Threat Brief: Windows Print Spooler RCE Vulnerability (CVE-2021-34527 AKA PrintNightmare)

A conceptual image representing a threat brief, such as this post, covering CVE-2021-34527

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Executive Summary

On July 1, 2021, Microsoft released a security advisory for a new remote code execution (RCE) vulnerability in Windows, CVE-2021-34527, referred to publicly as "PrintNightmare.” Security researchers initially believed this vulnerability to be tied to CVE-2021-1675 (Windows Print Spooler Remote Code Execution Vulnerability), which was first disclosed in the Microsoft Patch Tuesday release on June 8, 2021. Microsoft has since updated the FAQ section of the advisory that shows CVE-2021-34527 is similar but distinct from CVE-2021-1675, which addresses a different but related vulnerability in RpcAddPrinterDriverEx().

Systems Vulnerable to CVE-2021-34527

All Windows versions are affected by this vulnerability. Domain controllers, clients and member servers running the Print Spooler service on any Windows version are affected by this vulnerability. Microsoft has released an out-of-band update with the fixes for versions other than Windows 10 version 1607, Windows Server 2016 or Windows Server 2012. For these, the security update is expected to be released soon.

Mitigation Actions

Microsoft released an out-of-band security update to address this vulnerability on July 6, 2021. Please see the Security Updates table for the applicable update for your system. Administrators are strongly advised to install these updates. If you are unable to install these updates, see the FAQ and Workarounds sections in the 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 RCE exploit in the Windows Print Spooler service known as “PrintNightmare,” documented in CVE-2021-34527.


Palo Alto Networks provides protection against the exploitation of this vulnerability:

  • Next-Generation Firewalls with a Threat Prevention security subscription (running Applications and Threat content update version 8427+) can automatically block sessions related to this vulnerability (as well as CVE-2021-1675) using Threat IDs 91333, 91346 and 91349.
  • Cortex XDR agent 7.4.1 with content version 189-64538 and above is capable of preventing all currently known implementations of the exploits on vulnerable hosts, including patched hosts with the “Point and Print” feature enabled.

Palo Alto Networks will update this Threat Brief with new information and recommendations as they become available.

Additional Resources