How to remediate – Windows SeriousSAM HiveNightmare Registry Read Vulnerability

Contents

1. Introduction

The Windows SeriousSAM HiveNightmare Registry Read Vulnerability is a security flaw allowing privilege escalation on affected Windows systems. It occurs due to overly permissive access controls on critical system files, specifically the Security Accounts Manager (SAM) database. Successful exploitation allows an attacker to run code with SYSTEM level privileges, potentially compromising data confidentiality, integrity and availability. This typically affects standard Windows server and desktop installations.

2. Technical Explanation

The vulnerability stems from incorrect Access Control Lists (ACLs) on the %windir%system32config directory and related files. An attacker with local access can read sensitive information from the SAM database, enabling them to crack password hashes or otherwise compromise user accounts. The CVE identifier for this issue is CVE-2021-36934.

Root cause: Overly permissive ACLs on the %windir%system32config directory allow unauthorized read access to sensitive files, including the SAM database.
Exploit mechanism: An attacker uses local tools to enumerate and read the contents of the SAM file, then attempts offline password cracking or other credential abuse techniques. For example, an attacker could use a custom script to dump the SAM database and then crack the hashes using John the Ripper.
Scope: Windows operating systems are affected. Specific versions are detailed in Microsoft’s advisory (CVE-2021-36934).

3. Detection and Assessment

Confirming vulnerability requires checking file permissions and identifying shadow copies. A quick check involves verifying the presence of Volume Shadow Copy Service.

Quick checks: Run vssadmin list shadows to confirm VSS is enabled. If it returns a list of shadow copies, further investigation is needed.
Scanning: Nessus plugin ID 1873e7da can detect this vulnerability. This is an example only and may require updating.
Logs and evidence: Examine the System event log for unusual access attempts to %windir%system32config. Look for Event IDs related to file access or security policy changes.

vssadmin list shadows

4. Solution / Remediation Steps

Fixing this vulnerability requires restricting access to the SAM database and deleting shadow copies.

4.1 Preparation

Dependencies: No specific dependencies are required, but ensure you have administrative privileges. Roll back involves restoring from backup or snapshot.
Change window: A standard change window is recommended, with approval from the security team.

4.2 Implementation

Step 1: Restrict access to %windir%system32config using icacls. Run icacls "%windir%system32config" /inheritance:d to remove inherited permissions.
Step 2: Grant explicit permissions only to trusted accounts and groups. For example, run icacls "%windir%system32config" /grant SYSTEM:(OI)(CI)F /grant Administrators:(OI)(CI)F.
Step 3: Delete Volume Shadow Copy Service (VSS) shadow copies using vssadmin delete shadows /all /quiet.

4.3 Config or Code Example

Before

icacls "%windir%system32config"

After

icacls "%windir%system32config" /inheritance:d; icacls "%windir%system32config" /grant SYSTEM:(OI)(CI)F /grant Administrators:(OI)(CI)F

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

Practice 1: Least privilege – limit user and service account permissions to the minimum required for operation, reducing the impact of potential compromise.
Practice 2: Secure defaults – configure systems with secure default settings, including restrictive file permissions.

4.5 Automation (Optional)

A PowerShell script can automate access restriction and shadow copy deletion.

#Requires -RunAsAdministrator
$configPath = "$env:windirsystem32config"
icacls $configPath /inheritance:d
icacls $configPath /grant SYSTEM:(OI)(CI)F /grant Administrators:(OI)(CI)F
vssadmin delete shadows /all /quiet

5. Verification / Validation

Confirm the fix by re-checking file permissions and verifying shadow copies are deleted.

Post-fix check: Run icacls "%windir%system32config". The output should show only SYSTEM and Administrators with full control, and no inherited permissions.
Re-test: Re-run vssadmin list shadows. It should return “No shadow copies found”.
Smoke test: Verify core system functions like user login and application launch continue to work as expected.
Monitoring: Monitor the System event log for any unexpected access attempts to %windir%system32config (Event ID 4663). This is an example only.

icacls "%windir%system32config"

6. Preventive Measures and Monitoring

Update security baselines and implement patch management processes.

Baselines: Update your Windows security baseline to include restrictive file permissions for %windir%system32config, aligning with CIS controls or similar standards.
Asset and patch process: Implement a regular patch cycle, prioritizing critical security updates like this one. Review patches within 72 hours of release.

7. Risks, Side Effects, and Roll Back

Incorrectly restricting access could cause service failures.

Risk or side effect 2: Deleting VSS shadow copies removes restore points, increasing recovery time in case of data loss.
Roll back: Restore from backup or snapshot. If a snapshot is unavailable, carefully re-grant permissions to affected services based on documented requirements.

8. References and Resources

Link only to sources that match this exact vulnerability.

Vendor advisory or bulletin: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-36934
NVD or CVE entry: http://www.nessus.org/u?1873e7da
Product or platform documentation relevant to the fix: https://www.kb.cert.org/vuls/id/506989

Updated on October 26, 2025

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