Authentication error has occurred – Function requested is not supported

If when you try to use the Remote Desktop Connection between two Windows computers and you receive the error message; An authentication error has occurred – The requested function is not supported, this post is intended to help you. In this article, we will identify some known potential causes that could trigger the error, and then provide possible solutions that you can try to resolve this problem.

When Remote Desktop Connection authentication fails, you will receive the following error message;

Remote Desktop connection

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

Remote computer: Computer_Name or IP_Address
This may be due to the correction of the CredSSP encryption oracle.
For more information, see https://go.microsoft.com/fwlink/?linkid=866660

As you can see in the image above, the error message is caused by the CredSPP Encryption Oracle Remediation.

Recently, Microsoft has found that a remote code execution vulnerability (CVE-2018-0886: Oracle encryption attack) exists in versions of CredSSP. An attacker who successfully exploited this vulnerability could relay user credentials to execute code on the target system. Thus, any application that depends on CredSSP for authentication was vulnerable to this type of attack.

To address this security risk, Microsoft has released a security update that addresses the vulnerability by modifying the way CredSSP validates requests during the authentication process. The fix updated the CredSSP and Remote Desktop clients authentication protocol for all affected platforms.

After installing the update, the corrected clients could not communicate with the uncorrected servers. In other words, if the client computer installed the security update but the server computer was not updated with the security update (or vice versa), the remote connection failed and the user received the above error message.

RDP connection error: an authentication error has occurred

If you are faced with this An authentication error has occurred – The requested function is not supported problem, you can try our recommended solutions below to fix the problem.

1. Update Windows 10 with the latest security patches
2. Edit the Encryption Oracle remediation Politics
3. Create and configure the AllowEncryptionOracle registration key

Let’s take a look at the description of the process involved in relation to each of the solutions listed.

1]Update Windows 10 with the latest security patches

In this solution, it is recommended to install the CredSSP security patch in both computers (server and client). You can also click on Start > Settings > Update and security > Windows Update > Check for updates to download and install the latest cumulative update.

Once the CredSSP patch is installed on both computers, the An authentication error has occurred – The requested function is not supported the error message will be resolved.

If for some reason you cannot install the security update on the server or client computer, you can then use solutions 2 and 3 below.

2]Edit the Encryption Oracle remediation Politics

The error message can be resolved using the Group Policy Editor to modify the Encryption Oracle remediation Politics.

Note: This method does not apply to Windows 10 Home edition because the local Group Policy Editor is not installed by default. But you can work around this problem by adding the local Group Policy Editor to Windows 10 Home Edition.

To activate the Oracle Encryption remediation policy, follow these steps:

• Press Windows key + R.
• In the Run dialog box, type gpedit.msc and press Enter to open the Group Policy Editor.
• In the Local Group Policy Editor, use the left pane to navigate to the path below:

Computer Configuration> Administrative Templates> System> Delegation of Credentials

• In the right pane, double-click Encryption Oracle remediation to modify its properties.
• With the Oracle Remediation encryption policy open, set the radio button to Activated.
• Then scroll down to Protection level and change it to Vulnerable.
• Click on Apply > Okay to save the changes.

You can now exit the Local Group Policy Editor and restart your computer. At startup, try the RDP connection again and see if the problem is resolved.

3]Create and configure the AllowEncryptionOracle registration key

It’s the equivalent of activating Encryption Oracle remediation Politics. You can resolve the problem by creating and configuring the following registry key:

AllowEncryptionOracle: DWORD: 2

Since this is a registry operation, it is recommended that you back up the registry or create a system restore point in case the procedure does not work.

Once you have taken the necessary precautionary measures, you can do the following:

HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionPoliciesSystem

• Right click System, to select New > Key and define his name as CredSSP.
• Then right click CredSSP, to select New > Key and define his name as Settings.
• Now right click on the empty space in the right pane and then select New > DWORD value (32 bit).
• Rename the value name to AllowEncryptionOracle and press Enter.
• Double-click on the new value to modify its properties.
• Contribution 2 in the Value data box and press Enter to save the change.
• Exit Registry Editor and restart your PC.

You should now be able to establish the Remote Desktop connection successfully!

• Keywords: Errors, Remote Desktop