CVE-2026-20854: Windows LSASS Use After Free Vulnerability

CVE-2026-20854 Overview

CVE-2026-20854 is a use-after-free vulnerability in the Windows Local Security Authority Subsystem Service (LSASS). This critical security flaw allows an authorized attacker with network access to execute arbitrary code on affected Windows systems. LSASS is responsible for enforcing security policy, handling user logins, and managing authentication tokens, making vulnerabilities in this component particularly impactful.

Critical Impact

Successful exploitation allows attackers to execute arbitrary code with elevated privileges on domain controllers and member servers, potentially compromising entire Active Directory environments.

Affected Products

• Windows Local Security Authority Subsystem Service (LSASS)
• Windows Domain Controllers running LSASS
• Windows systems with exposed LSASS network authentication services

Discovery Timeline

• 2026-01-13 - CVE-2026-20854 published to NVD
• 2026-01-13 - Last updated in NVD database

Technical Details for CVE-2026-20854

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) occurs within the Windows Local Security Authority Subsystem Service (LSASS) when handling authentication requests over the network. The flaw stems from improper memory management where a memory object is accessed after it has been freed, leading to potential memory corruption and arbitrary code execution.

The vulnerability requires an authenticated attacker with network access to the target system. While the attack complexity is high due to specific conditions that must be met for successful exploitation, the impact is severe—allowing complete compromise of confidentiality, integrity, and availability on affected systems.

LSASS processes handle sensitive security operations including password verification, access token creation, and security policy enforcement. Exploiting this component could allow attackers to manipulate authentication mechanisms, extract credentials from memory, or pivot to additional systems within an Active Directory domain.

Root Cause

The root cause is a use-after-free condition (CWE-416) in the LSASS service's memory management routines. When processing specific network authentication requests, the service improperly references memory after it has been deallocated. This occurs when an object is freed but a dangling pointer remains, and subsequent operations attempt to use the now-invalid memory reference.

Use-after-free vulnerabilities in Windows system services typically arise from:

• Complex object lifecycle management in multi-threaded authentication code
• Race conditions between memory deallocation and access operations
• Improper reference counting in authentication token handling

Attack Vector

The attack vector is network-based and requires the attacker to have low-privilege authentication to the target system. The exploitation scenario involves:

1. Initial Access: Attacker obtains valid low-privilege credentials for the target Windows environment
2. Network Connection: Attacker establishes a network connection to the LSASS service on the target system
3. Trigger Condition: Attacker sends specially crafted authentication requests designed to trigger the use-after-free condition
4. Memory Corruption: The freed memory is reallocated with attacker-controlled content
5. Code Execution: When LSASS accesses the corrupted memory, attacker-supplied code executes with SYSTEM privileges

The vulnerability mechanism involves sending malformed or specifically timed authentication protocol messages that cause LSASS to free an object prematurely while still maintaining references to it. For detailed technical exploitation information, refer to the Microsoft Security Update.

Detection Methods for CVE-2026-20854

Indicators of Compromise

• Unusual LSASS process crashes or restart events logged in Windows Event Log
• Abnormal memory allocation patterns or access violations in lsass.exe
• Unexpected network authentication attempts from unusual source IP addresses
• Process injection or child process spawning from lsass.exe

Detection Strategies

• Monitor Windows Security Event Logs for Event ID 4625 (failed login attempts) and 4624 (successful logons) with anomalous patterns
• Deploy memory integrity monitoring to detect LSASS memory corruption attempts
• Enable Credential Guard to protect LSASS process memory from tampering
• Implement network segmentation to limit direct access to LSASS on critical systems

Monitoring Recommendations

• Configure SIEM rules to alert on multiple authentication failures followed by success from the same source
• Monitor for lsass.exe process behavioral anomalies including unusual module loading or memory access patterns
• Track network connections to port 445 and 389 (LDAP) for abnormal authentication traffic
• Enable Windows Defender Credential Guard and monitor for bypass attempts

How to Mitigate CVE-2026-20854

Immediate Actions Required

• Apply the latest Windows security updates from Microsoft immediately
• Enable Windows Defender Credential Guard on all domain controllers and sensitive servers
• Restrict network access to LSASS-related ports (445, 389, 636) using firewall rules
• Audit all service accounts and reduce privileges where possible
• Monitor LSASS process behavior using endpoint detection tools

Patch Information

Microsoft has released a security update to address this vulnerability. Administrators should apply the patch as soon as possible through Windows Update or WSUS. For detailed patch information, refer to the Microsoft Security Update CVE-2026-20854.

SentinelOne Singularity Platform provides automated detection and response capabilities for exploitation attempts targeting LSASS. The platform's behavioral AI can identify memory corruption attempts and process injection techniques commonly used in LSASS exploitation.

Workarounds

• Enable Protected Users security group membership for privileged accounts to reduce credential exposure
• Configure Windows Firewall to restrict remote access to authentication services from untrusted networks
• Implement network segmentation to isolate domain controllers from general user traffic
• Deploy SentinelOne agents with Credential Guard enforcement policies

# Enable Credential Guard via Group Policy (Registry Configuration)
reg add "HKLM\SYSTEM\CurrentControlSet\Control\LSA" /v RunAsPPL /t REG_DWORD /d 1 /f
reg add "HKLM\SYSTEM\CurrentControlSet\Control\DeviceGuard" /v EnableVirtualizationBasedSecurity /t REG_DWORD /d 1 /f
reg add "HKLM\SYSTEM\CurrentControlSet\Control\DeviceGuard" /v RequirePlatformSecurityFeatures /t REG_DWORD /d 1 /f