CVE-2021-26435: Windows 10 Use-After-Free Vulnerability

CVE-2021-26435 Overview

CVE-2021-26435 is a memory corruption vulnerability in the Windows Scripting Engine that could allow an attacker to execute arbitrary code on affected systems. This vulnerability affects a wide range of Microsoft Windows operating systems, including both client and server editions, making it a significant threat across enterprise environments.

The vulnerability requires local access and user interaction to exploit, but successful exploitation grants the attacker full control over the affected system with the same privileges as the current user.

Critical Impact

Successful exploitation allows attackers to execute arbitrary code with user privileges, potentially leading to complete system compromise, data theft, or lateral movement within enterprise networks.

Affected Products

• Microsoft Windows 10 (multiple versions including 1607, 1809, 1909, 2004, 20H2, 21H1)
• 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 2004, 20H2)
• Microsoft Windows Server 2019
• Microsoft Windows Server 2022

Discovery Timeline

• 2021-09-15 - CVE-2021-26435 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-26435

Vulnerability Analysis

This vulnerability stems from improper memory handling within the Windows Scripting Engine. The underlying issue is classified as CWE-787 (Out-of-Bounds Write), indicating that the scripting engine fails to properly validate memory boundaries during script processing operations.

When a user opens a specially crafted document or visits a malicious website, the scripting engine may write data beyond the allocated buffer boundaries. This out-of-bounds write condition can corrupt adjacent memory structures, potentially allowing an attacker to overwrite critical data or function pointers to hijack program execution flow.

The attack requires user interaction—typically opening a malicious file or visiting a compromised website that hosts specially crafted content designed to trigger the memory corruption condition.

Root Cause

The root cause of CVE-2021-26435 is an out-of-bounds write vulnerability (CWE-787) in the Windows Scripting Engine. The engine fails to properly validate buffer boundaries when processing certain script content, allowing memory writes to occur beyond allocated regions. This improper memory handling creates an exploitable condition that attackers can leverage for code execution.

Attack Vector

The attack requires local access with user interaction. An attacker would need to convince a user to perform an action that triggers the vulnerability, such as:

• Opening a specially crafted document that contains malicious script content
• Visiting a compromised or attacker-controlled website hosting exploit code
• Opening an email attachment designed to trigger the scripting engine vulnerability

Once triggered, the memory corruption can be leveraged to achieve arbitrary code execution within the context of the current user's privileges.

The vulnerability mechanism involves memory corruption through out-of-bounds write operations in the Windows Scripting Engine. When processing malicious input, the engine writes data past allocated buffer boundaries, corrupting adjacent memory regions. This can overwrite critical structures or function pointers, allowing attackers to redirect execution flow to attacker-controlled code. For detailed technical information, refer to the Microsoft Security Advisory.

Detection Methods for CVE-2021-26435

Indicators of Compromise

• Unusual process behavior from scripting engines (wscript.exe, cscript.exe) or web browsers
• Memory access violations or application crashes in script-heavy applications
• Unexpected child processes spawned by Windows Script Host components
• Suspicious document files that trigger scripting engine execution upon opening

Detection Strategies

• Monitor for abnormal memory access patterns in the Windows Scripting Engine components
• Deploy endpoint detection solutions to identify exploitation attempts targeting script processing
• Implement application whitelisting to control script execution in enterprise environments
• Use SentinelOne's behavioral AI to detect memory corruption exploitation techniques

Monitoring Recommendations

• Enable Windows Event Logging for script execution and Application Error events
• Monitor process creation events for suspicious scripting engine activity
• Implement network monitoring for connections to known malicious infrastructure following script execution
• Configure alerts for unexpected process relationships involving scripting components

How to Mitigate CVE-2021-26435

Immediate Actions Required

• Apply the latest Microsoft security updates addressing CVE-2021-26435 immediately
• Restrict access to Windows Scripting Host in environments where scripts are not required
• Implement user awareness training to prevent opening suspicious documents or links
• Deploy SentinelOne endpoint protection to detect and prevent exploitation attempts

Patch Information

Microsoft has released security updates to address this vulnerability as part of their September 2021 Patch Tuesday release. System administrators should apply the appropriate patches for their Windows versions through Windows Update, WSUS, or the Microsoft Update Catalog.

For detailed patch information and download links, refer to the Microsoft Security Advisory for CVE-2021-26435.

Workarounds

• Disable Windows Script Host on systems where scripting functionality is not required
• Configure email gateways to block or quarantine documents containing embedded scripts
• Implement application control policies to restrict scripting engine execution
• Use Microsoft Defender Attack Surface Reduction rules to limit script execution

# Disable Windows Script Host via Registry (run as Administrator)
reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Script Host\Settings" /v Enabled /t REG_DWORD /d 0 /f

# Enable Attack Surface Reduction rule to block script execution
# Requires Windows Defender and appropriate Group Policy configuration
# Rule GUID: d4f940ab-401b-4efc-aadc-ad5f3c50688a (Block Office applications from creating child processes)