CVE-2023-29335: Microsoft Word Auth Bypass Vulnerability

CVE-2023-29335 Overview

CVE-2023-29335 is a security feature bypass vulnerability in Microsoft Word that allows attackers to circumvent security protections designed to safeguard users from malicious document content. This vulnerability stems from improper input validation (CWE-20) within Microsoft Word's security mechanisms, potentially enabling threat actors to deliver malicious content that would otherwise be blocked by Word's built-in security features.

Critical Impact

Successful exploitation allows attackers to bypass Microsoft Word security features, potentially leading to unauthorized access to sensitive data, system compromise, or further attack propagation through document-based attack vectors.

Affected Products

• Microsoft Word 2013, 2016 (including RT and SP1 variants)
• Microsoft Office 2019, Office 2021 LTSC, and Microsoft 365 Apps Enterprise
• Microsoft Windows 10 (versions 1507, 1607, 1809, 20H2, 21H2, 22H2)
• Microsoft Windows 11 (versions 21H2, 22H2)
• Microsoft Windows Server 2008 (SP2 and R2 SP1), 2012, 2012 R2, 2016, and 2022

Discovery Timeline

• May 9, 2023 - CVE-2023-29335 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-29335

Vulnerability Analysis

This security feature bypass vulnerability affects Microsoft Word's protective mechanisms that are designed to prevent malicious content execution. The vulnerability requires network-based delivery of a specially crafted document and user interaction to open the malicious file. While exploitation complexity is considered high, a successful attack can result in complete compromise of confidentiality, integrity, and availability of the affected system.

The improper input validation weakness (CWE-20) indicates that Microsoft Word fails to properly validate or sanitize certain input data, allowing attackers to craft documents that evade security controls. This type of vulnerability is particularly dangerous in enterprise environments where document sharing is common and users may be targeted through phishing campaigns.

Root Cause

The root cause of CVE-2023-29335 is improper input validation within Microsoft Word's security feature implementation. The application fails to adequately validate certain document content or metadata, creating a condition where security features designed to protect users can be bypassed. This allows specially crafted documents to circumvent protections such as Protected View, macro blocking, or other security mechanisms that normally prevent malicious code execution.

Attack Vector

The attack vector for this vulnerability is network-based, requiring an attacker to deliver a malicious Microsoft Word document to the target. Successful exploitation requires user interaction—specifically, the victim must open the crafted document. Attack scenarios typically include:

• Phishing emails with malicious Word document attachments
• Compromised websites hosting weaponized documents
• File sharing platforms where malicious documents are uploaded
• Social engineering tactics to convince users to download and open malicious files

The vulnerability does not require authentication and can be exploited without elevated privileges, making it accessible to remote attackers who can successfully deliver the malicious document to potential victims.

Detection Methods for CVE-2023-29335

Indicators of Compromise

• Suspicious Microsoft Word documents received from unknown or unexpected sources
• Word documents with unusual file structures or embedded objects that trigger security warnings before bypassing them
• Network traffic showing document downloads from untrusted external sources followed by anomalous Word process behavior
• Unexpected outbound connections initiated by WINWORD.EXE processes after document opening

Detection Strategies

• Monitor Microsoft Word process (WINWORD.EXE) for unusual child process spawning or network connections after opening documents
• Implement email gateway scanning to detect potentially malicious Word documents with anomalous characteristics
• Deploy endpoint detection rules to identify Word documents attempting to bypass Protected View or other security features
• Use SentinelOne's behavioral AI to detect post-exploitation activity following document-based attacks

Monitoring Recommendations

• Enable enhanced logging for Microsoft Office applications to capture document opening events and security feature interactions
• Configure SIEM rules to correlate document downloads with subsequent suspicious endpoint activity
• Monitor for modifications to Microsoft Word security settings or Trust Center configurations
• Track and alert on Word processes accessing sensitive system locations or spawning unexpected executables

How to Mitigate CVE-2023-29335

Immediate Actions Required

• Apply the latest Microsoft security updates addressing CVE-2023-29335 immediately across all affected systems
• Ensure Microsoft 365 Apps and standalone Office installations are configured for automatic updates
• Review and reinforce email filtering rules to quarantine suspicious Word document attachments
• Educate users about the risks of opening Word documents from untrusted sources

Patch Information

Microsoft has released security updates to address this vulnerability. Detailed patch information and download links are available in the Microsoft Security Update Guide for CVE-2023-29335. Organizations should prioritize applying these updates to all affected Microsoft Word and Office installations, including standalone applications and Microsoft 365 Apps deployments.

Workarounds

• Enable Protected View for all documents from the Internet and other potentially unsafe locations in Word Trust Center settings
• Disable macros by default and implement strict macro policies through Group Policy
• Use Microsoft Defender Application Guard for Office to isolate potentially malicious documents in a sandboxed environment
• Restrict document downloads to approved file sharing platforms with built-in malware scanning

# Group Policy configuration to enforce Protected View
# Navigate to: User Configuration > Administrative Templates > Microsoft Word > Word Options > Security > Trust Center
# Enable: "Turn off Protected View for attachments opened from Outlook"
# Set to: Disabled (to keep Protected View enabled)

# Registry key to enforce Protected View for Internet files
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f