Pierluigi Paganini July 09, 2021

Microsoft confirmed that the emergency security updates (KB5005010) correctly address the PrintNightmare Print Spooler vulnerability (CVE-2021-34527).

Microsoft says that the emergency security patches released early this week correctly address the PrintNightmare Print Spooler vulnerability (CVE-2021-34527) for all supported Windows versions.

Immediately after the release of the updates (KB5004945) multiple researchers questioned its efficiency and explained that the updates don’t fully address the vulnerability.

Researchers have demonstrated that it is possible to bypass the emergency patch to achieve remote code execution and local privilege escalation on systems that have installed it.

Shortly after the release of the patch, the popular researcher Matthew Hickey noticed that the fix is incomplete and that threat actors and malware can still locally exploit the vulnerability to gain SYSTEM privileges.

The failure of the Microsoft update was also reported by Will Dormann, a vulnerability analyst for CERT/CC.

Other researchers started testing the patch and demonstrated that it was possible to entirely bypass the fix to achieve both RCE and local privilege escalation (LPE).

The expert Benjamin Delpy, also known for having developed the popular Mimikatz tool, discovered that when the Point and Print policy is enabled, attackers could bypass the patch to achieve Remote Code Execution.

The clarified guidance for CVE-2021-34527 Windows Print Spooler Vulnerability published by Microsoft encourages customers to update as soon as possible.

“Our investigation has shown that the OOB security update is working as designed and is effective against the known printer spooling exploits and other public reports collectively being referred to as PrintNightmare. All reports we have investigated have relied on the changing of default registry setting related to Point and Print to an insecure configuration.” states the guidance published by Microsoft Security Response Center.

The IT giant recommends that customers follow the following steps immediately:

• In ALL cases, apply the CVE-2021-34527 security update. The update will not change existing registry settings
• After applying the security update, review the registry settings documented in the CVE-2021-34527 advisory
• If the registry keys documented do not exist, no further action is required
• If the registry keys documented exist, in order to secure your system, you must confirm that the following registry keys are set to 0 (zero) or are not present:
  • 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)

Users that cannot immediately install the security updates are recommended to disable the Windows Print Spooler service to mitigate the PrintNightmare vulnerability temporarily.

The CERT Coordination Center (CERT/CC) has also published an alert that includes instructions on stopping and disabling the service in a separate Vulnerability Note.

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Pierluigi Paganini

(SecurityAffairs – hacking, PrintNightmare)

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