Vulnerability record · CVE-2021-34527 · published 2 July 2021
CVE-2021-34527: Windows Print Spooler privileged file operation RCE (PrintNightmare)
Microsoft · Windows 10 1507
The Windows Print Spooler service improperly performs privileged file operations, allowing an attacker to execute arbitrary code as SYSTEM. This is the PrintNightmare flaw, and it affects a broad range of Windows client and server versions. Because the spooler runs with high privileges and is reachable remotely, it is a high-value target for both initial access and lateral movement.
Description
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.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Automated analysis
critical priorityActively exploited in ransomware campaigns, near-maximum EPSS, and SYSTEM-level remote code execution across a wide Windows footprint.
What it is
The Windows Print Spooler service improperly performs privileged file operations, allowing an attacker to execute arbitrary code as SYSTEM. This is the PrintNightmare flaw, and it affects a broad range of Windows client and server versions. Because the spooler runs with high privileges and is reachable remotely, it is a high-value target for both initial access and lateral movement.
Impact
An attacker can run arbitrary code with SYSTEM privileges, enabling installation of programs, viewing or modifying data, and creation of accounts with full user rights. In practice this yields full control of the affected host.
Attack surface
Reachable over the network via the print spooler service (CVSS vector AV:N), requiring low privileges (PR:L) and no user interaction (UI:N). The description also notes that a misconfigured PointAndPrint registry setting (NoWarningNoElevationOnInstall = 1) makes systems vulnerable by design.
Exploitation
CISA KEV lists it as actively exploited with known ransomware campaign use (Black Basta), and EPSS is 0.99792 (99.956th percentile). Multiple references are tagged Exploit, confirming public exploit material exists.
What to do
- Apply the July 2021 Microsoft security updates for all affected Windows versions immediately.
- Verify PointAndPrint registry settings: NoWarningNoElevationOnInstall and UpdatePromptSettings must be 0 or not defined.
- Apply the Group Policy guidance in the vendor FAQ and KB5005010 to restrict installation of new printer drivers.
- If patching is not possible, disable the Print Spooler service on systems that do not require printing.
- Restrict network access to print spooler RPC endpoints where feasible.
Detection
- Monitor for spoolsv.exe spawning child processes or loading unexpected DLLs.
- Audit PointAndPrint registry values and alert on NoWarningNoElevationOnInstall set to 1.
- Watch for new printer driver installations and spooler-related file writes in system directories.
- Correlate print spooler activity with subsequent SYSTEM-level process creation or account creation events.
This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.
Exploitation in the wild
CISA added CVE-2021-34527 to the Known Exploited Vulnerabilities catalog on 3 November 2021 as "Microsoft Windows Print Spooler Remote Code Execution Vulnerability". CISA reports known use in ransomware campaigns. Required action: Apply updates per vendor instructions. Federal deadline 3 May 2022.
Ransomware crews whose documented playbooks reference this CVE: