessionOption -SkipCACheck -SkipCNCheck Enter-PSSession -ComputerName "<<Public IP>>" -port "5986" -Credential (Get-Credential) -useSSL -SessionOption $Skip
  4. Run the following command to change the Encryption Oracle Remediation policy setting by using the registry:
    Set-ItemProperty -Path 'HKLM:\Software\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters' -name "AllowEncryptionOracle" 2 -Type DWord