Rapid 7 - CVE-2021-1675 (PrintNightmare) Patch Does Not Remediate Vulnerability

On June 8, 2021, Microsoft released an advisory and patch for CVE-2021-1675 (“PrintNightmare”), a critical vulnerability in the Windows Print Spooler. Although originally classified as a privilege escalation vulnerability, security researchers have demonstrated that the vulnerability allows authenticated users to gain remote code execution with SYSTEM-level privileges. On June 29, 2021, as proof-of-concept exploits for the vulnerability began circulating, security researchers discovered that CVE-2021-1675 is still exploitable on some systems that have been patched. As of this writing, at least 3 different proof-of-concept exploits have been made public.

Rapid7 researchers have confirmed that public exploits work against fully patched Windows Server 2019 installations. The vulnerable service is enabled by default on Windows Server, with the exception of Windows Server Core. Therefore, it is expected that in the vast majority of enterprise environments, all domain controllers, even those that are fully patched, are vulnerable to remote code execution by authenticated attackers.

The vulnerability is in the RpcAddPrinterDriver call of the Windows Print Spooler. A client uses the RPC call to add a driver to the server, storing the desired driver in a local directory or on the server via SMB. The client then allocates a DRIVER_INFO_2 object and initializes a DRIVER_CONTAINER object that contains the allocated DRIVER_INFO_2 object. The DRIVER_CONTAINER object is then used within the call to RpcAddPrinterDriver to load the driver. This driver may contain arbitrary code that will be executed with SYSTEM privileges on the victim server. This command can be executed by any user who can authenticate to the Spooler service.
Mitigation Guidance
Since the patch is currently not effective against the vulnerability, the most effective mitigation strategy is to disable the print spooler service itself. This should be done on all endpoints, servers, and especially domain controllers. Dedicated print servers may still be vulnerable if the spooler is not stopped. Microsoft security guidelines do not recommend disabling the service across all domain controllers, since the active directory has no way to remove old queues that no longer exist unless the spooler service is running on at least one domain controller in each site. However, until this vulnerability is effectively patched, this should have limited impact compared to the risk.

On Windows cmd:

net stop spooler

On PowerShell:

Stop-Service -Name Spooler -Force
Set-Service -Name Spooler -StartupType Disabled

The following PowerShell command can be used to help find exploitation attempts:

Get-WinEvent -LogName 'Microsoft-Windows-PrintService/Admin' | Select-String -InputObject {$_.message} -Pattern 'The print spooler failed to load a plug-in module'

Rapid7 Customers
While InsightVM and Nexpose checks for CVE-2021-1675 were released earlier in June, we are currently investigating the feasibility of additional checks to determine whether the print spooler service has been disabled in customer environments.

We will update this blog as further information comes to light.

from Rapid7 Blog https://blog.rapid7.com/2021/06/30/cve-2021-1675-printnightmare-patch-does-not-remediate-vulnerability/