Unable to RDP to an Azure VM due to a CredSSP Encryption Oracle Remediation error

June 27, 2018 at 7:22 pm in Azure, Cloud, CredSSP, Encryption Oracle Remediation, RDP, Remote Desktop Connection, VM, Windows 10 by Wim Matthyssen

After applying some Windows updates on my Windows 10 Version 1803 home pc I was unable to make a Remote Desktop Connection (RDP) connection to some Microsoft Azure virtual machine(s) (VM).

When I made an RDP connection, I received the following error message:

An authentication error has occurred. The function requested is not supported. Remote computer: <computer name or IP>. This could be due to CredSSP encryption oracle remediation. For more information, see https://go.microsoft.com/fwlink/?linkid=866660.

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What is CredSSP and why did it cause the error

The Credential Security Support Provider protocol (CredSSP) is a security protocol utilized to process authentication requests for separate applications like RDP. It allows you to securely forward credentials encrypted from the Windows client to the target servers for remote authentication.

Because of a critical vulnerability that has been discovered in CredSSP, which affects all versions of Windows and could allow remote attackers to exploit RDP and WinRM to steal data and run malicious code, Microsoft has released security update(s).
You can find the list of the corresponding KB number(s) for each operating system here: https://portal.msrc.microsoft.com/en-us/security-guidance/advisory/CVE-2018-0886

In my case my recently updated Windows 10 pc could not communicate with a non-updated server (not allowed to setup an insecure RDP connection).

Workaround

To solve the error, first of all, I needed to temporarily change the policy settings on my Windows 10 to gain RDP access to the server.

To do so, open Run and execute gpedit.msc to change the settings in the Local Group Policy Editor. Browse to Computer Configuration / Administrative Templates / System. Open Credentials Delegation in the left pane.

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Change the Encryption Oracle Remediation policy to Enabled, and Protection Level to Vulnerable.

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After this change, I was able to setup an unsecure RDP connection to the server(s) where I installed the missing security update.

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After deploying the specific update on the server(s), I was able to connect to it without the error and with the Encryption Oracle Remediation settings reset to the default.

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This concludes this blog post, hope it helps if you face this error.

Wim Matthyssen (@wmatthyssen)

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