CVE-2021-24092: Windows Defender Privilege Escalation Flaw

CVE-2021-24092 Overview

CVE-2021-24092 is an Elevation of Privilege vulnerability affecting Microsoft Defender and related security products across a wide range of Windows operating systems. This vulnerability allows a local attacker with low privileges to escalate their access to gain elevated system privileges, potentially compromising the integrity, confidentiality, and availability of affected systems.

Critical Impact

A successful exploit of this vulnerability allows local attackers to elevate privileges from a low-privileged user account to SYSTEM level access, enabling complete control over the affected Windows system.

Affected Products

• Microsoft Windows Defender
• Microsoft Windows 10 (multiple versions including 1607, 1803, 1809, 1903, 1909, 2004, 20H2)
• Microsoft Windows 7 SP1
• Microsoft Windows 8.1
• Microsoft Windows RT 8.1
• Microsoft Windows Server 2008 SP2 and R2 SP1
• Microsoft Windows Server 2012 and R2
• Microsoft Windows Server 2016 (including 1909, 2004, 20H2)
• Microsoft Windows Server 2019
• Microsoft Endpoint Protection
• Microsoft Security Essentials
• Microsoft System Center Endpoint Protection (including 2012 and R2)

Discovery Timeline

• February 25, 2021 - CVE-2021-24092 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-24092

Vulnerability Analysis

This vulnerability is classified under CWE-269 (Improper Privilege Management), indicating a flaw in how Microsoft Defender handles privilege separation and access control. The vulnerability requires local access to the target system and low-level privileges to exploit, but does not require user interaction.

The attack exploits improper privilege management within the Microsoft Malware Protection Engine (mpengine.dll), which is the core scanning engine used across Microsoft's security products. When the engine processes certain inputs during scanning operations, it fails to properly validate privilege contexts, allowing an attacker to execute code with elevated SYSTEM privileges.

Root Cause

The root cause of CVE-2021-24092 lies in improper privilege management (CWE-269) within the Microsoft Malware Protection Engine. The vulnerability exists because the engine does not properly separate privileged operations from user-controlled inputs, creating a condition where a low-privileged user can influence operations that execute with SYSTEM privileges.

This type of vulnerability is particularly concerning in security software, as these products typically run with the highest system privileges to perform their protective functions, making them attractive targets for privilege escalation attacks.

Attack Vector

The attack requires local access to the vulnerable system. An attacker who has already gained initial access to a system with low-level privileges (such as through phishing or another vulnerability) can exploit this flaw to escalate their privileges to SYSTEM level.

The exploitation path involves:

1. Gaining initial low-privilege access to the target system
2. Preparing a malicious payload designed to trigger the vulnerability in the Malware Protection Engine
3. Causing the engine to process the malicious input during a scan operation
4. Leveraging the improper privilege handling to execute code with SYSTEM privileges

Since no verified proof-of-concept code is publicly available, the specific exploitation technique involves manipulating inputs processed by the Malware Protection Engine during scanning operations. Attackers would craft payloads that exploit the privilege management weakness to elevate their execution context from user to SYSTEM level.

Detection Methods for CVE-2021-24092

Indicators of Compromise

• Unexpected processes spawning with SYSTEM privileges from MsMpEng.exe or related Defender processes
• Anomalous behavior in the Microsoft Malware Protection Engine (mpengine.dll)
• Suspicious file operations or registry modifications occurring under Defender's process context
• Unusual privilege escalation events involving Microsoft Defender components

Detection Strategies

• Monitor Windows Event Logs for privilege escalation attempts involving Defender processes (Event IDs 4624, 4672, 4688)
• Implement behavioral analysis to detect unexpected child processes spawned by MsMpEng.exe
• Deploy endpoint detection rules that alert on anomalous SYSTEM-level activity originating from security software processes
• Utilize Windows Defender Application Control (WDAC) or AppLocker policies to detect unauthorized code execution

Monitoring Recommendations

• Enable advanced audit policies for privilege use and process creation events
• Configure SIEM rules to correlate events involving Defender processes with subsequent privilege changes
• Implement process integrity level monitoring to detect unexpected elevation patterns
• Review Defender update logs to ensure the Malware Protection Engine is current

How to Mitigate CVE-2021-24092

Immediate Actions Required

• Verify that Microsoft Defender and related security products are configured for automatic updates
• Check the Malware Protection Engine version using PowerShell: Get-MpComputerStatus | Select-Object AMEngineVersion
• Ensure Windows Update is enabled and functioning to receive security patches
• Review user privileges and implement least-privilege principles to limit initial access vectors

Patch Information

Microsoft has released security updates to address this vulnerability. The fix is delivered through automatic updates to the Microsoft Malware Protection Engine. Affected organizations should verify that their systems have received engine version 1.1.17800.5 or later, which contains the fix for this vulnerability.

For detailed patch information and remediation guidance, refer to the Microsoft Security Advisory CVE-2021-24092.

Workarounds

• Ensure automatic signature and engine updates are enabled for all Microsoft security products
• Implement network segmentation to limit the impact of potential privilege escalation
• Apply application whitelisting to restrict unauthorized code execution
• Monitor for unusual process activity involving Microsoft Defender components while awaiting verification of patch deployment

# Verify Microsoft Defender Malware Protection Engine version
powershell -Command "Get-MpComputerStatus | Select-Object AMEngineVersion, AntivirusSignatureVersion, AntivirusSignatureLastUpdated"

# Force Defender signature and engine update
powershell -Command "Update-MpSignature"

# Check Windows Defender service status
sc query WinDefend