CVE-2021-40465: Windows 10 Text Shaping RCE Vulnerability

CVE-2021-40465 Overview

CVE-2021-40465 is a Remote Code Execution vulnerability affecting the Windows Text Shaping component across a wide range of Microsoft Windows operating systems. This vulnerability exists in the way Windows processes specially crafted text, allowing an attacker to execute arbitrary code in the context of the current user. The vulnerability requires user interaction, where a victim must open a malicious document or visit a crafted webpage containing malformed text content.

Critical Impact
Successful exploitation allows an attacker to execute arbitrary code with the privileges of the current user, potentially leading to complete system compromise if the user has administrative privileges.

Affected Products

Microsoft Windows 10 (multiple versions including 1607, 1809, 1909, 2004, 20H2, 21H1)
Microsoft Windows 11 21H2
Microsoft Windows 7 SP1
Microsoft Windows 8.1
Microsoft Windows RT 8.1
Microsoft Windows Server 2008, 2008 R2, 2012, 2012 R2, 2016, 2019, 2022
Microsoft Windows Server 20H2

Discovery Timeline

October 13, 2021 - CVE-2021-40465 published to NVD
November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-40465

Vulnerability Analysis

This vulnerability resides in the Windows Text Shaping engine, a core component responsible for rendering complex scripts and text layouts in Windows applications. The text shaping engine handles the conversion of character sequences into properly positioned glyphs, managing complex operations such as ligature substitution, contextual alternates, and bidirectional text rendering.

The flaw allows remote code execution when the text shaping component improperly handles certain malformed text sequences. An attacker who successfully exploits this vulnerability could gain the same user rights as the current user, making the impact particularly severe when the target user has elevated privileges.

Root Cause

The vulnerability stems from improper handling of text sequences within the Windows Text Shaping engine. When processing specially crafted text content, the component fails to properly validate input data, leading to a condition where an attacker can manipulate memory operations during text rendering. This allows for controlled memory corruption that can be leveraged to achieve code execution.

The text shaping subsystem (USP10.DLL and related components) processes complex script rendering and is invoked by numerous Windows applications when displaying text content. The widespread use of this component across the Windows ecosystem significantly increases the attack surface.

Attack Vector

The attack vector is local, requiring user interaction to trigger the vulnerability. Exploitation scenarios include:

Opening a malicious Microsoft Office document containing specially crafted text
Viewing a webpage with embedded malformed text content through a web browser
Opening a malicious PDF or other document format that utilizes Windows text rendering
Previewing malicious files in Windows Explorer

The attacker must convince a user to open or view a malicious file or navigate to a compromised website. Social engineering techniques such as phishing emails with malicious attachments are common delivery mechanisms for this type of vulnerability.

The vulnerability can be exploited through malformed text sequences embedded in documents or web content. When the Windows Text Shaping engine processes these sequences, it triggers the vulnerability condition. Technical exploitation details involve crafting specific text content that causes the rendering engine to mishandle memory operations during glyph positioning and layout calculations. For complete technical details, refer to the Microsoft Security Advisory.

Detection Methods for CVE-2021-40465

Indicators of Compromise

Unexpected crashes or memory errors in applications that process text or documents
Suspicious process spawning from document viewer applications (Microsoft Office, Adobe Reader, web browsers)
Anomalous memory allocation patterns in text rendering components such as USP10.DLL
Unusual network connections initiated by document processing applications

Detection Strategies

Monitor for suspicious child processes spawned by document viewers and web browsers
Implement endpoint detection rules for anomalous behavior in text rendering subsystems
Deploy application whitelisting to prevent unauthorized code execution
Enable Windows Defender Exploit Guard and Attack Surface Reduction (ASR) rules

Monitoring Recommendations

Enable Windows Event logging for process creation events (Event ID 4688) with command-line auditing
Monitor for suspicious DLL loading activity in document processing applications
Implement file integrity monitoring for core Windows text rendering components
Deploy network monitoring to detect post-exploitation command and control traffic

How to Mitigate CVE-2021-40465

Immediate Actions Required

Apply the October 2021 cumulative security updates from Microsoft immediately
Restrict user privileges following the principle of least privilege to minimize impact
Enable Protected View in Microsoft Office applications to reduce exposure
Implement web browser isolation for high-risk browsing activities

Patch Information

Microsoft has released security updates to address this vulnerability as part of the October 2021 Patch Tuesday release. The updates are available through Windows Update, Microsoft Update Catalog, and Windows Server Update Services (WSUS). Organizations should prioritize deployment of these patches across all affected Windows systems.

Detailed patch information and download links are available in the Microsoft Security Advisory for CVE-2021-40465.

Workarounds

Enable Protected View in Microsoft Office to prevent automatic execution of malicious content in documents
Configure web browsers to prompt before processing complex fonts and text rendering
Implement application sandboxing solutions to isolate document viewers from the main system
Deploy email gateway filtering to block suspicious document attachments

bash

# Enable Protected View in Microsoft Office via Registry
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f