CVE-2021-42292: Microsoft 365 Apps Auth Bypass Flaw

CVE-2021-42292 Overview

CVE-2021-42292 is a Security Feature Bypass vulnerability affecting Microsoft Excel and related Office products. This vulnerability allows attackers to bypass security features in Excel that are designed to protect users from malicious content. When a user opens a specially crafted Excel file, the attacker can circumvent the security mechanisms that would normally prevent malicious code execution, potentially leading to full system compromise.

Critical Impact

This vulnerability is actively exploited in the wild and is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Organizations should prioritize immediate patching as attackers are leveraging this bypass to deliver malware through malicious Excel documents.

Affected Products

• Microsoft 365 Apps for Enterprise
• Microsoft Excel 2013 SP1
• Microsoft Office 2013 SP1, 2016, 2019 (Windows and macOS)
• Microsoft Office Long Term Servicing Channel 2021 (Windows and macOS)

Discovery Timeline

• 2021-11-10 - CVE-2021-42292 published to NVD
• 2025-10-30 - Last updated in NVD database

Technical Details for CVE-2021-42292

Vulnerability Analysis

This Security Feature Bypass vulnerability exists in Microsoft Excel's handling of document security controls. The flaw allows attackers to craft malicious Excel files that bypass Protected View and other security features designed to sandbox potentially dangerous content. When users open these specially crafted files, the security mechanisms that would typically warn users or restrict execution fail to engage properly.

The vulnerability requires local access, meaning an attacker must convince a user to open a malicious Excel file. This is commonly achieved through phishing campaigns, malicious email attachments, or drive-by downloads. Once opened, the bypassed security features allow subsequent malicious payloads to execute with the user's privileges.

Root Cause

The root cause lies in improper validation within Excel's security feature implementation. The application fails to properly enforce security boundaries when processing certain document structures, allowing crafted content to bypass the intended security controls. This represents a design flaw in how security features validate and process document content before determining trust levels.

Attack Vector

The attack vector is local, requiring user interaction to open a malicious Excel document. Attackers typically deliver these documents through:

1. Phishing emails containing malicious Excel attachments
2. Compromised websites hosting weaponized documents
3. File-sharing platforms distributing infected spreadsheets
4. Social engineering tactics to convince targets to download and open files

The vulnerability enables attackers to bypass Excel's Protected View, macro security warnings, and other defensive features. Once bypassed, embedded malicious content can execute, potentially installing malware, establishing persistence, or stealing sensitive data.

Detection Methods for CVE-2021-42292

Indicators of Compromise

• Unusual Excel processes spawning child processes such as cmd.exe, powershell.exe, or wscript.exe
• Excel files with suspicious embedded objects or unusual macro signatures
• Unexpected network connections originating from EXCEL.EXE to external IP addresses
• Excel documents received via email with characteristics suggesting phishing origin

Detection Strategies

• Monitor for Excel processes spawning unexpected child processes indicative of code execution
• Implement email gateway scanning for malicious Office document attachments
• Deploy endpoint detection rules to identify Protected View bypass attempts
• Analyze suspicious Excel documents in sandbox environments before user access

Monitoring Recommendations

• Enable advanced logging for Microsoft Office applications including macro execution events
• Configure SIEM alerts for process chains involving EXCEL.EXE with suspicious descendants
• Monitor network traffic for Excel-related processes making external connections
• Track file system events for newly created executables or scripts following Excel file opens

How to Mitigate CVE-2021-42292

Immediate Actions Required

• Apply Microsoft security updates immediately to all affected Office installations
• Block Excel file attachments from external sources at the email gateway until patching is complete
• Enable Attack Surface Reduction (ASR) rules to block Office applications from creating child processes
• Educate users about phishing risks and the dangers of opening unexpected Excel attachments

Patch Information

Microsoft has released security updates to address CVE-2021-42292. Administrators should apply the patches available through Microsoft Update or WSUS. For detailed patch information and download links, refer to the Microsoft Security Advisory CVE-2021-42292. Given that this vulnerability is listed in the CISA Known Exploited Vulnerabilities catalog, organizations should treat patching as an urgent priority.

Workarounds

• Configure Group Policy to force all Office documents from the internet to open in Protected View
• Disable macros for Office documents obtained from the internet
• Implement application allowlisting to prevent execution of unexpected processes spawned by Excel
• Use Microsoft Defender Application Guard to isolate potentially malicious Office documents

# Example: Configure Attack Surface Reduction rules via PowerShell
# Block Office applications from creating child processes
Add-MpPreference -AttackSurfaceReductionRules_Ids d4f940ab-401b-4efc-aadc-ad5f3c50688a -AttackSurfaceReductionRules_Actions Enabled

# Block Office applications from creating executable content
Add-MpPreference -AttackSurfaceReductionRules_Ids 3b576869-a4ec-4529-8536-b80a7769e899 -AttackSurfaceReductionRules_Actions Enabled