CVE-2021-34527 "PrintNightmare"

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.

UPDATE July 7, 2021: The security update for Windows Server 2012, Windows Server 2016 and Windows 10, Version 1607 have been released. Please see the Security Updates table for the applicable update for your system. We recommend that you install these updates immediately. If you are unable to install these updates, see the FAQ and Workaround sections in this CVE for information on how to help protect your system from this vulnerability.

In addition to installing the updates, in order to secure your system, you must confirm that the following registry settings are set to 0 (zero) or are not defined (Note: These registry keys do not exist by default, and therefore are already at the secure setting.), also that your Group Policy setting are correct (see FAQ):

• HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\Printers\PointAndPrint
• NoWarningNoElevationOnInstall = 0 (DWORD) or not defined (default setting)
• UpdatePromptSettings = 0 (DWORD) or not defined (default setting)

Having NoWarningNoElevationOnInstall set to 1 makes your system vulnerable by design.

UPDATE July 6, 2021: Microsoft has completed the investigation and has released security updates to address this vulnerability. Please see the Security Updates table for the applicable update for your system. We recommend that you install these updates immediately. If you are unable to install these updates, see the FAQ and Workaround sections in this CVE for information on how to help protect your system from this vulnerability. See also KB5005010: Restricting installation of new printer drivers after applying the July 6, 2021 updates.

Note that the security updates released on and after July 6, 2021 contain protections for CVE-2021-1675 and the additional remote code execution exploit in the Windows Print Spooler service known as “PrintNightmare”, documented in CVE-2021-34527.

Recent assessments:

zeroSteiner at July 08, 2021 5:09pm UTC reported:

CVE-2021-34527 is related to the previous CVE-2021-1675. This fixes a vulnerability whereby an authenticated attacker can connect to the remote print service (via either MS-RPRN or MS-PAR) and add a driver using a custom DLL. Upon successful exploitation, the Print Spool service would load the attacker controlled DLL from either a remote UNC path or a local path. In both cases, the DLL is then executed with NT AUTHORITY\SYSTEM privileges.

The patch for CVE-2021-34527 is effective at preventing this attack only when Point and Print is disabled, which is the default setting. This can be configured by ensuring the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\Printers\PointAndPrint NoWarningNoElevationOnInstall is 0. The system does not need to be rebooted to enforce the changed registry key. If that registry key is defined as 1, the vulnerability can still be exploited. With Point and Print enabled, a standard UNC path used over the MS-RPRN vector (via RpcAddPrinterDriverEx) will fail with ERROR_INVALID_PARAMETER. This can be bypassed by converting the UNC path from the standard syntax (\\1.2.3.4\public\payload.dll) to the alternative syntax (\??\UNC\1.2.3.4\public\payload.dll).

With the patches applied and Point and Print disabled, the affected calls to RpcAddPrinterDriverEx will return ERROR_ACCESS_DENIED.

ccondon-r7 at July 08, 2021 12:12am UTC reported:

CVE-2021-34527 is related to the previous CVE-2021-1675. This fixes a vulnerability whereby an authenticated attacker can connect to the remote print service (via either MS-RPRN or MS-PAR) and add a driver using a custom DLL. Upon successful exploitation, the Print Spool service would load the attacker controlled DLL from either a remote UNC path or a local path. In both cases, the DLL is then executed with NT AUTHORITY\SYSTEM privileges.

The patch for CVE-2021-34527 is effective at preventing this attack only when Point and Print is disabled, which is the default setting. This can be configured by ensuring the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\Printers\PointAndPrint NoWarningNoElevationOnInstall is 0. The system does not need to be rebooted to enforce the changed registry key. If that registry key is defined as 1, the vulnerability can still be exploited. With Point and Print enabled, a standard UNC path used over the MS-RPRN vector (via RpcAddPrinterDriverEx) will fail with ERROR_INVALID_PARAMETER. This can be bypassed by converting the UNC path from the standard syntax (\\1.2.3.4\public\payload.dll) to the alternative syntax (\??\UNC\1.2.3.4\public\payload.dll).

With the patches applied and Point and Print disabled, the affected calls to RpcAddPrinterDriverEx will return ERROR_ACCESS_DENIED.

Assessed Attacker Value: 5
Assessed Attacker Value: 5Assessed Attacker Value: 4