Windows Print Spooler Remote Code Execution Vulnerability [CVE-2021-34527]

CVE number – CVE-2021-34527 also known as PrintNightmare

Microsoft is aware of and investigating a remote code execution vulnerability that affects Windows Print Spooler and has assigned CVE-2021-34527 to this vulnerability. This is an evolving situation and we will update the CVE as more information is available.

A remote code execution vulnerability exists when the Windows Print Spooler service improperly performs privileged file operations. An attacker who successfully exploited this vulnerability could run arbitrary code with SYSTEM privileges. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights.

An attack must involve an authenticated user calling RpcAddPrinterDriverEx().

Please ensure that you have applied the security updates released on June 8, 2021, and see the FAQ and Workaround sections in this CVE for information on how to help protect your system from this vulnerability.

Determine if the Print Spooler service is running (run as a Domain Admin)

Run the following as a Domain Admin:

Get-Service -Name Spooler

If the Print Spooler is running or if the service is not set to disabled, select one of the following options to either disable the Print Spooler service, or to Disable inbound remote printing through Group Policy:

Option 1 – Disable the Print Spooler service

If disabling the Print Spooler service is appropriate for your enterprise, use the following PowerShell commands:

Stop-Service -Name Spooler -Force

Set-Service -Name Spooler -StartupType Disabled

Impact of workaround Disabling the Print Spooler service disables the ability to print both locally and remotely.

Option 2 – Disable inbound remote printing through Group Policy

You can also configure the settings via Group Policy as follows:

Computer Configuration / Administrative Templates / Printers

Disable the “Allow Print Spooler to accept client connections:” policy to block remote attacks.

Impact of workaround This policy will block the remote attack vector by preventing inbound remote printing operations. The system will no longer function as a print server, but local printing to a directly attached device will still be possible.

For more information see: Use Group Policy settings to control printers.

Jason Davies

UK based technology professional, with an interest in computer security and telecoms.

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