How to remediate – Windows PrintNightmare Registry Exposure CVE-2021-34527 OOB Se…

1. Introduction

Windows PrintNightmare Registry Exposure CVE-2021-34527 OOB System is a remote code execution vulnerability in the Windows Print Spooler service. An authenticated attacker can exploit this to run arbitrary code with SYSTEM privileges on affected systems. This affects servers and workstations running vulnerable versions of Windows, potentially leading to complete system compromise. The likely impact is high: confidentiality, integrity, and availability could all be lost.

2. Technical Explanation

The vulnerability occurs because the Windows Print Spooler service does not properly perform privileged file operations when handling point and print registry settings. An attacker with access to a vulnerable system can modify these settings to execute code as SYSTEM. The key registry locations involved are used to control how printers are installed, allowing for remote loading of drivers.

• Root cause: insecure configuration in the PointAndPrint registry keys allows arbitrary file operations.
• Exploit mechanism: An attacker modifies the PointAndPrint registry settings to point to a malicious executable or DLL. When a printer is installed, the system attempts to load this malicious code with elevated privileges.
• Scope: Windows 7 SP1 and later are affected.

3. Detection and Assessment

• Quick checks: Use reg query to check the following keys for insecure values:

reg query "HKLMSOFTWAREPoliciesMicrosoftWindows NTPrintersPointAndPrint" /v EnableRemoteInstalledDrivers

reg query "HKLMSOFTWAREPoliciesMicrosoftWindows NTPrintersPointAndPrintNoWarningNoElevationOnInstall" /v DisablePrompt

reg query "HKLMSOFTWAREPoliciesMicrosoftWindows NTPrintersPointAndPrintUpdatePromptSettings" /v UpdatePromptSetting

• Scanning: Nessus plugin ID 1638974 can detect this vulnerability.
• Logs and evidence: Look for event IDs related to printer installation failures or unexpected file operations in the System event log.

4. Solution / Remediation Steps

The solution involves applying a Microsoft security update that addresses the insecure registry settings. Follow these steps carefully to ensure successful remediation.

4.1 Preparation

• Ensure you have administrative privileges and a stable network connection. Roll back by restoring the backup or snapshot if needed.
• A change window may be required depending on your organisation’s policies.

4.2 Implementation

1. Step 1: Install the latest cumulative update for your version of Windows from Microsoft Update.
2. Step 3: Verify that the Print Spooler service is running correctly.

4.3 Config or Code Example

Before

reg query "HKLMSOFTWAREPoliciesMicrosoftWindows NTPrintersPointAndPrint" /v EnableRemoteInstalledDrivers

After

reg query "HKLMSOFTWAREPoliciesMicrosoftWindows NTPrintersPointAndPrint" /v EnableRemoteInstalledDrivers

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this type of vulnerability. Least privilege reduces the impact if exploited, and a regular patch cadence ensures systems are up-to-date with the latest security fixes.

• Practice 1: Implement least privilege principles to limit user access rights.
• Practice 2: Maintain a consistent patch management process for timely updates.

4.5 Automation (Optional)

PowerShell can be used to check and potentially remediate registry settings, but caution is advised as incorrect modifications can cause system instability.

# Check if the key exists
$keyPath = "HKLM:SOFTWAREPoliciesMicrosoftWindows NTPrintersPointAndPrint"
if (Test-Path $keyPath) {
    Write-Host "Key exists."
} else {
    Write-Host "Key does not exist."
}

5. Verification / Validation

Confirm the fix by checking the registry settings again and performing a smoke test of printer functionality.

• Post-fix check: Run reg query on the affected keys. The ‘EnableRemoteInstalledDrivers’ key should be absent or set to 0.
• Re-test: Re-run the quick checks from Section 3 to confirm the vulnerable settings are no longer present.
• Smoke test: Print a test page to verify basic printer functionality remains intact.
• Monitoring: Monitor the System event log for any errors related to printer installation or spooler service failures.

reg query "HKLMSOFTWAREPoliciesMicrosoftWindows NTPrintersPointAndPrint" /v EnableRemoteInstalledDrivers

6. Preventive Measures and Monitoring

Update security baselines to include the correct registry settings, and consider adding checks in your CI/CD pipelines to prevent similar misconfigurations.

• Baselines: Update your Windows security baseline or Group Policy Objects (GPOs) to enforce secure PointAndPrint settings.
• Pipelines: Add a check during deployment to verify the registry keys are configured correctly.
• Asset and patch process: Review and apply security updates on a regular schedule, ideally within 72 hours of release.

7. Risks, Side Effects, and Roll Back

Applying the update may require a system reboot, potentially causing temporary service interruption. Incorrect registry modifications can lead to printer installation issues.

• Risk or side effect 1: System reboot required, leading to brief downtime.
• Roll back: Restore your system from the backup created in Step 4.1 if any issues arise.

8. References and Resources

• Vendor advisory or bulletin: https://support.microsoft.com/en-us/help/5004945
• NVD or CVE entry: /cve/CVE-2021-34527
• Product or platform documentation relevant to the fix: https://support.microsoft.com/en-us/topic/kb5004945-how-to-identify-and-mitigate-the-printnightmare-vulnerability-1638974