CVE-2021-27090: Windows 10 Privilege Escalation Flaw

CVE-2021-27090 Overview

CVE-2021-27090 is a Windows Secure Kernel Mode Elevation of Privilege vulnerability affecting Microsoft Windows 10 and Windows Server 2016. This local privilege escalation flaw exists within the Windows Secure Kernel Mode, a critical security boundary that enforces isolation between different security levels in virtualization-based security (VBS) environments. An attacker who successfully exploits this vulnerability could gain elevated privileges on the target system, potentially compromising the integrity and confidentiality of sensitive data protected by secure kernel isolation.

Critical Impact

Successful exploitation enables local privilege escalation, allowing attackers to bypass secure kernel isolation and potentially gain SYSTEM-level privileges, undermining Windows virtualization-based security protections.

Affected Products

• Microsoft Windows 10 version 20H2
• Microsoft Windows 10 version 2004
• Microsoft Windows Server 2016 version 20H2
• Microsoft Windows Server 2016 version 2004

Discovery Timeline

• April 13, 2021 - CVE-2021-27090 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-27090

Vulnerability Analysis

This vulnerability resides in the Windows Secure Kernel Mode, which is a core component of Windows virtualization-based security (VBS). The Secure Kernel operates in Virtual Trust Level 1 (VTL1), providing a higher level of isolation from the standard Windows kernel running in VTL0. When VBS is enabled, the Secure Kernel enforces critical security boundaries including Credential Guard, Device Guard, and hypervisor-enforced code integrity (HVCI).

The elevation of privilege flaw allows a local attacker with low-privilege access to potentially break out of security boundaries enforced by the Secure Kernel. Since this requires local access and low privileges as a prerequisite, the attack surface is limited to scenarios where an adversary has already established a foothold on the target system. However, the impact is significant as successful exploitation could compromise the trust boundaries that VBS is designed to protect.

Root Cause

The vulnerability stems from improper handling within Windows Secure Kernel Mode operations. While Microsoft has not disclosed detailed technical specifics, the CWE classification indicates insufficient information about the exact weakness pattern (NVD-CWE-noinfo). Given the context of Secure Kernel Mode vulnerabilities, the root cause likely involves improper validation of requests or data passed between different virtual trust levels, allowing untrusted code in VTL0 to influence or corrupt operations in the higher-privileged VTL1 context.

Attack Vector

The attack vector for CVE-2021-27090 is local, requiring the attacker to have existing code execution capabilities on the target system with at least low-privilege access. The exploitation flow typically involves:

1. The attacker gains initial access to the target Windows system through any means (malware, phishing, compromised credentials)
2. From the low-privilege context, the attacker invokes specific system calls or operations that interact with Secure Kernel Mode
3. Through the vulnerability, the attacker manipulates the Secure Kernel's processing to elevate their privileges
4. With elevated privileges, the attacker can potentially access protected credentials, disable security features, or maintain persistent elevated access

The exploitation does not require any user interaction and has low attack complexity, meaning once an attacker has local access, exploitation is straightforward without additional prerequisites.

Detection Methods for CVE-2021-27090

Indicators of Compromise

• Unusual process activity attempting to interact with Secure Kernel Mode interfaces
• Unexpected privilege escalation events from low-privileged user accounts
• Anomalous system call patterns targeting virtualization-based security components
• Process execution anomalies indicating potential exploitation attempts against VBS features

Detection Strategies

• Monitor Windows Security Event logs for privilege escalation events (Event ID 4672, 4673)
• Enable and review Windows Defender Exploit Guard events for suspicious kernel-mode activity
• Deploy endpoint detection and response (EDR) solutions to identify anomalous process behavior patterns
• Implement behavioral analysis to detect unusual interactions with Secure Kernel Mode interfaces

Monitoring Recommendations

• Enable Windows Security logging with process creation auditing (Event ID 4688) including command-line logging
• Configure Sysmon to capture detailed process and driver activity on systems with VBS enabled
• Review audit logs for unexpected changes to security policies or credential access attempts
• Monitor for any attempts to disable or bypass virtualization-based security features

How to Mitigate CVE-2021-27090

Immediate Actions Required

• Apply the April 2021 security updates from Microsoft immediately to affected systems
• Prioritize patching systems where virtualization-based security (VBS) and Credential Guard are enabled
• Verify that all Windows 10 and Windows Server 2016 systems running versions 2004 and 20H2 are updated
• Implement network segmentation to limit lateral movement from potentially compromised endpoints

Patch Information

Microsoft has released security updates addressing this vulnerability as part of the April 2021 Patch Tuesday release. Detailed patch information and guidance are available in the Microsoft Security Advisory for CVE-2021-27090. Organizations should apply the appropriate cumulative update for their Windows version through Windows Update, WSUS, or manual download from the Microsoft Update Catalog.

Workarounds

• Restrict local access to systems with VBS features enabled to only trusted administrators
• Implement the principle of least privilege to minimize the number of users with local access
• Enable additional monitoring on systems where patching must be delayed for operational reasons
• Consider temporary isolation of unpatched systems from sensitive network segments until updates can be applied

# Verify Windows Secure Kernel Mode status
# Run in elevated PowerShell to check VBS configuration
Get-ComputerInfo | Select-Object -Property DeviceGuard*

# Check if system is patched - verify installed updates
Get-HotFix | Where-Object {$_.InstalledOn -ge "2021-04-13"} | Select-Object HotFixID, InstalledOn