CVE-2021-31941: Microsoft 365 Apps RCE Vulnerability

CVE-2021-31941 Overview

CVE-2021-31941 is a Remote Code Execution (RCE) vulnerability affecting Microsoft Office Graphics components. This vulnerability allows attackers to execute arbitrary code on affected systems by exploiting flaws in how Microsoft Office processes graphics elements. An attacker could leverage this vulnerability by crafting a malicious document that, when opened by a victim, triggers code execution in the context of the current user.

Critical Impact

Successful exploitation could allow an attacker to execute arbitrary code with the privileges of the current user, potentially leading to complete system compromise, data theft, or lateral movement within an enterprise network.

Affected Products

• Microsoft 365 Apps (Enterprise)
• Microsoft Office 2013 SP1 (including RT edition)
• Microsoft Office 2016
• Microsoft Office 2019 (Windows and macOS)
• Microsoft Outlook 2013 SP1 (RT edition)

Discovery Timeline

• 2021-06-08 - CVE-2021-31941 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-31941

Vulnerability Analysis

This vulnerability exists within the graphics rendering components of Microsoft Office applications. When Office applications process specially crafted documents containing malicious graphics elements, improper handling of these elements can lead to memory corruption conditions that enable arbitrary code execution.

The attack requires user interaction, specifically requiring a victim to open a malicious document. The exploitation occurs locally on the target system, meaning the attacker must deliver the malicious file to the victim through methods such as email attachments, malicious downloads, or shared network locations. Once the document is opened, the vulnerability can be triggered without additional user interaction beyond the initial file opening.

The impact of successful exploitation is severe, allowing complete compromise of confidentiality, integrity, and availability on the affected system. An attacker gaining code execution through this vulnerability would inherit the permissions of the user running the Office application.

Root Cause

The root cause of CVE-2021-31941 lies in the improper handling of graphics data within Microsoft Office applications. The vulnerability stems from insufficient validation or incorrect processing of graphics elements embedded in Office documents. This flaw in the graphics rendering pipeline creates an exploitable condition that attackers can leverage to achieve code execution.

Attack Vector

The attack vector for this vulnerability is local, requiring the attacker to convince a user to open a specially crafted Office document. Typical attack scenarios include:

1. Phishing campaigns - Attackers send emails with malicious Office documents attached, often using social engineering to convince recipients to open the files
2. Watering hole attacks - Hosting malicious documents on compromised websites frequented by target users
3. Supply chain attacks - Inserting malicious documents into legitimate document workflows or shared storage locations

The vulnerability requires no special privileges to exploit, but does require user interaction in the form of opening the malicious document.

Detection Methods for CVE-2021-31941

Indicators of Compromise

• Unusual Office application crashes or unexpected behavior when opening documents
• Office processes spawning unexpected child processes (e.g., cmd.exe, powershell.exe)
• Suspicious outbound network connections originating from Office applications
• Office documents with unusual file structures or embedded objects from untrusted sources

Detection Strategies

• Monitor process creation events for Office applications spawning suspicious child processes
• Implement email gateway scanning to detect and quarantine potentially malicious Office documents
• Enable Microsoft Office Protected View and Attack Surface Reduction (ASR) rules
• Deploy endpoint detection solutions capable of identifying exploitation attempts against Office applications

Monitoring Recommendations

• Configure SIEM alerts for unusual Office process behavior patterns
• Enable detailed logging for Office application activity on endpoints
• Monitor for document downloads from unusual or untrusted sources
• Review email security logs for blocked or suspicious Office document attachments

How to Mitigate CVE-2021-31941

Immediate Actions Required

• Apply the latest Microsoft security updates for affected Office products immediately
• Enable Microsoft Office Protected View to reduce exposure to malicious documents
• Implement Attack Surface Reduction (ASR) rules in Microsoft Defender to block Office applications from creating child processes
• Restrict Office macro execution and consider blocking macros from documents originating from the internet

Patch Information

Microsoft has released security updates to address this vulnerability. Organizations should apply the patches available through the Microsoft Security Advisory for CVE-2021-31941. Additional technical details are available in the ZDI Advisory ZDI-21-669.

Affected versions include:

• Microsoft 365 Apps for Enterprise
• Microsoft Office 2013 SP1
• Microsoft Office 2016
• Microsoft Office 2019 (Windows and macOS)
• Microsoft Outlook 2013 SP1 RT

Workarounds

• Enable Protected View for all Office documents from external sources
• Block Office documents containing certain object types at the email gateway
• Implement application whitelisting to prevent unauthorized code execution
• Configure Office applications to open files in sandboxed environments

# Enable Attack Surface Reduction rules via PowerShell (requires Windows Defender)
# 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

# Block Office applications from injecting code into other processes
Add-MpPreference -AttackSurfaceReductionRules_Ids 75668C1F-73B5-4CF0-BB93-3ECF5CB7CC84 -AttackSurfaceReductionRules_Actions Enabled