Credential Security Support Provider protocol (CredSSP) is an
authentication provider that processes authentication requests for
A remote code execution vulnerability exists in unpatched versions of
CredSSP. An attacker who successfully exploits this vulnerability
could relay user credentials to execute code on the target system. Any
application that depends on CredSSP for authentication may be
vulnerable to this type of attack.
March 13, 2018
The initial March 13, 2018, release updates the CredSSP authentication
protocol and the Remote Desktop clients for all affected platforms.
Mitigation consists of installing the update on all eligible client
and server operating systems and then using included Group Policy
settings or registry-based equivalents to manage the setting options
on the client and server computers. We recommend that administrators
apply the policy and set it to “Force updated clients” or “Mitigated”
on client and server computers as soon as possible. These changes
will require a reboot of the affected systems.
Pay close attention to Group Policy or registry settings pairs that
result in “Blocked” interactions between clients and servers in the
compatibility table later in this article.
April 17, 2018
The Remote Desktop Client (RDP) update update in KB 4093120 will
enhance the error message that is presented when an updated client
fails to connect to a server that has not been updated.
May 8, 2018
An update to change the default setting from Vulnerable to Mitigated.
Source: https://support.microsoft.com/en-us/help/4093492/credssp-updates-for-cve-2018-0886-march-13-2018 
See also this reddit thread:
- Update server and client. (requires restart, recommended)
Not recommended workarounds if your server is publicly available, or if you do NOT have strict traffic control in your internal network, but sometimes restarting RDP server in work hours is a no go.
- Set CredSSP patching policy via GPO or the Registry. (requires restart or gpupdate /force)
- Uninstall KB4103727 (no restart required)
- I think that disabling NLA (Network Layer Authentication) may work too. (no restart required)
Be sure to understand the risks when using those and patch your systems ASAP.
 All GPO CredSSP description and registry modifications are described here.
 examples of GPO and registry settings in case Microsoft's site goes down.