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
# 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
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
